A study of C + + pointers (iv) function objects

The function object is not a function pointer. However, in program code, it is called like a function pointer, followed by parentheses.

This is the entry-level essay, which is about the definition of a function object, the use of it, and its relationship to function pointers, member function pointers.

Mu Feng Xiao Build

A function object is essentially a class that implements the operator ()-bracket operator.

For example:

class Add
{
public:
　 int operator()(int a, int b)
　 {
　　　 return a + b;
　 }
};

Add add； // 定义函数对象
cout << add（3，2）； // 5

The function pointer version is:

int AddFunc(int a, int b)
{
　 return a + b;
}
typedef int (*Add) (int a, int b);

　Add add = &AddFunc;
cout << add(3,2); // 5

Oh, in addition to the definition of the same way, the use of the same way. are:

cout << Add (3,2);

Since function objects and function pointers are not different in how they are used, why use a function object? Quite simply, a function object can carry additional data, and the pointer is not.

Here's an example of using additional data:

class less
{
public:
　　　 less(int num):n(num){}
　　　 bool operator()(int value)
　　　 {
　　　　　　　 return value < n;
　　　 }
private:
　　　 int n;
};

When in use:

Less isless (10);

cout << isless (9) << "" << isless (12);//Output 1 0

This example seems too trifling to change one:

const int SIZE = 5;
int array[SIZE] = { 50, 30, 9, 7, 20};
// 找到小于数组array中小于10的第一个数的位置
int * pa = std::find_if(array, array + SIZE, less(10)); // pa 指向 9 的位置
// 找到小于数组array中小于40的第一个数的位置
int * pb = std::find_if(array, array + SIZE, less(40));　// pb 指向 30 的位置