s.c_str(), key[i].c_str()

These are two examples of using the c_str() function in C++.

The c_str() function is a member function of the string class in C++. It returns a const char* pointer to the string data, which can be used in C-style functions that expect a null-terminated string.

In the first example, s is a string object and we are using the c_str() function to convert it to a const char* pointer. This is useful when passing the string data to functions that expect a C-style string.

In the second example, key[i] is also a string object and we are using the c_str() function to convert it to a const char* pointer. This is useful when comparing the string data to other C-style strings or performing other operations that require a C-style string.