7.4 function pointers and references

函数指针和函数引用
norm_ptr is a pointer, a function pointer.
The function should have two float parameters, and returns float.

函数指针也是一个指针，指向函数，指向的是执行指令的地址；之前的指针指向的都是内存的数据

#include <iostream>
#include <cmath>
using namespace std;
float norm_l1(float x, float y);     //declaration
float norm_l2(float x, float y);     //declaration
float (*norm_ptr)(float x, float y); //norm_ptr is a function pointer
int main()
{
    norm_ptr = norm_l1; //Pointer norm_ptr is pointing to norm_l1
    cout << "L1 norm of (-3, 4) = " << norm_ptr(-3.0f, 4.0f) << endl; // function invoked
    norm_ptr = &norm_l2; //Pointer norm_ptr is pointing to norm_l2
    cout << "L2 norm of (-3, 4) = " << (*norm_ptr)(-3.0f, 4.0f) << endl;
    return 0;
}
float norm_l1(float x, float y)
{
    return fabs(x) + fabs(y);
}
float norm_l2(float x, float y)
{
    return sqrt(x * x + y * y);
}

常用案例：
A function pointer can be an argument and pass to a function.
把函数当作一个参数传递给另外一个参数

// <stdlib.h>
void qsort
( void *ptr, size_t count, size_t size,
           int (*comp)(const void *, const void *) // 自定义该排序标准的算法          
);

To sort some customized types, such as

• struct Point
• struct Person

Function references

#include <iostream>
#include <cmath>
using namespace std;
float norm_l1(float x, float y);               //declaration
float norm_l2(float x, float y);               //declaration
float (&norm_ref)(float x, float y) = norm_l1; //norm_ref is a function reference
int main()
{
    cout << "L1 norm of (-3, 4) = " << norm_ref(-3, 4) << endl;
    return 0;
}
float norm_l1(float x, float y)
{
    return fabs(x) + fabs(y);
}
float norm_l2(float x, float y)
{
    return sqrt(x * x + y * y);
}