When a variable is declared as a reference, it becomes an alternative name for an existing variable. A variable can be declared as a reference by putting ‘&’ in the declaration.
Also, we can define a reference variable as a type of variable that can act as a reference to another variable. ‘&’ is used for signifying the address of a variable or any memory. Variables associated with reference variables can be accessed either by its name or by the reference variable associated with it.
Prerequisite: Pointers in C++
Syntax:
data_type &ref = variable;
Example:
C++
#include <iostream>
using namespace std;
int main()
{
int x = 10;
int& ref = x;
ref = 20;
cout << "x = " << x << '\n';
x = 30;
cout << "ref = " << ref << '\n';
return 0;
}
|
Output:
x = 20
ref = 30
Applications of Reference in C++
There are multiple applications for references in C++, a few of them are mentioned below:
- Modify the passed parameters in a function
- Avoiding a copy of large structures
- In For Each Loop to modify all objects
- For Each Loop to avoid the copy of objects
1. Modify the passed parameters in a function:
If a function receives a reference to a variable, it can modify the value of the variable. For example, the following program variables are swapped using references.
Example:
C++
#include <iostream>
using namespace std;
void swap(int& first, int& second)
{
int temp = first;
first = second;
second = temp;
}
int main()
{
int a = 2, b = 3;
swap(a, b);
cout << a << " " << b;
return 0;
}
|
2. Avoiding a copy of large structures:
Imagine a function that has to receive a large object. If we pass it without reference, a new copy of it is created which causes a waste of CPU time and memory. We can use references to avoid this.
Example:
struct Student {
string name;
string address;
int rollNo;
}
// If we remove & in below function, a new
// copy of the student object is created.
// We use const to avoid accidental updates
// in the function as the purpose of the function
// is to print s only.
void print(const Student &s)
{
cout << s.name << " " << s.address << " " << s.rollNo
<< '\n';
} 3. In For Each Loop to modify all objects:
We can use references for each loop to modify all elements.
Example:
C++
#include <iostream>
#include <vector>
using namespace std;
int main()
{
vector<int> vect{ 10, 20, 30, 40 };
for (int& x : vect) {
x = x + 5;
}
for (int x : vect) {
cout << x << " ";
}
cout << '\n';
return 0;
}
|
4. For Each Loop to avoid the copy of objects:
We can use references in each loop to avoid a copy of individual objects when objects are large.
Example:
C++
#include <iostream>
#include <vector>
using namespace std;
int main()
{
vector<string> vect{ "geeksforgeeks practice",
"geeksforgeeks write",
"geeksforgeeks ide" };
for (const auto& x : vect) {
cout << x << '\n';
}
return 0;
}
|
Outputgeeksforgeeks practice
geeksforgeeks write
geeksforgeeks ide
References vs Pointers
Both references and pointers can be used to change the local variables of one function inside another function. Both of them can also be used to save copying of big objects when passed as arguments to functions or returned from functions, to get efficiency gain. Despite the above similarities, there are the following differences between references and pointers.
1. A pointer can be declared as void but a reference can never be void For example
int a = 10;
void* aa = &a; // it is valid
void& ar = a; // it is not valid
2. The pointer variable has n-levels/multiple levels of indirection i.e. single-pointer, double-pointer, triple-pointer. Whereas, the reference variable has only one/single level of indirection. The following code reveals the mentioned points:
3. Reference variables cannot be updated.
4. Reference variable is an internal pointer.
5. Declaration of a Reference variable is preceded with the ‘&’ symbol ( but do not read it as “address of”).
Example:
C++
#include <iostream>
using namespace std;
int main()
{
int i = 10;
int* p = &i;
int** pt = &p;
int*** ptr = &pt;
cout << "i = " << i << "\t"
<< "p = " << p << "\t"
<< "pt = " << pt << "\t"
<< "ptr = " << ptr << '\n';
int a = 5;
int& S = a;
int& S0 = S;
int& S1 = S0;
cout << "a = " << a << "\t"
<< "S = " << S << "\t"
<< "S0 = " << S0 << "\t"
<< "S1 = " << S1 << '\n';
return 0;
}
|
Outputi = 10 p = 0x7ffecfe7c07c pt = 0x7ffecfe7c080 ptr = 0x7ffecfe7c088
a = 5 S = 5 S0 = 5 S1 = 5
Limitations of References
- Once a reference is created, it cannot be later made to reference another object; it cannot be reset. This is often done with pointers.
