Q:

C++ string.c_str()

If using g++ and clang++, I get ++my string==my string##my string--. While MSVC and Intel Compiler, it is ++==my string##my string--. 
Why?
#include <string>
#include <iostream>

using namespace std;

string test()
{
    string s0 = "my string";
    return s0;
}

int main()
{
    string s = test();
    const char* s1 = test().c_str();
    const char* s2 = s.c_str();
    cout << "++" << s1 << "==" << s2 << "##" << test().c_str() << "--" << endl;
    return 0;
}

Is it an undefined behavior?

A:

In a comment, you asked:

Why test().c_str() can work but s1 not?

test().c_str() works only in some contexts, not all contexts.
std::cout << test().c_str() << std::endl;

is guaranteed to work since the temporary returned by test() is required to stay alive until the execution of the statement is complete.
On the other hand,
char const* s1 = test().c_str();
std:cout << s1 << std::endl;

is undefined behavior since the temporary is not required to live beyond completion of execution of the first line.

