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
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