問題描述
我看不到函數指針的效用.我想它在某些情況下可能很有用(畢竟它們存在)，但我想不出使用函數指針更好或不可避免的情況.
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.
你能舉一些很好地使用函數指針的例子嗎(在 C 或 C++ 中)?
Could you give some example of good use of function pointers (in C or C++)?
推薦答案
大多數例子都歸結為回調:你調用一個函數 f()code> 傳遞另一個函數的地址 g()，并且 f() 調用 g() 來執行某些特定任務.如果你把f()的地址傳給h()，那么f()會回調h()代碼> 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，這是一種參數化函數的方法:它的某些部分行為沒有硬編碼到f()中，但進入回調函數.調用者可以通過傳遞不同的回調函數使 f() 表現不同.一個經典的例子是來自 C 標準庫的 qsort()，它將排序標準作為一個指向比較函數的指針.
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，這通常使用函數對象(也稱為函子)來完成.這些是重載函數調用運算符的對象，因此您可以像調用函數一樣調用它們.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);
這背后的想法是，與函數指針不同，函數對象不僅可以攜帶算法，還可以攜帶數據:
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);
另一個優點是有時內聯調用函數對象比通過函數指針調用更容易.這就是為什么在 C++ 中排序有時比在 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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