从lambda中的容器中移除自我 [英] Remove self from the container in the lambda

问题描述

我想注册一个回调，该回调可能会将其从其容器中删除.但是我发现捕获的变量似乎已清除.像这样的代码:

I want to register a callback that may remove itself from its container. But I found the captured variables seem to be cleaned up. the code like this:

class A {
    int val_;
public:
    A(int val) : val_(val) {}
    void foo() {
        std::cout << val_ << std::endl;
    }
};

int main()
{
    const int func_index = 1;
    std::unordered_map<int, std::function<void(void)>> container;
    A a(10);
    container.insert(std::make_pair(func_index, [&container, func_index, &a] () {
        container.erase(func_index);
        for (int i = 0; i < 10; i++) {
            a.foo();
        }
    }));
    container[func_index]();
    return 0;
}

---

解决方案，将std :: function对象移到lambda中以延迟其破坏:

---

the solution, move the std::function object into the lambda to delay its destruction:

class A {
    int val_;
public:
    A(int val) : val_(val) {}
    void foo() {
        std::cout << val_ << std::endl;
    }
};

int main()
{
    const int func_index = 1;
    std::unordered_map<int, std::function<void(void)>> container;
    A a(10);
    container.insert(std::make_pair(func_index, [&container, func_index, &a] () {
        std::function<void(void)> self = std::move(container[func_index]);
        container.erase(func_index);
        for (int i = 0; i < 10; i++) {
            a.foo();
        }
    }));
    container[func_index]();
    return 0;
}

推荐答案

您的代码具有未定义的行为，因为您是在从容器中删除闭包后访问闭包的.在使用容器后，应将其从容器 中移除:

Your code has undefined behaviour, since you are accessing the closure after deleting it from the container. You should remove the closure from the container after doing work with it:

class A {
    int val_;
public:
    A(int val) : val_(val) {}
    void foo() {
        std::cout << val_ << std::endl;
    }
};

int main()
{
    const int func_index = 1;
    std::unordered_map<int, std::function<void(void)>> container;
    A a(10);
    container.insert(std::make_pair(func_index, [&container, func_index, &a] () {
        for (int i = 0; i < 10; i++) {
            a.foo();
        }
        container.erase(func_index);
    }));
    container[func_index]();
    return 0;
}

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