有没有typedef绝对必要的情况？ [英] Are there cases where a typedef is absolutely necessary?

问题描述

请参阅安全bool成语的以下摘录：

typedef void (Testable::*bool_type)() const;
operator bool_type() const;

是否可以在没有typedef的情况下声明转换函数？以下不编译：

Is it possible to declare the conversion function without the typedef? The following does not compile:

operator (void (Testable::*)() const)() const;

推荐答案

啊，我只记得 identity 元功能。可以写

Ah, I just remembered the identity meta-function. It is possible to write

operator typename identity<void (Testable::*)() const>::type() const;

with the following definition of identity:

template <typename T>
struct identity
{
    typedef T type;
};

您可以认为 identity typedef ，但这个解决方案对我来说是好。

You could argue that identity still uses a typedef, but this solution is "good" enough for me.

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