traits类如何工作？ [英] how do traits classes work?

问题描述

我正在阅读Scott Meyers的 有效的C ++ 。他正在谈论traits类，我知道我需要他们在编译期间确定对象的类型，但我不能理解他的解释这些类实际上做什么？ （从技术的角度）

I'm reading Scott Meyers' Effective C++. He is talking about traits classes, I understood that I need them to determine the type of the object during compilation time, but I can't understand his explanation about what these classes actually do? (from technical point of view)

推荐答案

也许你期待一种魔法，使类型特征工作。在这种情况下，失望 - 没有魔法。类型特征是为每种类型手动定义的 。例如，考虑 iterator_traits ，它为迭代器提供typedef（例如 value_type ）。

Perhaps you’re expecting some kind of magic that makes type traits work. In that case, be disappointed – there is no magic. Type traits are manually defined for each type. For example, consider iterator_traits, which provides typedefs (e.g. value_type) for iterators.

使用它们，您可以写入

iterator_traits<vector<int>::iterator>::value_type x;
iterator_traits<int*>::value_type y;
// `x` and `y` have type int.

但是要使这项工作，实际上有一个显式定义 < iterator> 头，它读取如下：

But to make this work, there is actually an explicit definition somewhere in the <iterator> header, which reads something like this:

template <typename T>
struct iterator_traits<T*> {
    typedef T value_type;
    // …
};

这是 iterator_traits的部分专业化 类型的类型 T * ，即一些通用类型的指针。

This is a partial specialization of the iterator_traits type for types of the form T*, i.e. pointers of some generic type.

同样， iterator_traits 专用于其他迭代器，例如 typename向量< T> :: iterator 。

In the same vein, iterator_traits are specialized for other iterators, e.g. typename vector<T>::iterator.

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