为什么从 C++11 中删除了 unary_function、binary_function?

问题描述

我发现 binary_function 已从 C++11 中删除.我想知道为什么.

I found that binary_function is removed from C++11. I am wondering why.

C++98:

template <class T> struct less : binary_function <T,T,bool> {
  bool operator() (const T& x, const T& y) const {return x<y;}
};

C++11:

template <class T> struct less {
  bool operator() (const T& x, const T& y) const {return x<y;}
  typedef T first_argument_type;
  typedef T second_argument_type;
  typedef bool result_type;
};

已修改---------------------------------------------------------------------------

MODIFIED----------------------------------------------------------------------------

template<class arg,class result>
struct unary_function
{
       typedef arg argument_type;
       typedef result result_type;
};

例如，如果我们想在 C++98 中编写函数适配器，

For example, if we want to write our adapter for function even in C++98,

template <class T> struct even : unary_function <T,bool> {
  bool operator() (const T& x) const {return 0==x%2;}
};

find_if(bgn,end,even<int>()); //find even number

//adapter
template<typename adaptableFunction >
class unary_negate
{
   private:
       adaptableFunction fun_;
   public:
       typedef adaptableFunction::argument_type argument_type;

       typedef adaptableFunction::result_type result_type;  
       unary_negate(const adaptableFunction &f):fun_(f){}

       bool operator()(const argument_type&x) 
       {
           return !fun(x);
       }
}

find_if(bgn,end, unary_negate< even<int> >(even<int>()) ); //find odd number

如果没有 unary_function，我们如何在 C++11 中改进这一点?

How can we improve this in C++11 without unary_function?

推荐答案

它没有被移除，只是在 C++11 中被弃用了.它仍然是 C++11 标准的一部分.您仍然可以在自己的代码中使用它.不过它在 C++17 中被删除了.

It isn't removed, it's just deprecated in C++11. It's still part of the C++11 standard. You can still use it in your own code. It was removed in C++17 though.

它在标准中不再使用，因为要求从binary_function派生的实现是过度规范.

It isn't used in the standard any more because requiring implementations to derive from binary_function is over-specification.

用户不应该关心less是否从binary_function派生，他们只需要关心它定义了first_argument_type、second_argument_type 和 result_type.如何提供这些 typedef 应该取决于实现.

Users should not care whether less derives from binary_function, they only need to care that it defines first_argument_type, second_argument_type and result_type. It should be up to the implementation how it provides those typedefs.

强制实现从特定类型派生意味着用户可能开始依赖该派生，这毫无意义且无用.

Forcing the implementation to derive from a specific type means that users might start relying on that derivation, which makes no sense and is not useful.

编辑

我们如何在没有 unary_function 的情况下在 c++11 中改进这一点?

How can we improve this in c++11 without unary_function?

你不需要它.

template<typename adaptableFunction>
class unary_negate
{
   private:
       adaptableFunction fun_;
   public:
       unary_negate(const adaptableFunction& f):fun_(f){}

       template<typename T>
           auto operator()(const T& x)  -> decltype(!fun_(x))
           {
               return !fun_(x);
           }
}

事实上你可以做得更好，参见 not_fn:一个广义的否定

In fact you can do even better, see not_fn: a generalized negator

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