How to pass a pointer to a function in C++

A pointer is a variable that stores the address of an object stored in memory. In this answer, we'll see how to pass a pointer to a function in C++. We can pass a pointer to the function in the following ways:

  • Passing variable as a pointer

  • Passing array as a pointer

Let's see the steps to pass a pointer to a function as a parameter.

Steps to pass a pointer to a function

To pass a variable as a pointer to a function, we can follow these three simple steps:

  1. Define a function that takes a pointer as a parameter. For example:

void function(int *pointer) {
    // function code goes here
}
Defining a function that takes a pointer as a parameter

  1. Declare a pointer variable and point it to the variable or an array we want to pass to the function like this:

// For variable
int value = 5;                    
int *pointer = &value;
// For array
int arr[] = {1, 2, 3, 4, 5};
int *pointer = arr;
Declaring a pointer variable and point to the memory location

This will create a pointer variable named pointer that contains the address of the value variable.

  1. Call the function by passing the pointer as an argument, as shown below:

function(pointer);
Calling the function and passing pointer as its argument

This will pass the pointer variable, pointer, to the function() function.

Note: We can access the value pointed by the pointer variable by using the * operator like *pointer.

Passing variable as a pointer

Let's see the code to pass the pointer pointing to the variable as a parameter to the function:

#include <iostream>
using namespace std;
void Increment(int *number)
{
  *number = *number + 1;
}
int main() {
  int num = 5;
  int *ptr = &num;
  cout << "Number before incrementing is " << num << endl;
  Increment(ptr);
  cout << "Number after incrementing is " << num << endl;
  return 0;
}

Explanation

  • Line 4: We define the Increment() function that takes number pointer to an integer as a parameter.

  • Line 5: We dereference the number pointer and increment its value.

  • Line 10: We declare a variable num and set it to 5.

  • Line 11: We declare a pointer variable, ptr, and point it to the num variable.

  • Line 12: We print the num variable.

  • Line 13: We call the Increment() function by passing the ptr pointer as an argument.

  • Line 14: We print the value of the num variable after calling the function.

Remember: If we pass the variable to the function instead of passing a pointer (address) to the variable, then the changes to the variable inside the function will not be reflected outside of the function (e.g. main()).

Passing an array as a pointer

Let's see the code to pass the pointer pointing to an array as a parameter to the function:

#include <iostream>
using namespace std;
void Increment(int *pointer, int size)
{
  for(int i = 0; i < size; i++)
  {
    (*(pointer + i))++;     // pointer[i]++;
  }
}
int main() {
  int arr[] = {2, 3, 5, 7, 11};
  int size = 5;
  int *ptr = arr;
  cout << "Array before incrementing is: ";
  for (int i = 0; i < size; i++)
    cout << arr[i] << " ";
  cout << endl;
  Increment(ptr, size);
  cout << "Array after incrementing is: ";
  for (int i = 0; i < size; i++)
    cout << arr[i] << " ";
  cout << endl;
  return 0;
}

Explanation

  • Line 4: We define the Increment() function that has two parameters—integer type number pointer and size variable.

  • Line 6: We loop to iterate through an array.

  • Line 8: We dereference pointer to access each index of the array and increment the value by 1.

  • Line 13: We declare an array, arr, and initialize it.

  • Line 14: We store the size of an array in a variable size.

  • Line 15: We create a pointer ptr to arr.

  • Lines 17–20: We print arr before calling the Increment() function.

  • Line 13: We pass ptr and size to the Increment() function.

  • Lines 24–27: We print arr after calling the Increment() function.

Note: The type of pointer variable should be same as the type of variable to be pointed.

Conclusion

Conclusively, to pass a pointer to a function in C++, we define a function that takes a pointer as a parameter, create a pointer variable that points to the value we want to pass, and pass it to the function. We can use the pointer to access or modify the original value inside the function.

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