String concatenation

Question

Why it is possible to do

const string exclam = \"!\";
const string str = exclam + \"Hello\" + \" world\";

And not possible to do this:

Answer 1

const string exclam = "!";    // Initialize a c++ string with an ansi c string
const string str = exclam + "Hello" + " world"; // Using the operator+ method of exclam

You can do it because the operator+ of exclam will return a new string containing "!Hello", on which you subsequently call the operator+ with " world" as parameter, so you get another string which, finally, gets assigned to str by means of the copy constructor.

On the other hand

const string str = "Hello" + " world" + exclam;

cannot be executed because "Hello" is just a const char *, which doesn't have a operator+ taking a const char * as parameter.