lazy_loading::DelayProxy : an approach to delay the creation of an object while still permitting to code with it

lazy_proxy.h

#include <functional>
#include <memory>
#include <optional>
#include <type_traits>
#include <utility>

namespace lazy {

template <typename T> class DelayProxy {
	std::unique_ptr<T> subject{};

  public:
	template <typename... P> void setup(P &&... v) {
		subject = std::make_unique<T>(std::forward<P>(v)...);
	}
	void reset() { subject.reset(); }

	template <typename Fn, typename FnRet = std::invoke_result_t<Fn, T &>,
			  typename Ret = std::conditional_t<!std::is_void_v<FnRet>, std::optional<FnRet>, void>>
	auto operator()(Fn fun) noexcept(std::is_nothrow_invocable_v<Fn, T &>)
		-> Ret {
		if (subject)
			return std::invoke(fun, *subject);

		if constexpr (!std::is_void_v<FnRet>)
			return Ret{};
	}
};

}

main.cpp

#include "lazy_proxy.h"
#include <iostream>

using namespace lazy;

struct SomeClass {
	int a;
	int c;
	SomeClass(int _a, int _c) : a{_a}, c{_c} {}
};

void fun() {
	DelayProxy<SomeClass> delay;

	auto res = delay([&](auto &sc) -> int {
		std::cout << "not invoked\n";
		std::cout << sc.a << " " << sc.c << "\n";
		return sc.a + sc.c;
	});

	std::cout << res.value_or(-1) << "\n";

	delay.setup(3, 2);
  
	delay([&](auto &sc) {
		std::cout << "actually invoked\n";
		std::cout << sc.a << " " << sc.c << "\n";
	});

	auto res_v = delay([&](auto &sc) -> int {
		std::cout << "actually invoked\n";
		sc.a *= 3;
		std::cout << sc.a << " " << sc.c << "\n";
		return sc.a + sc.c;
	});
	std::cout << res_v.value_or(-1) << "\n";

	delay.setup(7, 11);
	delay([&](auto &sc) {
		std::cout << "actually invoked\n";
		std::cout << sc.a << " " << sc.c << "\n";
	});

	std::cout << "now will reset\n";
	delay.reset();
	auto res_v2 = delay([&](auto &sc) -> int {
		std::cout << "again, not invoked\n";
		sc.a *= 3;
		std::cout << sc.a << " " << sc.c << "\n";
		return sc.a + sc.c;
	});
	std::cout << res_v2.value_or(-1) << "\n";
}

int main(){
  fun();
}

output.txt

$ ./lazy_loading
-1
actually invoked
3 2
actually invoked
9 2
11
actually invoked
7 11
now will reset
-1
$