什么是函数指针的地步？ [英] What is the point of function pointers?

问题描述

我无法看到函数指针的效用。我想这可能是在某些情况下（他们的存在，毕竟）是有用的，但我不认为它是好还是不可避免的要使用的函数指针的情况。

I have trouble seeing the utility of function pointers. I guess it may be useful in some cases (they exist, after all), but I can't think of a case where it's better or unavoidable to use a function pointer.

你能给良好的使用功能指针（在C或C ++）？一些例子

Could you give some example of good use of function pointers (in C or C++)?

推荐答案

大多数示例归结为 回调的：你调用一个函数 F（ ）通过另一个函数的地址克（）和 F（）调用克（）对于一些特定的任务。如果你通过 F（）地址 H（）来代替，那么 F（） 将回调 H（）来代替。

Most examples boil down to callbacks: You call a function f() passing the address of another function g(), and f() calls g() for some specific task. If you pass f() the address of h() instead, then f() will call back h() instead.

基本上，这是一种方法， 参数多态的功能：其行为的某些部分不硬coded到 F（ ），但进入回调函数。呼叫者可以让 F（）通过传递不同的回调函数表现不同。一个典型的是的qsort（）向变更后承受其排序标准为指针，以一个比较函数C标准库。

Basically, this is a way to parametrize a function: Some part of its behavior is not hard-coded into f(), but into the callback function. Callers can make f() behave differently by passing different callback functions. A classic is qsort() from the C standard library that takes its sorting criterion as a pointer to a comparison function.

在C ++中，这通常使用做的（又称仿函数）。这些都是重载函数调用操作的对象，所以你可以打电话给他们，好像他们是一个函数。例如：

In C++, this is often done using function objects (also called functors). These are objects that overload the function call operator, so you can call them as if they were a function. Example:

class functor {
  public:
     void operator()(int i) {std::cout << "the answer is: " << i << '\n';}
};

functor f;
f(42);

这背后的想法是，不像函数指针，一个功能对象可以携带不仅一种算法，而且数据

The idea behind this is that, unlike a function pointer, a function object can carry not only an algorithm, but also data:

class functor {
  public:
     functor(const std::string& prompt) : prompt_(prompt) {}
     void operator()(int i) {std::cout << prompt_ << i << '\n';}
  private:
     std::string prompt_;
};

functor f("the answer is: ");
f(42);

的另一个优点是，它是有时容易直列调用函数比通过函数指针调用对象。这就是为什么用C排序的理由++有时比在C

Another advantage is that it is sometimes easier to inline calls to function objects than calls through function pointers. This is a reason why sorting in C++ is sometimes faster than sorting in C.

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