Wgtteol.cpp

#define _CRT_SECURE_NO_WARNINGS
#include <cstdio>
#include <cassert>

#include <stdexcept>
#include <iostream>
#include <functional>

struct X {
    X() { puts("X()"); }
    X(const X&) { puts("X(const X&)"); }
    X(X&&) noexcept { puts("X(X&&)"); }

    ~X() { puts("~X()"); }
};

/*
struct scope_guard 
{
    std::function<void()> _f; // 多态包装类
    template<typename F, typename ...Args> 
    scope_guard(F&& f, Args&& ...args) :_f{ std::bind(f, std::forward<Args>(args)...) } {}
    ~scope_guard() { _f(); }
    scope_guard(const scope_guard&) = delete;
    scope_guard& operator=(const scope_guard&) = delete;
};
*/

template<typename F, typename... Args>
struct scope_guard {
    template<typename Fn, typename... Ts>
    scope_guard(Fn&& f, Ts&& ...args) :values{ std::forward<Ts>(args)... }, _f{ std::forward<Fn>(f) } {}
    ~scope_guard() { std::apply(_f, values); }
    std::tuple<Args...> values;
    F _f;
};

template<typename F, typename... Args>
scope_guard(F&&, Args&&...) -> scope_guard<std::decay_t<F>, std::decay_t<Args>...>;

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);
    }
}