Examples about function pointer and callback function definition - c++

Question

I'm learning about callback function in C++ and have some problems in understanding the initialization of a callback function such as
typedef void (CALLBACK *name)(int,int);
I think it looks very similar to the declaration of function pointer like this:
typedef void (*name)(int,int);
I have a simple example about how to call a function inside another function using the declaration of function pointer. The example converts a string to int and compare with anoter int. Then tells which one is bigger:
#include <stdio.h>
#include <stdlib.h>
int StrToInt(char* inputchar) //converting function
{
int outputint;
outputint = atoi(inputchar);
return outputint;
}
typedef int(*p)(char*); //declare function pointer
void IntCompare(p FuncP, char* inputchar, int b) //comparing function
{
int a;
a = FuncP(inputchar); //call converting function using function pointer
if (a<b)
{
printf("%d is bigger\n", b);
}
else
{
printf("%d is bigger\n", a);
}
}
void main()
{
char* StrNum = "1234";
p FuncP; //creat a function pointer
FuncP = StrToInt; //point to converting function
IntCompare(FuncP, StrNum, 21);
}
What I'm asking is:
Could somebody give me a similar example about how to use typedef void (CALLBACK *name)(int,int);? Please help me understand when and how to use it. Thank you for your attention.

Answer 1

CALLBACK is macro. It relates to callback functions but it's not the same. You can start looking at implementation of callback functions in C.
qsort for example uses this technique. qsort is a single function which can sort any array. But you have to tell qsort how to compare different data types. That's done by passing a function pointer to qsort.
int compare_int(const void * a, const void * b)
{
return ( *(int*)a - *(int*)b );
}
int compare_string(const void * a, const void * b)
{
const char *pa = *(const char**)a;
const char *pb = *(const char**)b;
return strcmp(pa, pb);
}
int main ()
{
int int_array[] = { 3, 2, 1 };
int count = sizeof(int_array) / sizeof(int);
qsort(int_array, count, sizeof(int), compare_int);
const char *string_array[] = { "234","123","456" };
count = sizeof(string_array) / sizeof(char*);
qsort(string_array, count, sizeof(char*), compare_string);
return 0;
}
Of course in C++ we use std::sort, which uses templates instead.
But we still need to pass functions in C++. See for example, the implementation of for_each
template<class InputIt, class UnaryFunction>
UnaryFunction for_each(InputIt first, InputIt last, UnaryFunction f)
{
for (; first != last; ++first) {
f(*first);
}
return f;
}
Usage:
std::vector<int> nums{ 3, 4, 2, 8, 15, 267 };
auto print = [](const int& n) { std::cout << " " << n; };
std::for_each(nums.begin(), nums.end(), print);
std::cout << '\n';