#include <string>

using namespace std;

​

int main(void)

{

    string str1 = "C++ is very nice!";

    string str2 = "nice";

    string str3 = "awesome";

    string::size_type index = str1.find(str2);

    if (index != string::npos)

    {

        str1.replace(index, str2.length(), str3);

    }

    cout << str1;

    return 0;

}