Initialize const char

Initialize const char. despite it only in parameter and can be in const memory - it not declared as const pointer. So if you pass this char[] to method who changes data you can have interesting behavior. So you can assign a pointer to a constant char array to const char* text; even a pointer to an array in PROGMEM. const char* somecstylefunction(); std::string imacppstring = somecstylefunction(); where it is constructing the string from My GDB just shows three std::string(const char *) constructors being called, and without knowing the theory, one can't tell if this is an initializer_list<const char *> being converted to an initializer_list<string>, or initializer_list<string> being constructed element-wise "" yields an array of const char, whereupon you want an array of or pointer to NON-const unsigned char, both the type and the cv-qualification don't fit. It does not have the same rules for initializing a char[] array from a non-literal const char[] array, or from a const char* pointer (which your PRESIDENT decays into). A value that you will actually need. c_str(); Note that I made the temporary string const, because any changes to it might cause it to re-allocate and thus render cstr invalid. 55. Hot Network Questions Prove that it is always possible for them to join the line so that the number of men in front of them is equal to the number of women behind them How to initialize a const char* and/or const std::string in C++ with a sequence of UTF-8 character? 0. for unicode: LPCTSTR lpszUnicode = L"Test String"; And for ASCII/MultiByte: LPCTSTR lpszMultibyte = "Test String"; Initialization from strings. Initialise std::string in uninitialised, unowned memory. In C++ an initializer is a part of object definition. Follow edited Jul 6, 2011 at 16:53. To initialize a vector of char arrays in C++, we can use the list initialization where each element is a string literal. invalid conversion from ‘char’ to ‘const char*’ c++. About; Products OverflowAI; Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & How do I achieve the dynamic equivalent of this static array initialisation: char c[2] = {}; // Sets all members to '\0'; In other words, create a dynamic array with all values initialised to the But, remember that string literals have type const char * in C++, but you declared your vector to hold elements of type char *. So, formally speaking, specifying initializers for any static members directly inside the class is "incorrect". However, the position of the const keyword matters in a pointer's declaration. Simply make it const char* sports[], and const char* parameter for processString(). Const array of const strings and ERROR "not a compile-time constant" Hot Network Questions OLS linear regression of +/-1 standard deviation and not the mean Can you sustain yourself with the water from Elementalism? How to pick part of a list of scripts In this article, the various functions of the const keyword which is found in C++ are discussed. QString CONSIGNMENT_TAG_DONATE = "DTag"; works because QString s have a non-explicit constructor taking const char* , so the literal can again decay to const char* which is then used to construct the QString . cpp // When built, the But strrchr will return a const char* if the input is a const char*, and a char * if the input is a char *. Your sprintf(ab, "abc%d", 123); line failed, because you did not initialize any memory for I have the following structure. 14. How do you define a char array as a constant? 6. This is one area where C and C++ differ; in C, string literal has type "array N of char", not "array N of I am having trouble initializing a constant array of constant strings. For all other types, the initialization must be done in the When you create a char* initialized to a string, the string data is compiled into the text segment and the program initializes the pointer to point into the text segment. but you don't write any char in between ''. – YES you can but only for int types. one character in total. Thanks! Archived post. Edit. A literal "yes" is actually a const char*, because the literals will be constant data in the programms data section. In C++ you have so many other alternatives: std::string being the obvious choice in your case. e. char *mgt_dev_name; /* and soon after */ mgt_dev_name = NULL; then it is always a better idea to use initialization instead of assignment. In this case, we declare a 5x5 char array and include five braced strings inside the outer curly braces. 4. Explanation of why this code is not undefined behaviour as suggested by some of the comments, using C11 standard references: This code does not violate 6. Segfault. But even if you were to remove the [26] from the assignment so that you are assigning to the array itself, that will still not work either, as you simply can't assign The following code works in VS2010. const double PI = 3. Actually this IS possible, and I used it many times, BUT I initialize it from a configuration file. 212k 37 37 gold badges 399 399 silver Why assigning string to const char* at the initialization is possible (but it's not for normal char*, what is understable since it is a pointer to char, not char array)? const char* word1 = "This is word 1" // This is possible char* word2 = "This is word 2" // This is not (what I fully understand) How does the Triant's answer is the best in my opinion as it achieves the shortest style with the least amount of code whilst still sticking to struct (not using an intermediate List to achieve the goal which is to have an array). See the definition, explanation, notes, example and defect reports of I'm creating module tests for an operation that takes in a const char *, and writes it to a buffer at a given offset. If you want to inizialise it to a string literal, since string literals are stored in read Learn the differences and advantages of three ways of initializing strings in C++: using char*, std::string and char[]. Initialize char** 1. 3 Constants. Due to its being const, const char data[6]; must be initialized to be usable, and it may only be initialized statically (static objects with no initializer get automatically zeroed), with a string literal, or with a brace-enclosed initializer list. See also const in C vs const in C++ Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company There are already constructors with arguments (e. Can I free the memory of the char* string when I assign it to std::string? 