std::stringstream with std::cout error

[mahmood n via gcc-help]

Hi,
A code is fine with gcc-4.8. However, with gcc-5.4, I get 

error: no match for ‘operator<<’ (operand types are ‘std::basic_ostream<char>’ and ‘std::stringstream {aka std::__cxx11::basic_stringstream<char>}’)
    std::cout << "Failed to execute: " << cmd << std::endl;
                                       ^
note: candidate: operator<<(int, int) <built-in>
note:   no known conversion for argument 2 from ‘std::stringstream {aka std::__cxx11::basic_stringstream<char>}’ to ‘int’


Part of the code for that error is

std::stringstream cmd;
cmd << "a string”;
int result = system(cmd.str().c_str());
if(result){
     std::cout << "Failed to execute: " << cmd << std::endl;
     exit(1);
}


I changed that cout line to  cmd.str() and the error disappeared. However, I am not if I have to use cmd.str().c_str()?

Regards,
Mahmood