forward capture

fwd_lambda.cpp

// https://vittorioromeo.info/index/blog/capturing_perfectly_forwarded_objects_in_lambdas.html
#include <iostream>
#include <functional>

#define FWD(...) std::forward<decltype(__VA_ARGS__)>(__VA_ARGS__)

namespace impl
{

template <typename... Ts>
auto fwd_capture(Ts&&... xs)
{
    return std::tuple<Ts...>(FWD(xs)...); 
}

}

template <typename T>
decltype(auto) access(T&& x) { return std::get<0>(FWD(x)); }

#define FWD_CAPTURE(...) impl::fwd_capture(FWD(__VA_ARGS__))

struct A
{
    int _value{0};
};

auto foo = [](auto&& a)
{
    return [a = FWD_CAPTURE(a)]() mutable 
    { 
        ++access(a)._value;
        std::cout << access(a)._value << "\n";
    };
};

int main()
{
    {
        auto l_inner = foo(A{});
        l_inner(); // Prints `1`.
        l_inner(); // Prints `2`.
        l_inner(); // Prints `3`.
    }
    
    {
        A my_a;
        auto l_inner = foo(my_a);
        l_inner(); // Prints `1`.
        l_inner(); // Prints `2`.
        l_inner(); // Prints `3`.

        // Prints '3', yay!
        std::cout << my_a._value << "\n";
    }
}