Concatenate text and variables C++

I passed mostly through all similar questions here, but can't make it work.

I have one const string and I need to add variable value to the end.

I tried to do that with thic C++ code:

std::stringstream volume_control;   
std::volume_control << "/usr/bin/amixer cset numid=1 " << volume_value << endl;
std::cout << volume_control << endl;

But nothing prints out, where I am wrong?

Thanks for the answer.