如何获取嵌套向量中的元素类型? [英] How to get the element type in a nesting vector?

问题描述

我有一个DataType供用户定义，它可以是int, vector, vector;>... 我想知道是否有一些模板技巧来获取int类型?我更喜欢使用 none c++11 方法，因为我的 g++ 版本是 4.1.2，我无法更新它.

I have a DataType for user to define, it can be int, vector<int>, vector<vector<int> >... I want to know whether there are some template tricks to get the type int? I prefer to use a none c++11 method because my g++ version is 4.1.2 and I can not update it.

推荐答案

这适用于 GCC 4.3，这是我可以轻松访问的最早版本的 GCC.您必须测试它是否适用于 4.1.2.

This works on GCC 4.3, which is the earliest version of GCC I have easy access to. You'll have to test if it works with 4.1.2.

template<class T> struct voider { typedef void type; };

template<class T, class = void>
struct data_type {
    typedef T type;
};

template<class T>
struct data_type<T, typename voider<typename T::value_type>::type>
       : data_type<typename T::value_type> {}; 

您将其用作例如

typename data_type<DataType>::type meow;
// type of 'meow' is int if DataType is vector<vector<int> >, or vector<int>, or int

这使用 void_t 技巧 并与定义 value_type 类型定义的所有内容.好处是它对 std::vector<std::deque<std::list<int> 开箱即用.>>;缺点是它可能太多了(data_type >::type 是 char).

This uses the void_t trick and works with everything that defines a value_type typedef. The benefit is that it works out of the box for std::vector<std::deque<std::list<int> > >; the drawback is that it might be too much (data_type<std::vector<std::string> >::type is char).

如果你只是想让它与向量一起工作，你可以这样做:

If you just want it to work with vectors, you can do:

template<class T>
struct data_type {
    typedef T type;
};

template<class T, class A>
struct data_type<std::vector<T, A> >
    : data_type<T> { };

// partially specialize for other containers if wanted

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