C++ STL: will c_str() be changed when creating const copy of std::string?

Question

This is question about difference between STL implementations in handling const copy of std::string. I have such short test, which does 2 const copies and prints addresses returned by c_str():

#include <stdio.h>
#include <string>
using namespace std;
int main()
{
          string a("Hello World!");
    const string b(a);
    const string c(b);

    printf("a: %p = %s\n", a.c_str(), a.c_str());
    printf("b: %p = %s\n", b.c_str(), b.c_str());
    printf("c: %p = %s\n", c.c_str(), c.c_str());
  return c.c_str() == b.c_str();
}

On my gcc 4.6.2 with libstdc++.so.6.0.16 STL all pointers are returned as equal.

Can I rely on this behaviour?

Is it portable or defined in recent standard?

Will this work on current or future versions of libstdc++, libc++, Microsoft STL, stdcxx (apache.org)?

Answer 1

You can't rely on that behavior at all. The only thing you're guaranteed with respect to c_str() is that the pointer returned is valid until the underlying string is mutated (specific methods/feature invalidate the returned pointer). You have no guarantee it will work on any particular architecture or compiler and shouldn't rely on such behavior.

Answer 2

This is what's known as COW (Copy on Write) semantics. It is an optimization strategy to avoid unnecessary copying of strings. But you cannot rely on this behavior - it is an implementation detail of GNU libstdc++. In fact, it is not allowed by the C++11 standard.