17. When you do this: str = 48 + v; You are attempting to change a pointer to either 48 or 49, depending on what v is. A string is a series of characters followed by a string terminator ('\0'). A constant is a class member that represents a constant value: a value that can be computed at compile-time. Either of the approaches can also be used when you have an array of strings (i. However, you can initialize a It's impossible to do it in a standard compliant way. This is a defect in the C language, because if you attempt a write access of a string literal, then it leads to undefined behaviour and the program might crash and burn. I've noticed that I can do the following: I have a confusion while dealing with char pointers. It is therefor safer to not to store the result of the call to str() at all and use cstr only until the end of the full expression: But, remember that string literals have type const char * in C++, but you declared your vector to hold elements of type char *. Here you refer to an expression like. ). Is there a way to do this with a initialization list? Use {{ }} Double Curly Braces to Initialize 2D char Array in C. Initialization from strings. Your options are: You store a special non-string value in your enjoy array set to NULL at the end of the array; Or you store the total size of the enjoy array in a separate value. This So it is impossible to use C struct initialization to initialize char array members in declaration of a struct, right? – tonga. C Program to Initialize Char Array in Struct. initialize const char[] as non-static class member. They are unfortunately not treated as constant type const char[] by the language. This non-const initialization is allowed in C99 code, but is deprecated in C++98 and removed in C++11. 17. MSVC++ is, for compatibility's sake, allowing the same license that plain C does, to treat string literals as having type char * rather than const char *. I am having trouble initializing a constant array of constant strings. 16 Constant expressions Please note, that char* is a pointer to a char, not a string object. The pointer to this string literal is then stored inside the vector of char arrays. It is on you to work in code with a pointer to PROGMEM the right way. This is dangerous because the string is not-modifiable. 9. DetermineElapsedTime(&tm, &tm2);. Initializing a char * from a string literal (e. When used with variables, the const keyword in C++ specifies that the value of the variable cannot be changed after initialization. You need to add null terminator when using strncpy, generally speaking. Commented Sep 5, 2016 at 3:47. given class with field const char* filename; so that the name of the class is for example MyClass. So it's preferred to do: const char *a = "test"; You are correct that "test" in this case is not allocated on the heap or the stack* and instead lies in static memory that is not-modifiable. Modified 6 years, 1 month ago. C++ is a compiled language, not interpreted. Explanation: String literals in C are character arrays, char []. vertexShaderCode_color = { "uniform mat4 uMVPMatrix;\n" "attribute vec4 vPosition;\n" "void main() { \n" " gl_Position = uMVPMatrix * vPosition; \n" "}\n" }; "football" is const char* in C++ (unlike C), so assigning it to a char* array element is fishy. I want it to contain the value {'T', 'E', 'S', 'T', '\0'} or something similar. class A{ private: static const int a = 4; // valid static const std::string t ; // can't be initialized here It initializes arr (a const char *) with a const char pointing at "foo" and discards the excess initalizers. As commend said I mixed a bit char[] with char*, that is not good as they differs a bit. Is there a way to do this with a initialization list? The declaration: static char const *program_name; says program_name is a (variable) pointer to constant characters. (3) substring constructor works because the "DTag" (which is of type const char[5]) decays to const char*, thus the assignment makes sense. String literal (optionally enclosed in braces) may be used as the initializer for an array of matching type: . Assign Unicode character to a char . Of course, if the array is declared as a local object, it is allocated locally and initialized at run-time, but that can be still thought of as a single-step process that cannot be meaningfully subdivided. My GDB just shows three std::string(const char *) constructors being called, and without knowing the theory, one can't tell if this is an initializer_list<const char *> being converted to an initializer_list<string>, or initializer_list<string> being constructed element-wise A value of type "const wchar_t *" cannot be used to initialize an entity of type "LPCSTR" 1. How to initialize a const char* and/or const std::string in C++ with a sequence of UTF-8 characters? I'm using a regular expression API that accepts UTF8 string as const char*. When a templatized function is being compiled for a particular concrete use, the entire function needs to be compiled, not just the branches that happen to Constructs a string object, initializing its value depending on the constructor version used: (1) empty string constructor (default constructor) Constructs an empty string, with a length of zero characters. @MichaelKrelin-hacker: Technically not, you cannot (ever) bind a reference to a value (or compile time constant), the standard is quite explicit as to what actually happens: Otherwise, a temporary of type “cv1 T1” is created and initialized from the initializer expression using the rules for a non-reference copy-initialization (8. This is kind of an abuse of the system, but if you REALLY want to define it in the header file (and you don't have C++17), you can do this. This has the same effect as how to initialize const char** array. The point of std::vector is dynamic growth at run-time, not any old syntax checking that should be done at compile-time. You are trying to initialize a WCHAR*, which is a pointer to a non-const character, to point at that array. However, assigning it to a char * discards this constraint; a simple char * suggests that the characters pointed to by the ptr1 pointer are not constant and you can now freely write ptr1[0] = 'A'; In Microsoft C++, you can use a string literal to initialize a pointer to non-const char or wchar_t. 3/6 because the space returned by malloc is not "an object defined with a const-qualified type". If it's OK for you works in a ValueOneHolder specialization, you first can develop a constexpr function that, given a C-style array, return the size of the array class MyClass { private: const char *filename; public: void func (const char *_filename); } void MyClass::func (const char *_filename) { filename = _filename; //This isn't going to work } What I want to achieve is not simply assign one memory address to another but to copy contents. If a static or thread-local (since C++11) variable is constant-initialized (see below), constant initialization is performed instead of zero initialization before all other initializations. char *pointer = "Hello world"; /* Danger Will Robinson !!!! */ or (same net effect) how initialize char* const argv [] in c++. Successive bytes of the string literal (including the terminating null character if there is room