取消引用后调用函数的函子? [英] Functor that calls a function after dereferencing?

问题描述

在 C++ 标准或 Boost 中是否有一个小函子环绕另一个函子，但在调用该函子之前取消对指针的引用?

Is there a small functor in the C++ standard or in Boost that wraps around another functor, but dereferences a pointer before it calls that functor?

我正在考虑这样的事情:

I'm thinking of something like this:

template<class F>
struct DerefCmp {
  template<class T>
  bool operator()(T* v) const {
    return F(*v);
  }
};

我会在指针容器中使用它，例如，我想按值进行比较:

I'd use it in a container of pointers, for example, where I want to compare by value:

std::set<int*, DerefCmp< std::equal<int> > > s;

推荐答案

我不知道 C++ 标准库或 Boost 中的任何函数对象执行此操作(这并不是说没有；我不是熟悉 Boost 库中的一切 :-P).

I am not aware of any function object in the C++ Standard Library or in Boost that does this (that's not to say there isn't one; I am not familiar with everything in the Boost libraries :-P).

但是，编写自己的代码相当简单.考虑以下几点:

However, writing your own is rather straightforward. Consider the following:

template <typename Predicate>
class indirect_binary_predicate
{
public:
    indirect_binary_predicate(const Predicate& pred = Predicate()) 
        : pred_(pred) 
    {
    }

    template <typename Argument0, typename Argument1>
    bool operator()(Argument0 arg0, Argument1 arg1) const 
    { 
        return pred_(*arg0, *arg1); 
    }

private:
    Predicate pred_;
};

使用示例:

std::set<int*, indirect_binary_predicate<std::equal_to<int> > > s;

请注意，如果指针指向动态分配的对象并且容器拥有指向对象的所有权，那么使用原始指针容器是不明智的；这样做不是异常安全的.也就是说，这个谓词适配器应该同样适用于智能指针、迭代器或任何其他支持取消引用的类型.

Note that it is ill-advised to have a container of raw pointers if the pointers are to dynamically allocated objects and the container has ownership of the pointed-to objects; it isn't exception-safe to do this. That said, this predicate adapter should work just as well for smart pointers, iterators, or any other type that supports dereferencing.

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