decltype 的行为

问题描述

假设我有一些 stl 容器类 obj 的对象.我可以通过这种方式定义其他相同类型的对象:

decltype(obj) obj2;

但我不能以这种方式为容器声明迭代器:

decltype(obj)::iterator it = obj.begin();

为什么?我做错了什么吗?

根据最终的 C++0x 草案 (FDIS)，您的代码格式良好.这是 Visual Studio 编译器尚未实现的后期更改.

与此同时，一种解决方法是使用 typedef:

typedef decltype(obj) obj_type;obj_type::iterator it = obj.begin();

相关章节和诗句是5.1.1/8:

为了完整起见:

原始核心问题

措辞建议

Say I have an object of some of stl container classes obj. I can define other object of same type this way:

decltype(obj) obj2;

But I can't declare iterator for the container this way:

decltype(obj)::iterator it = obj.begin();

Why? Am I doing something wrong?

Your code is well-formed according to the final C++0x draft (FDIS). This was a late change that's not yet been implemented by the Visual Studio compiler.

In the meantime, a workaround is to use a typedef:

typedef decltype(obj) obj_type;
obj_type::iterator it = obj.begin();

EDIT: The relevant chapter and verse is 5.1.1/8:

qualified-id:
    [...]
    nested-name-specifier templateopt unqualified-id

nested-name-specifier:
    [...]
    decltype-specifier ::

decltype-specifier:
    decltype ( expression )

And for completeness's sake:

The original core issue

Proposal for wording

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