or if the array is of unknown size) initialize the elements of the array. source is a const char *, a pointer to const characters, so the characters cannot be changed by dereferencing the pointer (i. error: non-const static data member must be initialized out of line. A really good rule of thumb regarding const: . if you want an array of size STR_SIZE: const char version[] = " version 0. Hot Network Questions Why 出 needs negative ません One flip coin game Why can it be that connectors do not cause reflections when they are short enough? Use the . Hot Network Questions Do airports conduct drug tests when you arrive from another country? Is this amount of chain slack normal? After traveling 250,000,000 miles, how much is the laser from the Psyche spacecraft attenuated by having to go through Earth's atmosphere? YES you can but only for int types. In the same way as every other array. How to initialize a const char* ? Using any of default, value, copy or list initialization. class Program { const string key = "6Z8FgpPBeXg="; static void Main(string[] args) { var buffer = Convert. I am writing a program and I need to initialize a message buffer which will hold text. Standard initialization of char [] with char (s); where ch is a variable or constant of type char : Terminating null character is ignored if the size of the char array is the same as the number of characters in the initializer. Giving char* to std::string for management (to eventually free memory) 2. const char means that the characters cannot change. My GDB just shows three std::string(const char *) constructors being called, and without knowing the theory, one can't tell if this is an initializer_list<const char *> being converted to an initializer_list<string>, or initializer_list<string> being constructed element-wise const std::string tmp = stringstream. Argument of type "LPCWSTR" is incompatible of the parameter of type "LPCSTR" 1. ordinary string literals and UTF-8 string literals (since C11) can initialize arrays of any character type (char, signed char, unsigned char) ; L-prefixed wide string literals can be used to initialize arrays of any type compatible with const char* and const wchar_t* are two different types with no implicit conversion between them. Ask Question Asked 6 years, 1 month ago. How to initialize a constant array in heap memory using C++? Hot Network Questions How to create a time delay (24h) using Why can I not do this: char* p = new char[10]; void SetString(char * const str) { p = str; } SetString("Hello"); I have a const pointer to a char, why can I not assign the const pointer to . argument of type "const char *" is incompatible with parameter of type "LPCWSTR" 8. answered Jul 6, 2011 at 16:22. Never is really long time, but you should avoid initialization char[] to string, because, "string" is const char*, and you are assigning it to char*. These are . Follow edited Feb 10, 2010 at 13:30. And that's what you're doing here (const added by me for modern correctness): There are already constructors with arguments (e. Example: Definition of const member in general, needs initialization of the variable too. If it fits your needs, you could use aggregate initialization like so: struct { const char* string; } myAnonymousStruct = { "some text" }; But if you're trying to default initialize more than just the one instance of the struct then you may want to give your struct a constructor and initialize members in it instead. c_str() method for const char *. It isn't clear what you intended to do with your code, but a valid initialization of a char* with no conversions would be In the statement: studentPtr->name[26] = { 'M','a','r','k'};, you are accessing the 27th char in the array (which is out of bounds!) and trying to assign a <braced-init_list> to that single char. Please have a look at following code: class Person { char* pname; public: Person(char* name) { //I want to initialize 'pname' with the person's name. There are different types of such references with different behaviors. Compare and contrast with: static char * const unalterable_pointer = "Hedwig"; #ifdef _UNICODE typedef wchar_t TCHAR; #else typedef char TCHAR; #endif // _UNICODE typedef const TCHAR* LPCTSTR; So you can initialize a LPCTSTR variable as you would a const wchar_t* or a const char* variable, e. #ifdef _UNICODE typedef wchar_t TCHAR; #else typedef char TCHAR; #endif // _UNICODE typedef const TCHAR* LPCTSTR; So you can initialize a LPCTSTR variable as you would a const wchar_t* or a const char* variable, e. Use the . A value of type const char* cannot be assigned to an entity of type char*? Hot Network Questions Why does it say the whole generation died, if Yocheved was still alive? looking for a word meaning trimming eyebrows Why do You are assigning a pointer to an array, which is not allowed. The point of auto is to resolve to the simplest possible type which "works" for the type of the assignment. 0"; Compiler concatenates the adjacent pieces of string-literals, making one bigger piece of string-literal. Hot Network Questions Why 出 needs negative ません One flip coin game Why can it be that connectors do not cause reflections when they are short enough? How to initialize a const char array data member with C++? 4. I am able to make it work, however I am writing below various ways used to initialize the strings in C and I want to understand the difference. You can however change where it points to. const unsigned char *b= (const unsigned char *)(a); I presume you mean const char * and char * const . str(); const char* cstr = tmp. Initializing a Struct's variable const char* const* with C++. especially native api. could not convert <string> from ‘const char []’ to <class name> 18. cpp // Build: cl /Zc:strictStrings /W3 C2440s. A better way is to store a constant string in Bin64 and convert back to byte[] on the fly. Is it possible to do in C++11 or above?. 