standard c_str() problem in C++

I have craeted following kind of function in a class:

void SetStr(string& s1, string& s2)
 {
 const char* s =(s1+s2).c_str();
 cout<<s<<endl;
 }

On calling it from main nothing is getting printed on console. While the following function works:

void SetStr(string& s1, string& s2)
 {
 cout<<(s1+s2).c_str()<<endl;
 }

Can someone please tell me what is the problem.
Thank you.