I have the following code to demonstrate a function been called inside another function.

The below code works correctly:

#include <iostream>

int thirds()
{
    return 6   1;
}
template <typename T, typename B>
int hello(T x, B y , int (*ptr)() ){
    
    int first = x   1;
    int second = y   1;
    int third = (*ptr) (); ;
    
    return first   second   third;
}
int add(){
    
     int (*ptr)() = &thirds;
    return hello(1,1, thirds);
}
int main()
{
    std::cout<<add();
    return 0;
}

Now I want to pass one number as a parameter from add function ie into thirds function (thirds(6)).

I am trying this way:

#include <iostream>

int thirds(int a){
    return a   1;
}

template <typename T, typename B>
int hello(T x, B y , int (*ptr)(int a)() ){
    
    int first = x   1;
    int second = y   1;
    int third = (*ptr)(a) (); ;
    
    return first   second   third;
}

int add(){
    
     int (*ptr)() = &thirds;
    return hello(1,1, thirds(6)); //from here pass a number
}

int main()
{
    
    std::cout<<add();

    return 0;
}

My expected output is:

11

But It is not working. Please can someone show me what I am doing wrong?

CodePudding user response：

1. If you want add to pass a value to hello, to be passed to the function given by the pointer ptr, you have to add a separate parameter.
2. In c it is usually advised to use std::function instead of old c style funtion pointers.

A complete example:

#include <iostream>
#include <functional>

int thirds(int a) 
{
    return a   1;
}

template <typename T, typename B>
//------------------------------------------------VVVVV-
int hello(T x, B y, std::function<int(int)> func, int a) 
{
    int first = x   1;
    int second = y   1;
    int third = func(a);
    return first   second   third;
}

int add() 
{
    std::function<int(int)> myfunc = thirds;
    return hello(1, 1, myfunc, 6);
}

int main() 
{
    std::cout << add();
    return 0;
}

Output:

11

Note: another solution could be to use std::bind to create a callable from thirds and the argument 6. But I think the solution above is more simple and straightforward.

CodePudding user response：

The code you posted does not seem to compile. Have you tried something like the following?

#include <stdio.h>
#include <iostream>

int thirds(int a){
    return a   1;
}

template <typename T, typename B>
int hello(T x, B y , int (*ptr)(int)){

    int first = x   1;
    int second = y   1;
    int third = ptr(first);

    return first   second   third;
}

int add(){

  int (*ptr)(int) = &thirds;
  return hello(1, 1, ptr); //from here pass a number
}

int main()
{

    std::cout << add();

    return 0;
}

In addition, I would suggest looking into std::function and lambdas.