模板模板成员的消歧器模板关键字：什么时候？ [英] Disambiguator template keyword for a template member of a template: when exactly?

问题描述

此处提供了有关模板消除歧义的问题：

A question regarding template disambiguator was given here:

模板消除歧义

，在答案中我们可以阅读：

and in the answer we can read:

ISO C ++ 03 14.2 / 4

ISO C++03 14.2/4

或者在后缀表达式中，或者在限定id中的嵌套名称说明符之后，并且postfix-expression或qualified-id显式地取决于模板参数（14.6.2），成员模板名称必须是前缀为关键字模板。

When the name of a member template specialization appears after . or -> in a postfix-expression, or after nested-name-specifier in a qualified-id, and the postfix-expression or qualified-id explicitly depends on a template-parameter (14.6.2), the member template name must be prefixed by the keyword template. Otherwise the name is assumed to name a non-template.

现在这里是我的conrete例子，我不太明白： / p>

Now here comes my conrete example that I don't quite understand:

template <class T>
class Base {
  public:


  template <int v>
    static int baseGet() {return v;}

  class InnerA {
    public:

    template <int v>
      static int aget() {return v;}

  };

  class InnerB {
    public:
      typedef Base BaseType;
      typedef BaseType::InnerA OtherType;

      template <int v>
        static int baseGet() {return BaseType::baseGet<v>();} //(A)

      template <int v>
        static int aget() {return OtherType::aget<v>();} //(B)
  };
};

这显然无法编译。您在行（B）中需要模板： OtherType :: template aget< v>（）; 。
然而，g ++（4.4.3）和clang ++（2.9）都没有抱怨在行（A）中缺少模板。为什么？ BaseType 取决于类型 T ，不是吗？

It obviously fails to compile. You need template in the line (B): OtherType::template aget<v>();. However, both g++ (4.4.3) and clang++ (2.9) don't complain about the lack of template in the line (A). Why? BaseType depends on the type T, does it not? Is it a small depart from the standard by those compilers, or do I misunderstand something in the standard?

推荐答案

它们实现了C语言++ 0x规范，其中 Base 是当前实例化。在这种情况下，C ++ 0x允许省略模板关键字。因为 BaseType 是 Base 的typedef，当您说 BaseType ，也命名当前实例化。

They implement the C++0x specification, where Base is the current instantiation. And C++0x allows to omit template keyword in such a case. Since BaseType is a typedef for Base, when you say BaseType, that names the current instantiation too.

引用规范，因为你似乎对spec refs感兴趣

To quote the spec, since you seem to be interested in spec refs

名称是<...>

A name is a member of the current instantiation if it is [...]

• 的成员 id，其中嵌套名称指定符指向当前实例化，并且当查找时，指代当前实例化的至少一个成员或其非依赖基类。

• A qualified-id in which the nested-name-specifier refers to the current instantiation and that, when looked up, refers to at least one member of the current instantiation or a non-dependent base class thereof.

和

名称是指当前实例化，如果它是[...]

A name refers to the current instantiation if it is [...]

• 定义类模板的[...] 嵌套类，[...]，类模板的注入类名（第9条）嵌套类

• in the definition of a [...] nested class of a class template, [...], the injected-class-name (Clause 9) of the class template or nested class

和（您修改的14.2 / 4）
$ b

and (the modified 14.2/4 that you quoted)

[...]或qualified-id中的nested-name-specifier是指依赖类型，但该名称不是当前实例化的成员（14.6.2.1），成员模板名称必须以关键字模板为前缀。 [...]

[...] or the nested-name-specifier in the qualified-id refers to a dependent type, but the name is not a member of the current instantiation (14.6.2.1), the member template name must be prefixed by the keyword template. [...]

---

注意：在C ++ 03中，因为 BaseType 和 OtherType 是相关的。规格说明：

---

Note: In C++03 your code is ill-formed because both BaseType and OtherType are dependent. The spec says:

一个类型是依赖的，如果它是[...]

A type is dependent if it is [...]

• 一个模板参数


• 一个具有嵌套名称说明符的限定ID，其中包含命名依赖类型的类名


• 一个template-id，其中模板名称是模板参数或任何模板参数是依赖类型

（注意 Base 等效于 Base< T> 这是 Base 和 BaseType :: InnerA 是依赖类型的基础）。

(note that Base is equivalent to Base<T>, which is the base on which Base and BaseType::InnerA are dependent types).

请注意，您的报价中的明确依赖是一个预标准术语，并且最近被清除了（我相信是在1996年12月）。它基本上意味着（在这个上下文中）限定符是依赖的限定符，或者是一个类成员访问（ a-> x / ）其中 a 是依赖。在显式依赖从草稿中删除后，它仍然潜伏在一些地方，甚至C ++ 0x仍然引用显式依赖在14.6.2p2的注释：

Note that "explicitly depends" in your quote is a pre-standard term, and was gotten rid of fairly lately (I believe it was at December1996). It basically meant (in this context) a qualified-id in which the qualifier is dependent or a class member access (a->x / a.x) where the a was dependent. After "explicitly depends" was removed from the draft, it was still lurking around at some places, and even C++0x has still references to "explicitly depends" in a note at 14.6.2p2:

基类名 B ，类型名 T :: A ，名称 B :: i 和 pb-> j 显式依赖。

the base class name B<T>, the type name T::A, the names B<T>::i and pb->j explicitly depend on the template-parameter.

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