14; Then, we have used this constant variable to calculate the area of a circle: double area = PI * radius * radius; Const References. Viewed 2k times 5 Initially I started trying to initialize a vector of To clarify the keyword const: A const is always related to the "item" left to it. It is possible to initialize a std::array from another std::array, but only if they have the same size line 161: Error: Cannot use char** to initialize const char**. So cmd will not have the null terminator. Programmatically calling main(int argc, char **argv) in C. To examine the type of char you have, use The default value of the char type is \0, that is, U+0000. Yacoby Yacoby. Read Declarations Right-to-Left. That means that what the pointer points to is a constant that should not be modified (if you want to modify it you must first copy it), and that is a good thing because it helps to detect many programming errors at compile time. The compiler doesn't know the difference between a PROGMEM pointer and a pointer in SRAM. Also note, that the unary * operator on a pointer dereferences the pointer. Fwiw, there is a compiler setting that is available to make it work for old code that did not get it Cannot initialize a vector of const char*/string array with an initializer-list on declaration. pstr = "rubbish"; where the red squiggly line is under the assignment symbol. My question is, how do I initialize that member? Here's what I'm trying to do: typedef Learn how to create and initialize C-style string objects with const char, const char* and constexpr std::string_view. It's a single pointer to const GLchar*. – M. But sometimes you want to keep underlying array uninitilized (maybe for performance reasons). It is a pointer to const char. initializing const char array . The initialization probably took place at compile time. Just a small addition to great JohannesD answer. You can use &mystring[0] to get a char * pointer, but there are a couple of gotcha's: you won't necessarily get a zero terminated string, and you won't be able to change the string's size. unsigned char *tempBuffer = NULL; If you want to initialize an array of unsigned chars, you can do either of following things: unsigned char *tempBuffer = new unsigned char[1024](); // and do not forget to delete it later delete[] tempBuffer; or To convert a char* to const OLECHAR* you can use the macro A2COLE. My comment to that answer makes it a tiny bit better in my opinion as it avoids using generics for the tuple and also (as a result), the tuple fields can now a value of type "const wchar_t *" cannot be used to initialize an entity of type "LPOLESTR" I have #include <string> at the top. See syntax, examples, pros and cons of each method and Learn how to use the const keyword in C++ to declare and initialize constant variables, pointers, and objects. mzimmers I'm working on a firmware app. char * string = "mystring"; You are right that doing string[1]='r'; is undefined. In general, if you want to play it safe, it's good to initialize to NULL all pointers; in this way, it's easy to spot problems derived from uninitialized pointers, since dereferencing a NULL pointer will yield a crash (actually, as far The logic implemented by the C++03 language standard is based on the following rationale. either it has an initializer or its default-initialization results in some initialization being performed, and You can also initialize a pointer to char with an array of chars: const char *bar = "good bye"; this works because of the “decay to pointer” feature of C and C++. But the type of the data in "hello" is const; so a char* does not want to point at it. class A{ private: static const int a = 4; // valid static const std::string t ; // can't be initialized here the first member of the struct MORSECODE is a char not a string so it should be initialzed with a char, the first one should be { 'E', ". So, to fix your array, it needs to be not an array of chars, but an array of const char*s. Moreover, for char operands, arithmetic and bitwise logical operators perform an operation on the corresponding character codes and produce the result of the int type. This sample generates C2440: // C2440s. : int const x; // x is a constant int const int x; // x is an int which is const // easy. How to point char *argv[] from Main function to the table. mzimmers. Argument type "WCHAR *" is incompatible with parameter of type "const char *" Hot Network Questions When used with variables, the const keyword in C++ specifies that the value of the variable cannot be changed after initialization. My question is, how do I initialize that member? Here's what I'm trying to do: typedef struct httpd_req {const char uri[HTTPD_MAX_URI_LEN + 1]; } httpd_req_t; class MyClass { private: const char *filename; public: void func (const char *_filename); } void MyClass::func (const char *_filename) { filename = _filename; //This isn't going to work } What I want to achieve is not simply assign one memory address to another but to copy contents. You’ll need storage for both the array of char const* as well as the entities pointed to. You are allowed to assign a const char[12] type to const char *options[2][100]; options[0][0] = "test1"; options[1][0] = "test2"; Re-reading your question and comments though I'm guessing that what you really want to do is this: const char *options[2] = { "test1", "test2" }; Share. Also, which is the most appropriate method for initializing a wchar_t/char string? Method I: wchar_t message[100]; Initializing a Vector of Char Arrays in C++ . e. An empty string ("") consists of no characters followed by a single string terminator character - i. static const int a = 10; //at declaration 2) Second way can be . If you want your static member to be any other type, you'll have to define it somewhere in a cpp file. – The structure being used effectively uses a C-style approach to defining a variable sized array of pointers to char (with const sprinkled over it). 7. This will imply a narrowing conversion const char * -> char *, which brace initialization doesn't allow. const int a; a = 5; // problem in C as code attempts to write `a` // Really should be `const char *fred`, but allowed for backwards compatibility. Rather than cast away the constness you should keep it const and make a copy if you want to edit its contents. You cannot initialize it with a pointer or another array. char *mgt_dev_name = NULL; However, initialization is only good when you can initialize your object with a meaningful useful value. The exception of this is, when const is the first word in the expression, then it relates to the item right to it. Note also that in C++ char != signed char and char != unsigned char . Initializing a char as 256. std::vector<std::string> strings = somehow_compute_the_strings(); The C++ standard has specific rules for initializing a char[] array from a string literal. In case of no arguments passed to foo constructor, array will be default initialized. Note that the final element is the NUL-terminator \0 which the language exploits as an "end of string" marker. The pointer can change (so it can be assigned in main()), but the string pointed at cannot be changed via this pointer. In general case, this is only Thank you, This answered the reason I came to this question. C/C++ Initialise char array to const char* 0. Commented Nov 16, 2016 at 20:29. #include <cstring> class MyClass{ const char* filename; public: MyClass(const char* name); }; How can I initialize the field filename by name? NOTE: I must do it with strcpy function. The brackets are used to initialized char* s, but char*s must be declared const as JL Borges indicated. 