函数指针的意义是什么?

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，这是一种参数化函数的方法:它的某些部分行为没有硬编码到f()中，但进入回调函数.调用者可以通过传递不同的回调函数使 f() 表现不同.一个经典的例子是来自 C 标准库的 qsort()，它将排序标准作为一个指向比较函数的指针.

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，这通常使用函数对象(也称为函子)来完成.这些是重载函数调用运算符的对象，因此您可以像调用函数一样调用它们.示例:

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 << '
';}
};

functor f;
f(42);

这背后的想法是，与函数指针不同，函数对象不仅可以携带算法，还可以携带数据:

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 << '
';}
  private:
     std::string prompt_;
};

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

另一个优点是有时内联调用函数对象比通过函数指针调用更容易.这就是为什么在 C++ 中排序有时比在 C 中排序更快的原因.

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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