C ++ 11和缺乏多态lambdas - 为什么？ [英] C++11 and the lack of polymorphic lambdas - why?

问题描述

我一直在审查 C ++ 11 标准的草稿版本。特别是关于 lambdas 的部分，我对不引入多态lambda语言的推理感到困惑。 / p>

例如，在100001种方式中，可以使用多态lambdas，我希望我们可以使用如下代码：

  template< typename Container> 
 void foo（Container c）
 {
 for_each（c.begin（），c.end（），[]（T& t）{++ t;}）; 
} 
  

原因是什么：

$ b $

这种多态性lambda是太难以实现的

或者也许他们被视为 PTB ？

注意：不是唯一的，它只是作为指南的类型的代码。

相关来源：

b

• 用于C ++的Lambda表达式和闭包 （文档编号N1968 = 06-0038）


• 可以将lambda函数作为模板吗？

解决方案

我们没有多态性lambdas的原因在此发布中得到了很好的解释。

它与从C ++ 11中提取的概念功能有关：实质上，多态lambda是普通的，无约束的函数模板，我们不知道如何类型检查概念 - 使用无约束模板的约束模板。然而，解决这个问题很容易，如这里所示，所以我不要认为还有任何障碍。

I've been reviewing the draft version of the C++11 standard. Specifically the section on lambdas, and I am confused as to the reasoning for not introducing polymorphic lambdas.

For example, amongst the 100001 ways polymorphic lambdas could be used, I had hoped we could use code such as the following:

template<typename Container>
void foo(Container c)
{
    for_each(c.begin(), c.end(), [](T& t) { ++t; });
}

What were the reasons:

• Was it that the committee ran out of time?

• That polymorphic lambdas are too hard to implement?

• Or perhaps that they are seen as not being needed by the PTB?

Note: Please remember the example above is not the only one, and it is only provided as a guide to the types of code. Answers that solely concentrate on providing a workaround for the above piece of code will not be considered as valid!

Related sources:

• Lambda expressions and closures for C++ (document number N1968=06-0038)
• Can lambda functions be templated?

解决方案

The reason we don't have polymorphic lambdas is explained pretty well in this posting.

It has to do with the concepts feature that was pulled from C++11: essentially, polymorphic lambdas are ordinary, unconstrained function templates and we didn't know how to typecheck a concept-constrained template that used an unconstrained template. However, solving that problem turns out to be easy as shown here, so I don't think there's any obstacle remaining.

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