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