C ++字符串文字的类型

Question

Out of curiosity, I'm wondering what the real underlying type of a C++ string literal is.

Depending on what I observe, I get different results.

A typeid test like the following:

std::cout << typeid("test").name() << std::endl;

shows me char const[5] .

Trying to assign a string literal to an incompatible type like so (to see the given error):

wchar_t* s = "hello";

I get a value of type "const char *" cannot be used to initialize an entity of type "wchar_t *" from VS12's IntelliSense.

But I don't see how it could be const char * as the following line is accepted by VS12:

char* s = "Hello";

I have read that this was allowed in pre-C++11 standards as it was for retro-compatibility with C, although modification of s would result in Undefined Behavior. I assume that this is simply VS12 having not yet implemented all of the C++11 standard and that this line would normally result in an error.

Reading the C99 standard ( from here , 6.4.5.5) suggests that it should be an array:

The multibyte character sequence is then used to initialize an array of static storage duration and length just sufficient to contain the sequence.

So, what is the type underneath a C++ string literal?

Thank you very much for your precious time.

Answer 1

The type of a string literal is indeed const char[SIZE] where SIZE is the length of the string plus the null terminating character.

The fact that you're sometimes seeing const char* is because of the usual array-to-pointer decay.

But I don't see how it could be const char * as the following line is accepted by VS12: char* s = "Hello";

This was correct behaviour in C++03 (as an exception to the usual const-correctness rules) but it has been deprecated since. A C++11 compliant compiler should not accept that code.

Answer 2

The type of a string literal is char const[N] where N is the number of characters including the terminating null character. Although this type does not convert to char* , the C++ standard includes a clause allowing assignments of string literal to char* . This clause was added to support compatibility especially for C code which didn't have const back then.

The relevant clause for the type in the standard is 2.14.5 [lex.string] paragraph 8:

Ordinary string literals and UTF-8 string literals are also referred to as narrow string literals. A narrow string literal has type “array of n const char”, where n is the size of the string as defined below, and has static storage duration (3.7).

Answer 3

First off, the type of a C++ string literal is an array of n const char . Secondly, if you want to initialise a wchar_t with a string literal you have to code:

wchar_t* s = L"hello"