8. The copy can be mutable, so there is no need for const. initializing const char array. g. It won't be a static member, but it will be a constant that only takes up storage per compilation unit (rather than per class instance): My understanding is as follows: char * points to a string constant, modifying the data it points to is undefined. The curly braced list can also be utilized to initialize two-dimensional char arrays. Improve this answer. Following article focus on differentiation and usage of all of these. Using ASCII characters in C++ program. ordinary string literals and UTF-8 string literals (since C11) can initialize arrays of any character type (char, signed char, unsigned char) ; L-prefixed wide string literals can be used to initialize arrays of any type compatible with const char * and const wchar_t * are not the same and not implicitly convertible. The first, const char *, is a pointer to a constant character. In this example I initialized the struct with the bracket notation which zeroes everything past the specified how initialize char* const argv [] in c++. To make it work with gcc without fixing the types, you need to write (char *)"data" everywhere you have bare "data". the rule becomes really useful in the following: int const * const p; // p is const Cannot initialize a vector of const char*/string array with an initializer-list on declaration. arrays in C++ are strange beasts, they do not behave like most other types, in particular they decay to pointers and do not have copy-constructors (unless wrapped in a structure/class). If you don't like old-style casts, const_cast<char I am writing a program and I need to initialize a message buffer which will hold text. Now that you do here is You cannot rely on data produced after your main has started for initialization of static variables, because static initialization in the translation unit of main happens before main gets control, and static initialization in other translation units may happen before or after static initialization of main translation unit in unspecified order. Attempting to do so results in undefined behavior. Obviously, I'm missing something. M. You especially have to be careful not to add characters past the end of the string or you'll get a buffer overrun (and probable crash). static const char *_myArray[10]; I have created a child Child1 class's CPP file and can initialize _myArray[10] using the below syntax. If it compiled, it would point to random memory The class template basic_string stores and manipulates sequences of character-like objects, which are non-array objects of TrivialType and StandardLayoutType. The string type represents text as a sequence of For what you trying to accomplish I'd recommend declaring a non-const pointer and a const pointer and assigning the non-const one to the const one after the initialization: int _tmain(int argc, _TCHAR* argv[]) { const int *tempTempInt = new int[4]; TryInitialize(tempInt); const int* const tempInt = tempTempInt; std::cout << tempInt[1] << endl; //this is now constant. Note that each string literal in this example initializes the five-element rows of the matrix. but say LsaOpenPolicy (yet one known exception) by some reason require not const OBJECT_ATTRIBUTES pointer – When initializing an object of array type, the initializer must be either a string literal (optionally enclosed in braces) or be a brace-enclosed list of initialized for array members. The pointer itself is mutable. In first function warning is return discards 'const' qualifiers from pointer target type. New comments cannot be posted and votes The const is not a problem, but you need to cast the pointer - this isn't recommended generally but if you don't have another choice, you can do one of the following. How to create a char array from string constant? 0. The class is dependent neither on the character type nor on the nature of operations on that type. See also const in C vs const in C++ You've pretty much answered your own question. The char type supports comparison, equality, increment, and decrement operators. In the C code, it declares a 'char * foo = "string literal" ', and foo is expected to be a char *, not a const char *, in a later function. c++ static const struct initialization. Instead you need a char const*, or a pointer to const chars. 2. , char *s = "whatever";) is allowed even though it violates this general rule (the literal itself is basically const, but you're creating a non-const pointer to it). Starting from C++14 you can use std::make_index_sequence and std::index_sequence. const char* abc = "abcabc"; You can't initialize the variable inside the class, because it technically hasn't been defined yet (only declared). We can also make references using the const keyword. So, if you have a 'char* pChar' it should be possible to do it like this: const OLECHAR* pOleChar = A2COLE( pChar ); BSTR str = SysAllocString( pOleChar ); // do something with the 'str' SysFreeString( str ); // need to cleanup the allocated BSTR Newer compilers/code could take advantage that const provides. Can ASCII code for a char be printed in C++ like in C? 17. non-ASCII character C++ does not allow direct decay of a const char[N] type to anything other than a const char* (or a pointer to a typedef of const char*), so in your case it is required to issue a diagnostic. (std::ios_base::openmode, const Allocator&)), so we'd need a placeholder to distinguish such arguments from values we're trying to format into the stream, or a weirder workaround like requiring the values to be formatted into the stream be passed in as an initialiser list. char hello[] = "Hello"; char world[] = "World"; Let's initialize 2 pointers to above arrays. LPCWSTR ERROR C++ A string literal is of type const char[N], its contents must not be modified. Therefore, 'Initializing': Cannot convert from 'const wchar_t[35]' to 'LPWSTR' 2. you are allocating not an array of chars but one char with this line: const char * myStr = new char(STR_SIZE); and that one allocated char is initialized with the value of STR_SIZE, causing a "char overflow" in this case. The definitions of the operations are supplied via the Traits template parameter - a specialization of It's a bit late but I think your issue may be that you've created a zero-length array, rather than an array of length 1. The relevant section in the C11 standard (n1570) is 6. and in second warning is assignment discards 'const' qualifiers from pointer target type. for example big family of functions which got POBJECT_ATTRIBUTES as in parameter. Geez, what a waste that would be just for the sake of a const sanity-check. The initialization code should be platform independent. 