在两个Lambda之间共享变量 [英] Share variable between two lambdas

问题描述

我希望能够在两个lambda函数之间共享包含范围内的变量.我有以下内容:

I want to be able to share a variable in the containing scope between two lambda functions. I have the following:

void holdAdd(const Rect& rectangle, Hold anonymousHeld, Hold anonymousFinish) {
    std::map<int,bool> identifierCollection;

    HoldFinish holdFinish = [=](const int& identifier) mutable {
        if (identifierCollection.count(identifier) == 0) return;

        identifierCollection.erase(identifier);
        anonymousFinish();
    };

    holdCollisionCollection.push_back([=](const int& identifier, const Vec2& point) mutable {
        if (rectangle.containsPoint(point)) {
            identifierCollection[identifier] = true;
            anonymousHeld();
        } else {
            holdFinish(identifier);
        }
    });
    holdFinishCollection.push_back(holdFinish);
}

我在调试器中看到，holdFinish指向的identifierCollection实现与第二个lambda函数中的实现不同.

I can see in the debugger that holdFinish is pointing to a different implementation of identifierCollection than in the 2nd lambda function.

如果我使用[=, &identifierCollection]，则无论我是否使用mutable，都会抛出EXC_BAD_ACCESS.

If I use [=, &identifierCollection] it throws a EXC_BAD_ACCESS whether I use mutable or not.

我对其他实现内联函数的语言的经验是，这应该是可能的.例如在javascript中:

My experience with other languages that implement inline functions is that this should be possible. For instance in javascript:

var a = 10;
var b = function() {
    a += 2;
}
var c = function() {
    a += 3;
}
b();
c();
alert(a);

会警报15.

我该怎么做才能使两个lambda函数都引用相同的identifierCollection实现?因此它的行为与javascript示例相同.

What do I have to do to get both lambda functions to reference the same identifierCollection implementation? So that it behaves in the same way as the javascript example.

推荐答案

与某些脚本语言不同，identifierCollection的生存期不会仅仅因为将其捕获到闭包中而延长.因此，只要将[=]更改为要通过引用捕获的[&]，它便是对要捕获的局部变量的悬挂引用.

Unlike in some scripting languages, identifierCollection's lifetime won't be extended simply because you captured it into a closure. So as soon as you change that [=] for a [&] to capture by reference, it's a dangling reference to a local variable that you're capturing.

您必须自己管理identifierCollection的生命周期；坦白说，这听起来像是共享指针的绝佳机会，它可以按值捕获到每个lambda中.它包装的动态分配的地图实际上会存在您需要的时间.

You'll have to manage the lifetime of identifierCollection yourself; frankly, this sounds like the perfect opportunity for a shared pointer, captured by value into each lambda. The dynamically-allocated map it wraps will literally exist for as long as you need it to.

void holdAdd(const Rect& rectangle, Hold anonymousHeld, Hold anonymousFinish)
{
    auto identifierCollection = std::make_shared<std::map<int,bool>>();

    HoldFinish holdFinish = [=](const int& identifier) mutable {
        if (identifierCollection->count(identifier) == 0) return;

        identifierCollection->erase(identifier);
        anonymousFinish();
    };

    holdCollisionCollection.push_back([=](const int& identifier, const Vec2& point) mutable {
        if (rectangle.containsPoint(point)) {
            (*identifierCollection)[identifier] = true;
            anonymousHeld();
        } else {
            holdFinish(identifier);
        }
    });
    holdFinishCollection.push_back(holdFinish);
}

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