C++ reference in detail

The reference variable is 1 individual name, that is, it is another name for an existing variable. Once a reference is initialized to a variable, the reference name or variable name can be used to point to the variable.

C++ refers to the vs pointer

References are easily confused with Pointers, and there are three major differences between them:

There is no empty reference. References must be connected to a valid block of memory. Once a reference is initialized to one object, it cannot be pointed to another. A pointer can point to another object at any time. References must be initialized at creation time. Pointers can be initialized at any time.

Create a reference in C++

Imagine that a variable name is a tag attached to a variable in a memory location, and you can think of a reference as a second tag attached to a variable in a memory location. Therefore, you can access the contents of a variable by its original name or reference. Such as:

int i = 17;

We can declare reference variables for i as follows:

int& r = i;
double& s = d;

In these statements, & Read as a quote. Thus, the first statement can be read as "r is an integer reference initialized to i ", and the second statement can be read as "s is an double reference initialized to d ". The following examples use int and double references:

#include <iostream>
 
using namespace std;
 
int main ()
{
  //  Declare simple variables 
  int  i;
  double d;
 
  //  Declare reference variables 
  int&  r = i;
  double& s = d;
  
  i = 5;
  cout << "Value of i : " << i << endl;
  cout << "Value of i reference : " << r << endl;
 
  d = 11.7;
  cout << "Value of d : " << d << endl;
  cout << "Value of d reference : " << s << endl;
  
  return 0;
}

When the above code is compiled and executed, it produces the following results:

[

Value of i : 5
Value of i reference : 5
Value of d : 11.7
Value of d reference : 11.7

]

References are commonly used for function argument lists and function return values. Here are two important concepts that C++ programmers must be aware of in relation to C++ references:

Take the reference as an argument

C++ supports passing references to functions as arguments, which is safer than passing 1-like arguments.

#include <iostream>
using namespace std;
 
//  Function declaration 
void swap(int& x, int& y);
 
int main ()
{
  //  Local variable declaration 
  int a = 100;
  int b = 200;
 
  cout << " Exchange of before, a  Value: " << a << endl;
  cout << " Exchange of before, b  Value: " << b << endl;
 
  /*  Call functions to exchange values  */
  swap(a, b);
 
  cout << " After exchanging, a  Value: " << a << endl;
  cout << " After exchanging, b  Value: " << b << endl;
 
  return 0;
}
 
//  The function definitions 
void swap(int& x, int& y)
{
  int temp;
  temp = x; /*  Save the address  x  The value of the  */
  x = y;  /*  the  y  Assigned to  x */
  y = temp; /*  the  x  Assigned to  y */
 
  return;
}

When the above code is compiled and executed, it produces the following results:

[

Before switching, the value of a: 100
Value of b before switching: 200
After swapping, the value of a: 200
After swapping, the value of b: 100

]

Take the reference as the return value

You can return a reference from the C++ function just as you would return any other data type 1. Using a reference instead of a pointer makes the C++ program easier to read and maintain. The C++ function can return 1 reference in a similar way to returning 1 pointer.

When a function returns a reference, an implicit pointer to the return value is returned. This way, the function can be placed to the left of the assignment statement. For example, look at this simple program:

#include <iostream>
 
using namespace std;
 
double vals[] = {10.1, 12.6, 33.1, 24.1, 50.0};
 
double& setValues( int i )
{
 return vals[i];  //  Returns the first  i  A reference to an element 
}
 
//  To call the main function that defined the function above 
int main ()
{
 
  cout << " Change the value before " << endl;
  for ( int i = 0; i < 5; i++ )
  {
    cout << "vals[" << i << "] = ";
    cout << vals[i] << endl;
  }
 
  setValues(1) = 20.23; //  Change the first  2  An element 
  setValues(3) = 70.8; //  Change the first  4  An element 
 
  cout << " The changed value " << endl;
  for ( int i = 0; i < 5; i++ )
  {
    cout << "vals[" << i << "] = ";
    cout << vals[i] << endl;
  }
  return 0;
}

When the above code is compiled and executed, it produces the following results:

[

Change the value before
vals[0] = 10.1
vals[1] = 12.6
vals[2] = 33.1
vals[3] = 24.1
vals[4] = 50
The changed value
vals[0] = 10.1
vals[1] = 20.23
vals[2] = 33.1
vals[3] = 70.8
vals[4] = 50

]

When returning a reference, be careful that the referenced object does not go out of scope. It is illegal to return a reference to a local variable, but you can return a reference to a static variable.

int& func() {
  int q;
  //! return q; //  An error occurred at compile time 
  static int x;
  return x;   //  Security, x  It is still valid outside the scope of the function 
}

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