c++ - Is storage for the same content string literals guaranteed to be the same?

Question

Is the code below safe? It might be tempting to write code akin to this:

#include <map>

const std::map<const char*, int> m = {
    {"text1", 1},
    {"text2", 2}
};

int main () {
    volatile const auto a = m.at("text1");
    return 0;
}

The map is intended to be used with string literals only.

I think it's perfectly legal and seems to be working, however I never saw a guarantee that the pointer for the literal used in two different places to be the same. I couldn't manage to make compiler generate two separate pointers for literals with the same content, so I started to wonder how firm the assumption is.

I am only interested whether the literals with same content can have different pointers. Or more formally, can the code above except?

I know that there's a way to write code to be sure it works, and I think above approach is dangerous because compiler could decide to assign two different storages for the literal, especially if they are placed in different translation units. Am I right?

Answer 1

Whether or not two string literals with the exact same content are the exact same object, is unspecified, and in my opinion best not relied upon. To quote the standard:

[lex.string]

16 Evaluating a string-literal results in a string literal object with static storage duration, initialized from the given characters as specified above. Whether all string literals are distinct (that is, are stored in nonoverlapping objects) and whether successive evaluations of a string-literal yield the same or a different object is unspecified.

If you wish to avoid the overhead of std::string, you can write a simple view type (or use std::string_view in C++17) that is a reference type over a string literal. Use it to do intelligent comparisons instead of relying upon literal identity.