Passing Reference to a Pointer in C++
Last Updated :
09 Dec, 2021
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
Similar Reads
Different ways to use Const with Reference to a Pointer in C++ Before moving forward with using const with Reference to a Pointers, let us first see what they are one by one: Pointers are used to store the address of variables or a memory location. A variable can be declared as a pointer by putting ‘*’ in the declaration. datatype *var_name; Example: CPP // C++
5 min read
How to Return a Pointer from a Function in C++? In C++, we can return a pointer from a function which is useful when we want to return large data structures that cannot be returned by value. However, it must be done carefully to avoid memory leaks, dangling pointers, and other issues related to dynamic memory management. In this article, we will
2 min read
Array of Pointers to Strings in C++ In C++, an array is a homogeneous collection of data that is stored in a contiguous memory location. We can store almost all types of data as array elements. In this article, we will learn how to store the array of pointers to strings in C++. Array of Pointers to Strings in C++A pointer to a string
6 min read
How to add reference of an object in Container Classes We all are familiar with an alias in C++. An alias means another name for some entity. So, a reference variable is an alias that is another name for an existing variable/object etc. Below is the program for adding reference to a variable: CPP // C++ program to illustrate // aliasing in variable #inc
4 min read
Overloads of the Different References in C++ This article focuses on function/method overloads by references, as well as the types of arguments that can be passed. Prerequisites: l-value references.r-value references.Move semantics - std::move(). Overview:l-value refers to a memory location that identifies an object. r-value refers to the data
12 min read
Const Reference vs Normal Parameter Passing in C++ In C++, when we call a function we can also pass the parameters to the function in many ways like pass by value, pass by reference, pass by pointer, and by const reference. Each parameter-passing technique has its own advantages and disadvantages. In this article, we will learn the difference betwee
4 min read
How to Create a Smart Pointer in C++? A smart pointer in C++ simulates a pointer while also providing automatic garbage collection as it deallocates or frees associated memory when it goes out of scope, which helps prevent memory leaks and dangling pointers. In this article, we will learn how to create a smart pointer in C++. Creating a
4 min read
How to Delete a Pointer in C++? In C++, memory management is essential to avoid memory leaks, crashes, and undefinable behavior. In this article, we will learn how to properly delete a pointer in C++. Deleting a Pointer in C++By deleting a pointer, we refer to deleting the memory allocated to the pointer. To delete a pointer in C+
2 min read
How to Create a Pointer to a Function in C++? In C++, a function pointer is a variable that stores the address of a function that can later be called through that function pointer. It is useful for passing functions as parameters to other functions(callback functions) or storing them in data structures. In this article, we will learn how to use
2 min read
Data type of a Pointer in C++ A pointer is a variable that stores the memory address of an object. The pointer then simply “points†to the object. The type of the object must correspond with the type of the pointer. Pointers are used extensively in both C and C++ for three main purposes: To allocate new objects on the heap.To pa
5 min read