"C and pointer" - Chapter 13 function of function pointer 1 - callback function 1

Be careful ： This is an original article , Without consent , Please do not reprint at will .

1. Questions and ideas

Q： Implement a type independent comparison function
A： Declare a function pointer , Each type implements its own comparison function , The function pointer points to the comparison function of a specific type , You can realize functions similar to templates ;
One thing that deserves special attention ： The formal parameter type in the function pointer is declared as void, In this way, any type can be passed in , in other words , Parameters passed to function pointers are pointers to certain types of data , such , There are no restrictions on the type of input parameters ~*

Upgraded version ： In fact, you can also consider templates , Reduce duplicate code , See the next blog ：《C And a pointer 》—— The first 13 Chapter function pointer function 1： Callback function 2（ Combined with formwork , Simplify the code ）

2. Concrete realization

Realized int、char、string、 Class type comparison function .

#pragma once
#include <iostream>
#include <string>
using namespace std;

//  Write a type independent comparison function , Note that it is not a template , 
//  Method ： Declare a function pointer , Each type implements its own comparison function , The function pointer points to the comparison function of a specific type , You can realize functions similar to templates .
int(*compare)(const void*, const void*);
/*  Agree that the return value of a specific type represents the meaning ,  return 0： equal ;  return -1： Parameters 1< Parameters 2  return 1: Parameters 1> Parameters 2; */
int compare_int(const void* a1, const void* a2)
{
    
    if (*(int*)a1 < *(int*)a2)// First the void*  Convert to int*;  Then dereference * Take the value in the address indicated by the pointer 
    {
    
        return -1;
    }
    else if (*(int*)a1 == *(int*)a2)
    {
    
        return 0;
    }
    else
    {
    
        return 1;
    }
}

int compare_char(const void* a1, const void* a2)
{
    
    if (*(char*)a1 < *(char*)a2)
    {
    
        return -1;
    }
    else if (*(char*)a1 == *(char*)a2)
    {
    
        return 0;
    }
    else
    {
    
        return 1;
    }
}

int compare_string(const void* a1, const void* a2)
{
    
    if (*(string*)a1 < *(string*)a2)
    {
    
        return -1;
    }
    else if (*(string*)a1 == *(string*)a2)
    {
    
        return 0;
    }
    else
    {
    
        return 1;
    }
}

int compare_Student(const void* a1, const void* a2);
class Student
{
    
public:
    Student() :name(""), score(0) {
    }
    Student(const string& _name, const int& _score) :name(_name), score(_score) {
    }
    friend ostream& operator<<(ostream& os, const Student& stu)
    {
    
        os << stu.name << "\t" << stu.score;
        return os;
    }
    friend int compare_Student(const void* a1, const void* a2)
    {
    
        if ((*(Student*)a1).score < (*(Student*)a2).score)
        {
    
            return -1;
        }
        else if ((*(Student*)a1).score == (*(Student*)a2).score)
        {
    
            return 0;
        }
        else
        {
    
            return 1;
        }
    }

private:
    string name;
    int score;
};

void TestFunctionPointer()
{
    
    int a[] = {
     4,2,5 };
    char chars[] = "ascii";
    string s[] = {
     "Anne","Zoe","Mary" };
    Student stus[] = {
     {
    "Anne",80},{
    "Zoe",95},{
    "Mary",90} };

    cout << " Function pointer to int Type comparison function " << endl;
    int nCountA = sizeof(a) / sizeof(a[0]);
    for (int i = 0; i < nCountA; ++i)
    {
    
        cout << a[i] << "\t";
    }
    cout << endl;
    compare = compare_int;
    int *pa = a;
    while (pa != a + nCountA - 1)
    {
    
        cout << compare(pa++, pa) << endl;
    }
    cout << endl;


    cout << " Function pointer to char Type comparison function " << endl;
    int nCountChar = sizeof(chars) / sizeof(chars[0]);
    int nTmp = nCountChar - 1;
    char *pc = &chars[0];
    while (nTmp--)
    {
    
        cout << *pc++;
    }
    cout << endl;
    compare = compare_char;
    pc = &chars[0];
    while (pc != &chars[nCountChar - 2])
    {
    
        cout << compare(pc++, pc) << endl;
    }
    cout << endl;

    cout << " Function pointer to string Type comparison function " << endl;
    int nCountS = sizeof(s) / sizeof(s[0]);
    for (int i = 0; i < nCountS; ++i)
    {
    
        cout << s[i] << endl;
    }
    compare = compare_string;
    string *ps = &s[0];
    while (ps != &s[nCountS - 1])
    {
    
        cout << compare(ps++, ps) << endl;
    }
    cout << endl;
    
    cout << " Function pointer points to class type Student The comparison function of " << endl;
    int nCountStus = sizeof(stus) / sizeof(stus[0]);
    for (int i = 0; i < nCountStus; ++i)
    {
    
        cout << stus[i] << endl;
    }
    compare = compare_Student;
    Student *pStus = &stus[0];
    while (pStus != &stus[nCountStus - 1])
    {
    
        cout << compare(pStus++, pStus) << endl;
    }
}

3. Results screenshots