模板，类型名，lambda - >依赖名称不依赖？ [英] templates, typename, lambda -> dependent names not dependent?

问题描述

请考虑：

template < typename Something >
boost::function<void()> f()
{
  typedef typename Something::what type;
  return [](){};
}

在这段代码中，你需要typename，因为'what'但请考虑：

In this code you need the typename because 'what' is a dependent name. But consider this:

template < typename Something >
boost::function<void()> f()
{
  return []()
  { 
    typedef typename Something::what type;
  };
}

编译程序bitches：typename不能在模板声明外使用

Compiler bitches: "typename cannot be used outside a template declaration"

WTF？

这项工作：

template < typename Something >
boost::function<void()> f()
{
  return []()
  { 
    typedef Something::what type;
  };
}

关于lambda的创建意味着什么一个依赖名称了吗？或者这只是一个错误？

What is it about the creation of a lambda that means "what" is not a dependent name anymore? Or is this just a bug?

嘿...修正。后者不工作。它说，东西不存在。这个修改版本的DOES工作，但仍然不直接不需要，不会接受typename。

Heh...correction. The latter doesn't work. It says that "Something" doesn't exist. This modified version DOES work though and still unintuitively doesn't need and won't accept "typename".

template < typename T > struct wtf { typedef typename T::what type; };

template < typename Something >
boost::function<void()> f()
{
  return []() { typedef wtf<Something>::type type; };
}

当然，现在我有两个问题：原来的，WTF doesn'除非用作模板参数，否则找到Something。

Of course, now I have TWO questions: the original and, WTF doesn't it find "Something" unless it's used as a template parameter??

推荐答案

这是一个非常有趣的问题。从我的理解，第一个WTF（在lambda主体中 typename 的一个）应该是正确的根据N3225 5.1.2 / 7：

That's a very interesting question. From my understanding, the first 'WTF' (the one with typename in the lambda body) should be the correct according to N3225 5.1.2/7 :

的复合语句会生成 function-body <函数调用运算符
，但是出于名称查找的目的，确定此类型和值并转换 id表达式
对于非静态类成员到类成员访问表达式中使用（* this），
将复合语句考虑在 lambda- 。

The lambda-expression’s compound-statement yields the function-body of the function call operator, but for purposes of name lookup, determining the type and value of this and transforming id-expressions referring to non-static class members into class member access expressions using (*this), the compound-statement is considered in the context of the lambda-expression.

由于在lambda表达式的上下文中，它应该也是在根据这个引号的lambda函数体的上下文中的依赖名称。

As Something is a dependent-name in the context of the lambda expression, it should also be a dependent name in the context of the lambda function body according to this quote.

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