16):. What you can do instead is to declare your array as "readonly". You fix this by passing a pointer to the variables by just passing their memory addresses using the & operator as shown below . typedef struct _person { int age; char sex; char name[]; }person; I have done some basic internet search (but unsuccessful) on how to create an instance and initialize a structure with a flexible array member without using malloc(). ) #include <algorithm> #include <vector> int main(int, char*[]) { // Initial test data const char* testdata = "the quick brown fox jumps over the lazy dog. It's a very dangerous thing, has been deprecated basically forever, and was finally removed in C++11. See examples of nested arrays, empty initialization, and Learn how to write and use character constants in C and C++, with different types and encodings. Second one is almost exactly the same as const int * const so you will be copying sizeof(int*) and in const int you will be copying sizeof(int), so probably exactly the same amount of data. That can never work. We have 2 char arrays:. You can't create a 'const' array because arrays are objects and can only be created at runtime and const entities are resolved at compile time. Any way to get szPathName as a char * instead of const char* automatically? If not, copy [path UTF8String] to a dynamically allocated char *. unfortunately old flaw of windows api. The second, char * const is a constant pointer to a character. for unicode: LPCTSTR lpszUnicode = L"Test String"; And for ASCII/MultiByte: LPCTSTR lpszMultibyte = "Test String"; I would like to initialize constexpr char[] member with another constexpr char [] member. When written as an array, the compiler places the initialized string data in the data segment instead, which is the same place that your global variables and such live. 3. Using reference to int has no sense - you probably shouldn't use const char *pointer = "Hello world"; The const represents the fact that modifying a string literal gives undefined behaviour (and means the above prevents using pointer to modify that string literal). T = char*). net and the Windows API and I'm currently writting a class which talks the serial port. L"TEST" is a string literal of type const wchar_t[5], which is an array of const characters (since the literal exists in read-only memory). In the class, keep static const set<char> mySet;. 9/14: . However, note that default initializing a pointer leaves it with an indeterminate value and therefore not useful for anything except comparing with null, and assigning a new value. I Learn how to initialize arrays of known or unknown size in C, using string literals, brace-enclosed lists, or designators. Skip to main content. But that is not because of the char *, but because of the string literal involved in a way that it is put We parenthesis character constant between ' ', e. The reason you can't "write" to your auto variable is that it's a const char * or const char [1], because that is the type of any string constant. This seems like it should be a simple thing but I've been Googling all morning and can't find the answer. Newer compilers/code could take advantage that const provides. For a std::string, when you pass "hello" to it, it copies the characters into a buffer owned by the std::string. Using reference to int has no sense - you probably shouldn't use Every const char* ends in '\0' would be better stated something like every C style string end in '\0'. So I would try the following: Doesn't const char *s means that "s is a pointer which is pointing towards a constant char " then why it is giving me this warning? I am not trying to change values. so remove PROGMEM from const PROGMEM char* text; Note that the array is read/write and thus not constant. The ability to implicitly convert string literals to char* was only ever there in C++ for backwards compatibility with C. Insights appreciated. Well The compile time string concatenation almost selled me on using #defines for strings but I'll stick to const char* std::vector uses the heap. And then there is const char * const where the pointer and character cannot change. Value of type "const char *" cannot be assigned to an entity of type "LPSTR" 0. The reference is then bound to It initializes arr (a const char *) with a const char pointing at "foo" and discards the excess initalizers. Reference Type char* abc = (char*) ("abcabc"); A string literal is a constant and, as such, may be stored in write protected memory. For example: for normal structures like Initialize a const char* Array. How to convert ASCII value into char in C++? 1. Thus you cannot initialize an array (member of std::array of char[4]) by an object of another array. Whenever const keyword is attached with any method(), variable, pointer variable, and with the object of a class it prevents that specific object/method()/variable to modify its data items value. 9 initialization says: 14 An array of character type may be initialized by a character string literal or UTF-8 string literal, optionally enclosed in braces. An array of character type may be initialized by a character string literal or UTF−8 string literal, optionally enclosed in braces. Prerequisite: Pointers There is a lot of confusion when char, const, *, p are all used in different permutations and meanings change according to which is placed where. From week. You cannot do following: const char *arr = "ABC"; const char d_arr[25] = arr; The code in the question is wrong, we all agree to that, but it has nothing to do with 'constant expressions' as you say in this answer, but rather with the fact that you can only initialize constant static members of a class in the declaration if they are of integer or enum type. 3 and 14. Now if I now have a This is a char array declaration and initialization, you assign char array initial value: char name[25] = "String literal"; You cannot do this assignment latter in a code, because name is a unmodifiable lvalue , so you can use its value (which is string literal address in our case) but you cannot modify it (assign a new value). Why should it be a problem to add constness? I am stuck, please help me out memory: [m y _ c h a r _ a r r a y | inbufptr | inbufptr_pos] ^ ^ | (1) | (2) inbufptr inbufptrpos The pointer char* inbufptr points to the beginning of the array, and does not promise to keep anything constant. const GLchar** vertexShaderCode_color; That's not a const char** array. The pointer is not const, so it can change. A constant expression is an expression that shall be fully evaluated at compile-time. (2) copy constructor Constructs a copy of str. Where an expression is required to be constant this is indicated in the grammar by using constant-expression. Initialize String in C++. Hot Network Questions How strongly would a Harris administration be committed to opposing "fracking?" Ramifications of having each chapter be a different 'episode' in a novel? If you exile a Dryad Arbor with Hazel's Brewmaster can all your foods tap for a green mana? initializing const char array . cpp file): It occurred to me that since std::string AKA std::basic_string<char> accepts string literals which are char *s just as a char [] does, it can accept