C ++类型traits在T1和T2之间选择 [英] C++ type traits to select between T1 and T2

问题描述

我想根据一些条件从两种类型中选择一个模板。例如

  struct Base {}; 
 
 template< typename T1，typename T2> 
 struct test 
 {
 //例如这里应当选择is_base_of< Base>的T1 / T2。 
 typename select_base< T1，T2> :: type m_ValueOfBaseType; 
}; 
  

当然，将条件传递给select_base是非常有用的，

下面是一个我尝试的示例解决方案，但它总是选择T1： http://ideone.com/EnVT8

问题是如何实施select_base模板。

 
 
 code> template< bool，typename T，typename U> struct if_ {typedef T type; }; 
 
 template< typename T，typename U> 
 struct if_< false，T，U> {typedef U型; }; 
  

然后你只需使用 is_base_of p>

  template< typename T，typename U> 
 struct select_base：if_< std :: is_base_of< T，Base> :: value，T，U> {}; 
  

就是这样。

I want a template to select from two types based on some condition. E.g.

struct Base {};

template <typename T1, typename T2>
struct test
{
    // e.g. here it should select T1/T2 that is_base_of<Base>
    typename select_base<T1, T2>::type m_ValueOfBaseType;
};

Of course to pass condition to the select_base (to make it generic) would be useful, but hard-coded solution is easier and good as well.

Here's a sample solution that I tried but it always selects T1: http://ideone.com/EnVT8

The question is how to implement the select_base template.

解决方案

First you need a facility to selection a type depending on a boolean value.

template <bool, typename T, typename U> struct if_ { typedef T type; };

template <typename T, typename U>
struct if_<false, T, U> { typedef U type; };

Then you just use is_base_of:

template <typename T, typename U>
struct select_base: if_<std::is_base_of<T, Base>::value, T, U> {};

And that is it.

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