C++实现Golang defer功能

引用自 zltoolkit
头文件onceToken.h

/*
 * Copyright (c) 2016 The ZLToolKit project authors. All Rights Reserved.
 *
 * This file is part of ZLToolKit(https://github.com/xia-chu/ZLToolKit).
 *
 * Use of this source code is governed by MIT license that can be found in the
 * LICENSE file in the root of the source tree. All contributing project authors
 * may be found in the AUTHORS file in the root of the source tree.
 */

#ifndef UTIL_ONCETOKEN_H_
#define UTIL_ONCETOKEN_H_

#include <functional>
#include <type_traits>
using namespace std;

namespace toolkit {

class onceToken {
public:
    typedef function<void(void)> task;

    template<typename FUNC>
    onceToken(const FUNC &onConstructed, function<void(void)> onDestructed = nullptr) {
        onConstructed();
        _onDestructed = std::move(onDestructed);
    }

    onceToken(nullptr_t, function<void(void)> onDestructed = nullptr) {
        _onDestructed = std::move(onDestructed);
    }

    ~onceToken() {
        if (_onDestructed) {
            _onDestructed();
        }
    }

private:
    onceToken() = delete;
    onceToken(const onceToken &) = delete;
    onceToken(onceToken &&) = delete;
    onceToken &operator=(const onceToken &) = delete;
    onceToken &operator=(onceToken &&) = delete;

private:
    task _onDestructed;
};

} /* namespace toolkit */
#endif /* UTIL_ONCETOKEN_H_ */

适用使用场景:在函数作用域结束后析构对应对象，或避免因抛异常时没有对象。

 socket->setOnErr([weak_self, weak_session, id](const SockException &err) {
        // 在本函数作用域结束时移除会话对象
        // 目的是确保移除会话前执行其 onError 函数
        // 同时避免其 onError 函数抛异常时没有移除会话对象
        onceToken token(nullptr, [&]() {
            // 移除掉会话
            auto strong_self = weak_self.lock();
            if (!strong_self) {
                return;
            }
            assert(strong_self->_poller->isCurrentThread());
            //从共享map中移除本session对象
            lock_guard<std::recursive_mutex> lck(*strong_self->_session_mutex);
            strong_self->_session_map->erase(id);
        });

        // 获取会话强应用
        if (auto strong_session = weak_session.lock()) {
            // 触发 onError 事件回调
            strong_session->onError(err);
        }
    });