Passing Reference to a Pointer in C++

Prerequisite: Pointers vs References in C++.

For clear understanding, let’s compare the usage of a “pointer to pointer” VS “Reference to pointer” in some cases.

Note: It is allowed to use “pointer to pointer” in both C and C++, but we can use “Reference to pointer” only in C++.

Passing pointer to a function

If a pointer is passed to a function as a parameter and tried to be modified then the changes made to the pointer does not reflects back outside that function. This is because only a copy of the pointer is passed to the function. It can be said that “pass by pointer” is passing a pointer by value. In most cases, this does not present a problem. But the problem comes when you modify the pointer inside the function. Instead of modifying the variable, you are only modifying a copy of the pointer and the original pointer remains unmodified.

Below program illustrate this:

CPP

#include <iostream>

using namespace std;

int global_Var = 42;

// function to change pointer value
void changePointerValue(int* pp)
{
    pp = &global_Var;
}

int main()
{
    int var = 23;
    int* ptr_to_var = &var;

    cout << "Passing Pointer to function:" << endl;

    cout << "Before :" << *ptr_to_var << endl; // display 23

    changePointerValue(ptr_to_var);

    cout << "After :" << *ptr_to_var << endl; // display 23

    return 0;
}

Output:

Passing Pointer to function:
Before :23
After :23

Passing “pointer to a pointer” as a parameter to function

The above problem can be resolved by passing the address of the pointer to the function instead of a copy of the actual function. For this, the function parameter should accept a “pointer to pointer” as shown in the below program:

CPP

#include <iostream>

using namespace std;

int global_var = 42;

// function to change pointer to pointer value
void changePointerValue(int** ptr_ptr)
{
    *ptr_ptr = &global_var;
}

int main()
{
    int var = 23;
    int* pointer_to_var = &var;

    cout << "Passing a pointer to a pointer to function " << endl;

    cout << "Before :" << *pointer_to_var << endl; // display 23

    changePointerValue(&pointer_to_var);

    cout << "After :" << *pointer_to_var << endl; // display 42

    return 0;
}

Output:

Passing a pointer to a pointer to function 
Before :23
After :42

How to call a function with “Reference to pointer” parameter?

A reference allows called function to modify a local variable of the caller function. For example, consider the following example program where fun() is able to modify local variable x of main().

CPP

#include<iostream>

using namespace std;

void fun(int &x) {
    x = 20;
}

int main() {
    int x = 10;
    fun(x);
    cout<<"New value of x is "<<x;
    return 0;
}

Output:

New value of x is 20

Below program shows how to pass a “Reference to a pointer” to a function:

CPP

#include <iostream>

using namespace std;

int gobal_var = 42;

// function to change Reference to pointer value
void changeReferenceValue(int*& pp)
{
    pp = &gobal_var;
}

int main()
{
    int var = 23;
    int* ptr_to_var = &var;

    cout << "Passing a Reference to a pointer to function" << endl;

    cout << "Before :" << *ptr_to_var << endl; // display 23

    changeReferenceValue(ptr_to_var);

    cout << "After :" << *ptr_to_var << endl; // display 42

    return 0;
}

Output:

Passing a Reference to a pointer to function
Before :23
After :42

Returning a pointer from a function

CPP

#include <iostream>

using namespace std;

int global_var = 42;

// function to return a pointer
int* returnPointerValue()
{
    return &global_var;
}

int main()
{
    int var = 23;
    int* ptr_to_var = &var;

    cout << "Return a pointer from a function " << endl;

    cout << "Before :" << *ptr_to_var << endl; // display 23

    ptr_to_var = returnPointerValue();

    cout << "After :" << *ptr_to_var << endl; // display 42

    return 0;
}

Output:

Return a pointer from a function 
Before :23
After :42

Returning reference from function

CPP

#include <iostream>

using namespace std;

int global_var = 42;

// function to return reference value
int& ReturnReference()
{
    return global_var;
}

int main()
{
    int var = 23;
    int* ptr_to_var = &var;

    cout << "Returning a Reference " << endl;

    cout << "Before :" << *ptr_to_var << endl; // display 23

    ptr_to_var = &ReturnReference();

    cout << "After :" << *ptr_to_var << endl; // display 42

    return 0;
}

Output:

Returning a Reference 
Before :23
After :42