a char just like a char [] would. Although it has const-qualified type, it denotes an object which I'm wonder, what is the proper way of initializing a string? Well, since the second snippet defines an uninitialized pointer to string, I'd say the first one. Hi all - This isn't purely a C++ problem, but it could arise in a C++ program. Stack Overflow. We cannot directly store the char array into the vector, but we can store them as pointers. The C-type code has functions that return const char* and the C++ code has in numerous places things like . " } Nowadays, the letters between double quotes like "string" are not plain char arrays as in the past, but const char *. I would like to know if it is possible to initialise the System::String^ from a std::string. *The standard says nothing about the stack or heap, though that's how it's usually A value of type "const char*" cannot be used to initialize an entity of type "char *" 0. i. In order to write a line to the serial port, I use the WriteLine function which takes a parameter, System::String^. c[i] = 'A'; char A. I am sorry to disagree with the comments and answers saying that it is not possible for a static const symbol to be initialized at program startup rather than at compile time. Hot Network Questions What is the quantum mechanical root of causality? Japanese When used with variables, the const keyword in C++ specifies that the value of the variable cannot be changed after initialization. ; You can also use both options -- this is the case, for example, with arguments given to int main(int argc, "Hello world" is a read-only literal with a const char[12] type. Viewed 2k times 5 Initially I started trying to initialize a vector of One method would be to use the array to initialize the vector. The expression mydeneme->a is not an object, it is an expression. String literals are constant data in C++ (compilers tend to store them in read-only memory when possible). C++: initialize char array with a string? 5. char *(*)[MAX] The compiler is warning you that you're initializing a pointer of one type from one of another type. FromBase64String(key); } } When initialization is performed with constant expressions, the process is essentially performed at compile time. Here's the deal with const char *. See examples, rules, and explanations for different scenarios Learn how to set the initial values of static or thread-local variables to a compile-time constant in C++. 16 Constant expressions. but say LsaOpenPolicy (yet one known exception) by some reason require not const OBJECT_ATTRIBUTES pointer – There are already constructors with arguments (e. char *a = buffer->data(); const unsigned char *b= reinterpret_cast<unsigned char *>(a); or plain C style. 5). 1. An attempt to modify the string Initialize const char * with out any memory leaks. and. class A { static const int a; //declaration }; const int A::a = 10; //defining the static member outside the class initializing const char array . answered Feb 10, 2010 at 11:00. Add a comment | 4 Answers Sorted by: Reset to default 33 The Windows CreateFile function is actually a macro that expands to one of: CreateFileA, which takes a file path of type const char* CreateFileW, which takes a file path of type const A constant is a class member that represents a constant value: a value that can be computed at compile-time. Therefore writing to it can crash your program, it is undefined behaviour. The pointer cannot change, the character it points to can. Commented Sep 23, 2013 at 19:23. (I created a std::string from your const char* array, but you could probably avoid that if you really wanted to. Initialize char** 22. So to be super-correct, one would define the string as static char const * const values[], which can be read right to left: values is an array of const pointers to const chars and all of How to initialize a const char [] with a string literal. In general, if you want to play it safe, it's good to initialize to NULL all pointers; in this way, it's easy to spot problems derived from uninitialized pointers, since dereferencing a NULL pointer will yield a crash (actually, as far I have a const char array in Base class. char *pointer = "Hello world"; /* Danger Will Robinson !!!! */ or (same net effect) In the above example, we have initialized a constant variable PI. Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Explanation: String literals in C are character arrays, char []. See syntax, examples, and notes on multicharacter constants. source[0] = 'A'; is a constraint violation). If you want to write really bad code you could also do. My To clarify the keyword const: A const is always related to the "item" left to it. See examples, advantages and disadvantages of each The following defines a pointer to constant text: const char* hello="hello"; The following defines an array: char myChars[100]; You are trying to assigning a pointer to a single const char* word1 = "word1"; This is syntactic sugar for something like: static const char __string_1[6] = {'w', 'o', 'r', 'd', '1', 0x00}; const char* word1 = __string_1; Assigning a string Learn how to express literal values of various string and character types in C++, using different encodings, prefixes, and escape characters. So if you pass this char[] to method who const std::array<unsigned char, 8> getByte; and initialize it in this way (note the double braces): ABCD() : ObjNum(3), getByte{{'a','b','c','d','e','f','g','h'}} {} Thanks for your explanation. 1) Inside the class , if you want to initialize the const the syntax is like this. The qualifier const can be applied to the declaration of Initialize a const char* Array. In C++11 and later, you can no longer assign a string literal directly to a pointer-to-non-const-char (char*) 1. Or find some way to use std::string instead, which would be preferred. I'm working on a firmware app. C and C++ allow you to type a literal using " surrounding the alphanumeric characters for convenience and that NUL-terminator is added for you. How to initialize a static array in C++. char* c[MAX]; The type of this expression &c is "pointer to a length-MAX array of pointers to char", or . But initializing an array of pointers with an array of chars simply does not make sense. Now what about line 29: b. There are certain set of rules for the declaration and const char **addresses = {"0xabcdefgh"}; produces only a warning. There's nothing about const char* that says it has to be used for C style strings. Also, which is the most appropriate method for initializing a wchar_t/char string? Method I: wchar_t message[100]; unfortunately old flaw of windows api. As a sidenote, const char* (which is A char* is just a pointer; as every pointer, you need a (owned) memory area to initialize it to. 5. char *fred = "sally"; C++ took a stronger approach and demands the initialization. 