fix.cpp

#include <iostream>
#include <functional>

template <class F>
struct Fixed {
    F f;
    Fixed(F f): f{f} {}
    template <class... Args>
    decltype(auto) operator()(Args&&... args) const {
        return f(std::ref(*this), std::forward<Args>(args)...);
    }
};

// for C++14
template <class F>
Fixed<std::decay_t<F>>
fix(F&& f) { return {std::forward<F>(f)}; }


int
main(){
    // C++14: use fix function template
    auto fib14 = fix([](auto f, int n) -> int {
        switch ( n ) {
            case 0: return 0;
            case 1: return 1;
            default: return f( n - 2 ) + f( n - 1 );
        }
    });
    
    std::cout << fib14(7) << std::endl;
    
    // C++17: class template deduction
    Fixed fib17 = [](auto f, int n) -> int {
        switch ( n ) {
            case 0: return 0;
            case 1: return 1;
            default: return f( n - 2 ) + f( n - 1 );
        }
    };
    
    std::cout << fib17(7) << std::endl;
}