std :: function的说明 [英] Explanation of std::function

问题描述

std :: function 的目的是什么？据我了解， std :: function 将函数，函子或lambda转换为函数对象。

我不太了解这个目的……Lambda和Functors都已经是函数对象，我相信它们可以用作排序和转换等算法的谓词。作为附带说明，Lambda实际上是（内部）函子。因此，我只能看到 std :: function 有用的是将常规函数转换为函数对象。

而且我不太清楚为什么我也想将常规函数转换为函数对象。如果我想使用一个函数对象，那么我首先会做一个函子或lambda ...，而不是编写一个函数，然后用std :: function对其进行转换，然后将其作为谓词传递给<... / p>

我猜想 std :: function 还有更多...乍看之下很明显。

对 std :: function 的解释将不胜感激。

解决方案

std :: function ？据我了解， std :: function 将函数，函子或lambda转换为函数对象。




std :: function 是一个更广泛的概念（称为类型擦除）的示例。您的描述不太准确。 std :: function< void（）> 所做的选择特定的专业表示 any 可调用，无需参数即可调用。它可以是具有具体类型的函数指针或函数对象，也可以是从lambda构建的闭包。只要符合合同，源类型是什么都可以，它就可以工作。而不是使用具体的源类型，我们擦除它-我们只处理 std :: function 。

<现在，为什么我们要使用类型擦除？毕竟，我们没有模板以便可以直接使用具体类型吗？那不是更有效吗，不是C ++就是效率吗？！

有时，您不能使用具体类型。一个可能更熟悉的示例是常规的面向对象的多态性。为什么当我们可以存储 Derived * 时，为什么还要存储 Base * ？好吧，也许我们不能存储 Derived * 。也许我们有许多不同的 Derived * 供不同的用户使用。也许我们正在编写一个甚至都不知道派生的库。这也是 类型擦除，只是一种与 std :: function 使用的技术不同的擦除方法。

用例的非穷尽列表：

• 需要存储当我们只关心满足特定接口的对象时，它们可能是异构的对象列表。对于 std :: function ，也许我只有一个 std :: vector< std :: function< void（）>>回调-可能都有不同的具体类型，但我不在乎，我只需要调用它们即可。


• 需要跨API边界使用（例如，我可以使用 virtual 函数并使用 std :: function< void（）> ，但是我可以t有一个虚拟函数模板）。


• 从工厂函数返回-我们只需要一些满足某些概念的对象，我们不需要不需要具体的事情（同样，在OO多态中很常见，这也是类型擦除）。


• 可能实际上在任何地方都使用模板，但是性能提升不值得编译击中。

What is the purpose of std::function? As far as I understand, std::function turns a function, functor, or lambda into a function object.

I don't quite understand the purpose of this... Both Lambdas and Functors are function objects already and I do believe that they can be used as predicates for algorithms like sort and transform. As a side note, Lambdas are actually Functors (internally). So the only thing I can see std::function being useful for is to turn regular functions into function objects.

And I don't quite see why I would want to turn a regular function into a function object either. If I wanted to use a function object I would have made one in the first place as a functor or lambda... rather than code a function and then convert it with std::function and then pass it in as predicate...

I'm guessing that there is much more to std::function... something that isn't quite obvious at first glance.

An explanation of std::function would be much appreciated.

解决方案

What is the purpose of std::function? As far as I understand, std::function turns a function, functor, or lambda into a function object.

std::function is an example of a broader concept called Type Erasure. The description you have isn't quite accurate. What std::function<void()> does, to pick a specific specialization, is represent any callable that can be invoked with no arguments. It could be a function pointer or a function object that has a concrete type, or a closure built from a lambda. It doesn't matter what the source type is, as long as it fits the contract - it just works. Instead of using the concrete source type, we "erase" it - and we just deal with std::function.

Now, why would we ever use type erasure? After all, don't we have templates so that we can use the concrete types directly? And wouldn't that be more efficient and isn't C++ all about efficiency?!

Sometimes, you cannot use the concrete types. An example that might be more familiar is regular object-oriented polymorphism. Why would we ever store a Base* when we could instead store a Derived*? Well, maybe we can't store a Derived*. Maybe we have lots of different Derived*s that different users use. Maybe we're writing a library that doesn't even know about Derived. This is also type erasure, just a different technique for it than the one std::function uses.

A non-exhaust list of use-cases:

• Need to store a potentially heterogenous list of objects, when we only care about them satisfying a concrete interface. For std::function, maybe I just have a std::vector<std::function<void()>> callbacks - which might all have different concrete types, but I don't care, I just need to call them.
• Need to use across an API boundary (e.g. I can have a virtual function taking a std::function<void()>, but I can't have a virtual function template).
• Returning from a factory function - we just need some object that satisfies some concept, we don't need a concrete thing (again, quite common in OO polymorphism, which is also type erasure).
• Could potentially actually use templates everywhere, but the performance gain isn't worth the compilation hit.

这篇关于std :: function的说明的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！