operator+

template<class E, class T, class A>
    basic_string<E, T, A> operator+(
        const basic_string<E, T, A>& lhs,
        const basic_string<E, T, A>& rhs);
template<class E, class T, class A>
    basic_string<E, T, A> operator+(
        const basic_string<E, T, A>& lhs,
        const E *rhs);
template<class E, class T, class A>
    basic_string<E, T, A> operator+(
        const basic_string<E, T, A>& lhs,
        E rhs);
template<class E, class T, class A>
    basic_string<E, T, A> operator+(
        const E *lhs,
        const basic_string<E, T, A>& rhs);
template<class E, class T, class A>
    basic_string<E, T, A> operator+(
        E lhs,
        const basic_string<E, T, A>& rhs);

The functions each overload operator+ to concatenate two objects of template class basic_string. All effectively return basic_string<E, T, A>(lhs).append(rhs).