为什么我们需要typename这里？ [英] Why do we need typename here?

问题描述

template<class T>
class Set 
{
public:
   void insert(const T& item);
   void remove(const T& item);
private:
   std::list<T> rep;
}

template<typename T>
void Set<T>::remove(const T& item)
{
   typename std::list<T>::iterator it =  // question here
    std::find(rep.begin(),rep.end(),itme);
   if(it!=rep.end()) rep.erase(it);

}

为什么需要remove / p>

Why the typename in the remove() is needed?

推荐答案

一般来说，C ++需要 typename ，因为不幸的语法[ ]它继承自C，这使得不可能没有非本地信息，例如 - 在 A * B; 是否 A 命名一个类型（在这种情况下，这是一个 B 作为指向它的指针的声明）或不是（在这种情况下，这是一个乘法表达式 - - 很可能，因为 A ，对于所有你可以告诉没有非本地信息，可以是重载 operator *

In general, C++ needs typename because of the unfortunate syntax [*] it inherits from C, that makes it impossible without non-local information to say -- for example -- in A * B; whether A names a type (in which case this is a declaration of B as a pointer to it) or not (in which case this is a multiplication expression -- quite possible since A, for all you can tell without non-local information, could be an instance of a class that overloads operator* to do something weird;-).

在大多数情况下，编译器确实具有消除歧义所需的非本地信息（虽然不幸的语法仍然意味着低级解析器需要来自更高层的反馈，保持符号表信息）...但是使用模板它不（通常不是，虽然在这种特定情况下，专门化 std :: list< T> ，以使其 :: iterator 不是类型名称; - ）。

In most cases the compiler does have the non-local information needed to disambiguate (though the unfortunate syntax still means the low-level parser needs feedback from the higher-level layer that keeps the symbol table info)... but with templates it doesn't (not in general, though in this specific case it might be technically illegal to specialize a std::list<T> so that its ::iterator is NOT a type name;-).

[*]不仅仅是我的意见，还包括Ken Thompson和Rob Pikes的意见，目前我的同事，他们忙于设计和实现一种新的内部使用的编程语言：新的编程语言，而其语法大多是C样，不重复C的语法设计错误 - 它是新的语言（如eg在好的老Pascal），语法足以区分标识符必须命名一个类型从不必须; - ）。

[*] not just my opinion, but also the opinion of Ken Thompson and Rob Pikes, currently my colleagues, who are busy designing and implementing a new programming language for internal use: that new programming language, while its syntax is mostly C-like, does NOT repeat C's syntax design errors -- it the new language (like e.g. in good old Pascal), syntax is sufficient to distinguish identifiers that must name a type from ones that must not;-).

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