Issue with string::operator+=

Question

I tried to append two letters to a string, but it seems that the string is not changed:

void fun()
{
    string str;
    str += 'a' + 'b';

    cout << str;
}

I checked the source code of STL and found the implementation of operator+=, but I still don't know why.

basic_string&
operator+=(_CharT __c)
{
    this->push_back(__c);
    return *this;
}

Answer 1

By adding 'a' + 'b' you will have 2 chars added together to form another char. Then you add it to the string with +=.

This code will do what you want:

std::string str;
( str += 'a' ) += 'b';

std::cout << str;