String Literal Differences Between C and C++

Question

As far as I can tell, before C++11, string literals were handled in almost exactly the same way between C and C++.

Now, I acknowledge that there are differences between C and C++ in the handling of wide string literals.

The only differences that I have been able to find are in the initialization of an array by string literal.

char str[3] = "abc"; /* OK in C but not in C++ */
char str[4] = "abc"; /* OK in C and in C++. Terminating zero at str[3] */

And a technical difference that only matters in C++. In C++ "abc" is const char [4] while in C it is char [4]. However, C++ has a special rule that allows the conversion to const char * and then to char * to retain C compatibility up until C++11 when that special rule is no longer applied.

And a difference in allowed lengths of literals. However, as a practical matter any compiler that compiles both C and C++ code will not enforce the lower C limit.

I have some interesting links that apply:

• http://david.tribble.com/text/cdiffs.htm
• http://www.coding-guidelines.com/cbook/c90c++.pdf

Are there any other differences?

Answer 1

Raw strings

A noticeable difference is that C++'s string literals are a superset of C ones. Specifically C++ now supports raw strings (not supported in C), technically defined at §2.14.15 and generally used in HTML and XML where " is often encountered.

Raw strings allow you to specify your own delimiter (up to 16 characters) in the form:

R"delimiter(char sequence)delimiter"

This is particularly useful to avoid unnecessary escaping characters by providing your own string delimiter. The following two examples show how you can avoid escaping of " and ( respectively:

std::cout << R"(a"b"c")";      // empty delimiter
std::cout << '\n';
std::cout << R"aa(a("b"))aa";  // aa delimiter
// a"b"c"
// a("b")

Live demo

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char vs const char

Another difference, pointed out in the comments, is that string literals have type char [n] in C, as specified at §6.4.5/6:

For character string literals, the array elements have type char, and are initialized with the individual bytes of the multibyte character sequence.

while in C++ they have type const char [n], as defined in §2.14.5/8:

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).

This doesn't change the fact that in both standard (at §6.4.5/7 and 2.14.5/13 for C and C++ respectively) attempting to modify a string literal results in undefined behavior.

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Unspecified vs Implementation defined (ref)

Another subtle difference is that in C, wether the character arrays of string literals are different is unspecified, as per §6.4.5/7:

It is unspecified whether these arrays are distinct provided their elements have the appropriate values.

while in C++ this is implementation defined, as per §2.14.5/13:

Whether all string literals are distinct (that is, are stored in nonoverlapping objects) is implementation- defined.