为什么在存在decltype的情况下范围解析失败？ [英] Why does scope resolution fail in presence of decltype?

问题描述

我的理解是，decltype用于查询对象/变量的类型等。

It is my understanding that decltype is used to query the type of an objects/variables and so on.

从维基百科中的示例，如下：

From the examples present on wikipedia, such as the following:

int i;
decltype(i) x3; // type is int

我假设我可以这样做：

class A
{
public:
    int a, b;
};

template<typename T>
struct IsClass
{
    enum { Yes = std::is_class<T>::value };
    enum { No = !Yes };
};

std::vector<A> v;
auto it = v.begin();
IsClass<decltype(it)::value_type>::Yes

是合法的：

IsClass<std::vector<A>::iterator::value_type>::Yes

不能编译，引用如下：错误C2039：'value_type'：不是'全局命名空间'的成员

Alas it wouldn't compile, citing the following: error C2039: 'value_type' : is not a member of 'global namespace''`

有关为什么范围解析在decltype存在时的行为的任何想法？

Any ideas as to why scope resolution was made to behave this way in presence of decltype?

PS：如果它有什么区别我使用MSVC2012（没有 Nov CTP）

P.S: If it makes any difference I'm using MSVC2012 (without the Nov CTP)

推荐答案

这是Visual C ++编译器中的一个已知错误。它还没有被固定为Visual C ++ 2013预览。您可以使用 std :: common_type 解决此问题：

This is a known bug in the Visual C++ compiler. It has not yet been fixed as of the Visual C++ 2013 Preview. You can work around this issue using std::common_type:

IsClass<std::common_type<decltype(it)>::type::value_type>::Yes
        ^^^^^^^^^^^^^^^^^            ^^^^^^^

（ std :: common_type template参数产生该参数类型;它是标准化的C ++ 11等价于在元编程中早已使用的 identity 模板。）

(std::common_type with a single template argument yields that argument type; it's the standardized C++11 equivalent of the identity template that has long been used in metaprogramming.)

您可以在Microsoft Connect上找到公共错误报告：无法在scope操作符之前使用decltype 。如果这个问题对您很重要，请考虑将该错误报告。

You can find the public bug report on Microsoft Connect: Cannot use decltype before scope operator. If this issue is important to you, please consider upvoting that bug report.

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