Question

I mean the way that is done in editors with insert key on.

So having string like:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The effect will be:

~~~~~~~~~~Hello!~~~~~~~~~~~~~~~~~~

that is without changing length of the string.

Answer 1

Overwriting a portion of a string is done with one of the several overloads of std::string‘s replace member function, for example:

    string str = "~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~";
    string rep = "Hello!";
    cout << str.replace(5, rep.size(), rep) << endl;

You can play with this example at ideone [link].