在两个 lambda 表达式之间共享变量 [英] Share variable between two lambdas
问题描述
我希望能够在两个 lambda 函数之间的包含范围内共享一个变量.我有以下几点:
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);
}
我可以在调试器中看到 holdFinish
指向的 identifierCollection
实现与第二个 lambda 函数不同.
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]
它会抛出一个 EXC_BAD_ACCESS
无论我是否使用 mutable
.
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);
会提醒 15
.
我该怎么做才能让两个 lambda 函数引用相同的 identifierCollection 实现?这样它的行为方式与 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
的生命周期不会仅仅因为您将其捕获到闭包中而延长.因此,一旦您将 [=]
更改为 [&]
以通过引用捕获,它就是对您正在捕获的局部变量的悬空引用.
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;坦率地说,这听起来像是共享指针的绝佳机会,通过值捕获到每个 lambda 中.只要您需要,它所包装的动态分配的地图就会一直存在.
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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