- References cannot be NULL. Pointers are often made NULL to indicate that they are not pointing to any valid thing.
- A reference must be initialized when declared. There is no such restriction with pointers.
Due to the above limitations, references in C++ cannot be used for implementing data structures like Linked List, Tree, etc. In Java, references don’t have the above restrictions and can be used to implement all data structures. References being more powerful in Java is the main reason Java doesn’t need pointers.
Advantages of using References
- Safer: Since references must be initialized, wild references like wild pointers are unlikely to exist. It is still possible to have references that don’t refer to a valid location (See questions 5 and 6 in the below exercise)
- Easier to use: References don’t need a dereferencing operator to access the value. They can be used like normal variables. The ‘&’ operator is needed only at the time of declaration. Also, members of an object reference can be accessed with the dot operator (‘.’), unlike pointers where the arrow operator (->) is needed to access members.
Together with the above reasons, there are a few places like the copy constructor argument where a pointer cannot be used. Reference must be used to pass the argument in the copy constructor. Similarly, references must be used for overloading some operators like ++.
Exercise with Answers
Question 1 :
C++
#include <iostream>
using namespace std;
int& fun()
{
static int x = 10;
return x;
}
int main()
{
fun() = 30;
cout << fun();
return 0;
}
|
Question 2
C++
#include <iostream>
using namespace std;
int fun(int& x) { return x; }
int main()
{
cout << fun(10);
return 0;
}
|
Output:
./3337ee98-ae6e-4792-8128-7c879288221f.cpp: In function 'int main()':
./3337ee98-ae6e-4792-8128-7c879288221f.cpp:8:19: error: invalid initialization of non-const reference of type 'int&' from an rvalue of type 'int'
cout << fun(10);
^
./3337ee98-ae6e-4792-8128-7c879288221f.cpp:4:5: note: in passing argument 1 of 'int fun(int&)'
int fun(int& x) { return x; }Question 3
C++
#include <iostream>
using namespace std;
void swap(char*& str1, char*& str2)
{
char* temp = str1;
str1 = str2;
str2 = temp;
}
int main()
{
char* str1 = "GEEKS";
char* str2 = "FOR GEEKS";
swap(str1, str2);
cout << "str1 is " << str1 << '\n';
cout << "str2 is " << str2 << '\n';
return 0;
}
|
Outputstr1 is FOR GEEKS
str2 is GEEKS
Question 4
C++
#include <iostream>
using namespace std;
int main()
{
int x = 10;
int* ptr = &x;
int&* ptr1 = ptr;
}
|
Output:
./18074365-ebdc-4b13-81f2-cfc42bb4b035.cpp: In function 'int main()':
./18074365-ebdc-4b13-81f2-cfc42bb4b035.cpp:8:11: error: cannot declare pointer to 'int&'
int&* ptr1 = ptr;Question 5
C++
#include <iostream>
using namespace std;
int main()
{
int* ptr = NULL;
int& ref = *ptr;
cout << ref << '\n';
}
|
Output:
timeout: the monitored command dumped core
/bin/bash: line 1: 34 Segmentation fault timeout 15s ./372da97e-346c-4594-990f-14edda1f5021 < 372da97e-346c-4594-990f-14edda1f5021.in
Question 6
C++
#include <iostream>
using namespace std;
int& fun()
{
int x = 10;
return x;
}
int main()
{
fun() = 30;
cout << fun();
return 0;
}
|
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