String Concatenation

It's because you are concatanating const char[6] + const char[6], which is not allowed, as you said.

In C++, string literals (stuff between quotes) are interpreted as const char[]s.

You can concatenate a string with a const char[] (and vice-versa) because the + operator is overridden in string, but it can't be overridden for a basic type.

The + operator is left-associative (evaluated left-to-right), so the leftmost + is evaluated first.

exclam is a std::string object that overloads operator+ so that both of the following perform concatenation:

exclam + "Hello"
"Hello" + exclam

Both of these return a std::string object containing the concatenated string.

However, if the first two thing being "added" are string literals, as in:

"Hello" + "World"

there is no class type object involved (there is no std::string here). The string literals are converted to pointers and there is no built-in operator+ for pointers.