在定义模板时，始终定义`value_type`是一个好习惯 [英] Is it a good practice to always define `value_type` when we define a template

问题描述

template<typename T>
class Point
{
public:
    typedef T value_type;
    ...
};

我在 book，pp176 。

问题1 > value_type ？

问题2 > 将被使用？

Question2> Where this defined value_type will be used?

例如：

Point< int> :: value_type ？

推荐答案

它通常只对容器（如 std :: vector ）有意义，因为所有容器都提供 typedef 接口用于访问包含的值（ begin / end ， > / back ），虽然这在C ++ 11中已经过时了 auto 和 decltype 。更清楚地说 some_template< typename container :: value_type> ...

It doesn't hurt to have one, but it mostly only makes sense for containers (like std::vector), as all containers provide this typedef and a uniform interface for accessing the contained values (begin/end, front/back), although this has mostly gotten obsolete in C++11 with auto and decltype. It's still cleaner to say some_template<typename container::value_type> ..., though.

这意味着它们可以在通用代码中互换使用（这是为什么这样做的主要原因） ）。如果你的 Point 类知道包含的值是什么类型是有意义的，那么 typedef 。正如我所说，它不伤害。但是，我有一种感觉，它对这个特定的例子没有太大的意义。

That in turn means they can be used interchangeably in generic code (the main reason why things were done that way). If it makes sense for your Point class to know what types the contained values are, well, have that typedef. As I said, it doesn't hurt. However, I have a feeling that it doesn't make too much sense for that particular example.

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