What Is the Type of a String Literal in C++

What is the type of string literals in C and C++?

In C the type of a string literal is a char[] - it's not const according to the type, but it is undefined behavior to modify the contents. Also, 2 different string literals that have the same content (or enough of the same content) might or might not share the same array elements.

From the C99 standard 6.4.5/5 "String Literals - Semantics":

In translation phase 7, a byte or code of value zero is appended to each multibyte character sequence that results from a string literal or literals. The multibyte character sequence is then used to initialize an array of static storage duration and length just sufficient to contain the sequence. For character string literals, the array elements have type char, and are initialized with the individual bytes of the multibyte character sequence; for wide string literals, the array elements have type wchar_t, and are initialized with the sequence of wide characters...

It is unspecified whether these arrays are distinct provided their elements have the appropriate values. If the program attempts to modify such an array, the behavior is undefined.

In C++, "An ordinary string literal has type 'array of n const char'" (from 2.13.4/1 "String literals"). But there's a special case in the C++ standard that makes pointer to string literals convert easily to non-const-qualified pointers (4.2/2 "Array-to-pointer conversion"):

A string literal (2.13.4) that is not a wide string literal can be converted to an rvalue of type “pointer to char”; a wide string literal can be converted to an rvalue of type “pointer to wchar_t”.

As a side note - because arrays in C/C++ convert so readily to pointers, a string literal can often be used in a pointer context, much as any array in C/C++.

Additional editorializing: what follows is really mostly speculation on my part about the rationale for the choices the C and C++ standards made regarding string literal types. So take it with a grain of salt (but please comment if you have corrections or additional details):

I think that the C standard chose to make string literal non-const types because there was (and is) so much code that expects to be able to use non-const-qualified char pointers that point to literals. When the const qualifier got added (which if I'm not mistaken was done around ANSI standardization time, but long after K&R C had been around to accumulate a ton of existing code) if they made pointers to string literals only able to be be assigned to char const* types without a cast nearly every program in existence would have required changing. Not a good way to get a standard accepted...

I believe the change to C++ that string literals are const qualified was done mainly to support allowing a literal string to more appropriately match an overload that takes a "char const*" argument. I think that there was also a desire to close a perceived hole in the type system, but the hole was largely opened back up by the special case in array-to-pointer conversions.

Annex D of the standard indicates that the "implicit conversion from const to non-const qualification for string literals (4.2) is deprecated", but I think so much code would still break that it'll be a long time before compiler implementers or the standards committee are willing to actually pull the plug (unless some other clever technique can be devised - but then the hole would be back, wouldn't it?).

What is the type of a string literal in C++?

The type of the string literal "Hello" is "array of 6 const char".

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 [...]

It can, however, be converted to a const char* by array-to-pointer conversion. Array-to-pointer conversion results in a pointer to the first element of the array.

What is the type of string literal in C?

String literals in C are not pointers, they are arrays of chars. You can tell this by looking at sizeof("hello, world"), which is 13, because null terminator is included in the size of the literal.

C99 allows string literals to be assigned to char *, which is different from C++, which requires const char *.

What is the data type of a string literal in C++?

Expressions have type. String literals have type if they are used as an expression. Yours isn't.

Consider the following code:

#include <stdio.h>

#define STR "HelloHelloHello"

char global[] = STR;

int main(void)
{
    char local[] = STR;
    puts(STR);
}

There are three string literals in this program formed using the same tokens, but they are not treated the same.

The first, the initializer for global, is part of static initialization of an object with static lifetime. By section 3.6.2, static initialization doesn't have to take place at runtime; the compiler can arrange for the result to be pre-formatted in the binary image so that the process starts execution with the data already in place, and it has done so here. It would also be legal to initialize this object in the same fashion as local[], as long as it was performed before the beginning of dynamic initialization of globals.

The second, the initializer for local, is a string literal, but it isn't really an expression. It is handled under the special rules of 8.5.2, which states that the characters within the string literal are independently used to initialize the array elements; the string literal is not used as a unit. This object has dynamic initialization, resulting in loading the value at runtime.

The third, an argument to the puts() call, actually does use the string literal as an expression, and it will have type const char[N], which decays to const char* for the call. If you really want to study object code used to handle the runtime type of a string literal, you should be using the literal in an expression, like this function call does.

What is a literal string & char array in C?

A string literal is an unnamed string constant in the source code. E.g. "abc" is a string literal.

If you do something like char str[] = "abc";, then you could say that str is initialized with a literal. str itself is not a literal, since it's not unnamed.

A string (or C-string, rather) is a contiguous sequence of bytes, terminated with a null byte.

A char array is not necessarily a C-string, since it might lack a terminating null byte.

What is the datatype of string literal in C++?

It is a const char[N] (which is the same thing as char const[N]), where N is the length of the string plus one for the terminating NUL (or just the length of the string if you define "length of a string" as already including the NUL).

This is why you can do sizeof("hello") - 1 to get the number of characters in the string (including any embedded NULs); if it was a pointer, it wouldn't work because it would always be the size of pointer on your system (minus one).

Type of a C++ string literal

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.

Addressing the address of a string literal in C

Your example is not valid:

char **s = &"Is this valid?";   // Not valid, wrong type

This is valid:

char (*s)[15] = &"Is this valid?";  // OK

The type of "Is this valid?" is char[15]. The type of a pointer to an array 15 of char is char (*)[15]. So the type of &"Is this valid?" is char (*)[15].

The type of a string literal is char[N+1] where N is the length of the string.