7 @Agnel It will work fine My personal preference is to use the declaration line initialization because: Where no other variables must be constructed this allows the generated default constructor to be used; Where multiple constructors are required this allows the variable to be initialized in only one place rather than in all the constructor initialization lists I have a confusion while dealing with char pointers. The reason to use enums instead of macro constants or const int for constant integer values is the fact this is the only type-safe way to declare a constant integer which can be portable used as an array boundary. Therefore, this constructor is discarded, no other is found, and your compiler complains. "; I've recently started learning about . I am trying to convert a piece of C code to C++. Here is how you could build it from a std::vector<std::string>:. static const int arr[] = {16,2,77,29}; vector<int> vec (arr, arr + sizeof(arr) / sizeof(arr[0]) ); Share. Initializing a pointer to non-const character data to point at an array of const character data is deprecated in C++98 (to I'm wonder, what is the proper way of initializing a string? Well, since the second snippet defines an uninitialized pointer to string, I'd say the first one. Commented Jul 29, 2009 at 15:10. . 12. In the implementation (i. (see Vandevoorde/Josutiss "C++ Templates: The Complete Guide") E. 0. Incompatible pointer types initializing 'const char **' with an expression of type 'char [11]' [clang -Wincompatible-pointer-types] and it fails when accessing addresses[0]. How can I initialize this? It needs to end with a null terminating character. However, assigning it to a char * discards this constraint; a simple char * suggests that the characters pointed to by the ptr1 pointer are not constant and you can now freely write ptr1[0] = 'A'; That is because you initialized char *ab to a read-only string. Empty space is nothing but suppose if you wants to assigned space then do: c[i] = ' '; // ^ space if wants to assigned nul char then do: c[i] = '\0'; // ^ null symbol In C++14 and before, use template specialization instead of conditionals. Constant Variables:. . This is nonsensical. However, the position of the const keyword I'm working on a firmware app. So if you try to change it, kaboom. What you write inside the class for static members is actually only a declaration. Paul R Paul R. A library uses a struct with a const char array member. I get "warning: initialization from incompatible pointer type" and "warning: excess elements in scalar initializer" – CB Bailey. const char *book[] = {"hello", "good I'm creating module tests for an operation that takes in a const char *, and writes it to a buffer at a given offset. A variable or temporary object obj is constant-initialized if . This seems like it should be straight-forward, but I've not been able to find any documentation showing how it should be done. The reference is then bound to We typically access them via pointers, of type const char* (in the olden days you could use char*, but that's no longer true; perhaps you're using an ancient compiler). Doesn't const char *s means that "s is a pointer which is pointing towards a constant char " then why it is giving me this warning? I am not trying to change values. In C, the type of a string literal is array of char, but in C++, it's array of const char. So to be super-correct, one would define the string as static char const * const values[], which can be read right to left: values is an array of const pointers to const chars and all of From the C# Language Specification (§ 17. The compiler does not "look forward to see what you are doing with the variable", so it The function is expecting a pointer to 2 variables but you are passing the variables themselves, that is the issue. h (showing only relevant parts): class Week { private: static const char *const *days = { "mon", "tue", "wed Say I have this: int x; int x = (State Determined By Program); const char * pArray[(const int)x]; // ?? How would I initialize pArray before using it? Because the initial size of the Array is I've got a const char * returned from a processing function, and I'd like to convert/assign it to an instance of std::string for further manipulation. I want to have filename as "const char*" and not as "char*". This is one area where C and C++ differ; in C, string literal has type "array N of char", not "array N of A value of type "const char*" cannot be used to initialize an entity of type "char *" 0. See examples of raw string struct T { static const char* const str = "hi"; // ^ error: 'constexpr' needed for in-class initialization of // static data member 'const char* const T::str' of non-integral type }; but it is of a Never is really long time, but you should avoid initialization char[] to string, because, "string" is const char*, and you are assigning it to char*. – Xirema. My Explanation. This is simply because there's lots of code that depends on doing this, and nobody's been willing to break that code, so they have (from 2 simple variable initialization question). 3k 16 16 gold badges 117 117 silver badges 121 121 bronze badges. For compatibility with C C++ still allows to initialize a char* with a const char*. Initializing a pointer to non-const character data to point at an array of const character data is deprecated in C++98 (to The relevant part of C11 standard draft n1570 6. Hot Network Questions Why is tetrazole acidic? What is the wire between these switches and how are they able to work independently? works because the "DTag" (which is of type const char[5]) decays to const char*, thus the assignment makes sense. h (showing only relevant parts): class Week { private: static const char *const *days = { "mon", "tue", "wed const char *pointer = "Hello world"; The const represents the fact that modifying a string literal gives undefined behaviour (and means the above prevents using pointer to modify that string literal). Last edited on JLBorges > Now what about line 29: where the red squiggly line is under the assignment symbol. To "properly" initialize a pointer (unsigned char * as in your example), you need to do just a simple. An array of pointers to char could be initialized as. 1: though some C++11 compilers may allow it as a non-standard extension for backwards compatibility, which may need to be enabled manually via L"TEST" is a string literal of type const wchar_t[5], which is an array of const characters (since the literal exists in read-only memory). If you're not going to grow then create a You have different option, depending on what you want to achieve. const char *Base:: C2440 can be caused if you attempt to initialize a non-const char* (or wchar_t*) by using a string literal in C++ code, when the compiler conformance option /Zc:strictStrings is set. crzplz hsise gcxteybg mnt gca qhjatks hlwa sgbwl owlye ysuk