Templates, decltype and Overloads All at Once

1#include <iostream>
2#include <utility>
3 
4template <typename T, typename U>
5auto combine(T first, U second) -> decltype(first + second) {
6    return first + second;
7}
8 
9template <typename T>
10void identify(T& value) {
11    std::cout << "L " << value << "\n";
12}
13 
14template <typename T>
15void identify(T&& value) {
16    std::cout << "R " << value << "\n";
17}
18 
19template <typename T>
20class Foo {
21    T m_value;
22public:
23    static int counter;
24    Foo(T value) : m_value(value) {}
25    T value() const { return m_value; }
26};
27 
28template <typename T>
29int Foo<T>::counter = 10;
30 
31template <typename T>
32std::ostream& operator<<(std::ostream& os, Foo<T>& foo) {
33    Foo<T>::counter += 1;
34    return os << "Foo& " << foo.value() << " " << Foo<T>::counter;
35}
36 
37template <typename T>
38std::ostream& operator<<(std::ostream& os, Foo<T>&& foo) {
39    Foo<T>::counter += 2;
40    return os << "Foo&& " << foo.value() << " " << Foo<T>::counter;
41}
42 
43template <typename T>
44void reveal(T&& item) {
45    std::cout << "reveal: " << item << "\n";
46}
47 
48int main() {
49    int n = 4;
50    decltype(combine(n, 0.5)) total = combine(n, 0.5);
51    std::cout << total << "\n";
52    identify(n);
53    identify(n++);
54    identify(++n);
55    std::cout << n << "\n";
56    Foo<int> f(n);
57    std::cout << f << "\n";
58    std::cout << Foo<int>(1) << "\n";
59    reveal(Foo<int>(2));
60    std::cout << f << "\n";
61    std::cout << std::move(f) << "\n";
62    return 0;
63}