mq卢瑟.cxx

#include <functional>
#include <cassert>
#include <iostream>


template<typename F, typename...Args>
    requires requires(F f, Args...args) { std::invoke(f, args...); }
struct scope_guard {
    F f;
    std::tuple<Args...>values;

    scope_guard(auto&& func, auto&&...args) :f{ std::forward<decltype(func)>(func) }, values{ std::forward<decltype(args)>(args)... } {}
    ~scope_guard() {
        std::apply(f, values);
    }
    scope_guard(const scope_guard&) = delete;
};

template<typename F, typename...Args>//推导指引非常重要
scope_guard(F&&, Args&&...) -> scope_guard<std::decay_t<F>, std::decay_t<Args>...>;
struct X {
    X() { puts("X()"); }
    X(const X&) { puts("X(const X&)"); }
    X(X&&) noexcept { puts("X(X&&)"); }
    ~X() { puts("~X()"); }
};

int main() {
    
    {
        // scope_guard的作用之一，是让各种C风格指针接口作为局部变量时也能得到RAII支持
        // 这也是本题的基础要求
        FILE* fp = nullptr;
        try {
            fp = fopen("test.txt", "a");
            auto guard = scope_guard([&] {
                fclose(fp);
            fp = nullptr;
                });

            throw std::runtime_error{ "Test" };
        }
        catch (std::exception& e) {
            puts(e.what());
        }
        assert(fp == nullptr);
    }
    puts("----------");
    {
        // 附加要求1，支持函数对象调用
        struct Test {
            void operator()(X* x) {
                delete x;
            }
        } t;
        auto x = new X{};
        auto guard = scope_guard(t, x);
    }
    puts("----------");
    {
        // 附加要求2，支持成员函数和std::ref
        auto x = new X{};
        {
            struct Test {
                void f(X*& px) {
                    delete px;
                    px = nullptr;
                }
            } t;
            auto guard = scope_guard{ &Test::f, &t, std::ref(x) };
        }
        assert(x == nullptr);
    }
}