C++ Tutorial/Function/reference parameter

Pass by reference by using pointer

<source lang="cpp">#include <iostream>

void swap(int *x, int *y);

int main()
{
    int x = 5, y = 10;

    std::cout << "Main. Before swap, x: " << x << " y: " << y << "\n";
    swap(&x,&y);
    std::cout << "Main. After swap, x: " << x << " y: " << y << "\n";
    return 0;
}

void swap (int *px, int *py)
{
    int temp;

    std::cout << "Swap. Before swap, *px: " << *px << " *py: " << *py << "\n";

    temp = *px;
    *px = *py;
    *py = temp;

    std::cout << "Swap. After swap, *px: " << *px << " *py: " << *py << "\n";
}</source>

Main. Before swap, x: 5 y: 10
Swap. Before swap, *px: 5 *py: 10
Swap. After swap, *px: 10 *py: 5
Main. After swap, x: 10 y: 5

Pass by reference using references

<source lang="cpp">#include <iostream>

void swap(int &x, int &y);

int main()
{
    int x = 5, y = 10;

    std::cout << "Main. Before swap, x: " << x 
                                << " y: " << y << "\n";
    swap(x,y);
    std::cout << "Main. After swap, x: " << x 
                               << " y: " << y << "\n";
    return 0;
}

void swap (int &rx, int &ry)
{
    int temp;

    std::cout << "Swap. Before swap, rx: " << rx 
                                << " ry: " << ry << "\n";

    temp = rx;
    rx = ry;
    ry = temp;

    std::cout << "Swap. After swap, rx: " << rx 
                               << " ry: " << ry << "\n";
}</source>

Main. Before swap, x: 5 y: 10
Swap. Before swap, rx: 5 ry: 10
Swap. After swap, rx: 10 ry: 5
Main. After swap, x: 10 y: 5

Passing references to objects

<source lang="cpp">#include <iostream>

class MyClass
{
public:
    MyClass();
    MyClass(MyClass&);
    ~MyClass();

    int GetAge() const { return itsAge; }
    void SetAge(int age) { itsAge = age; }

private:
    int itsAge;
};

MyClass::MyClass()
{
    std::cout << "Simple Cat Constructor...\n";
    itsAge = 1;
}

MyClass::MyClass(MyClass&)
{
    std::cout << "Simple Cat Copy Constructor...\n";
}

MyClass::~MyClass()
{
    std::cout << "Simple Cat Destructor...\n";
}

const MyClass & f (const MyClass & obj);

int main()
{
    MyClass myObject;
    std::cout << "myObject is " << myObject.GetAge() << " years old\n";

    int age = 5;
    myObject.SetAge(age);
    std::cout << "myObject is " << myObject.GetAge() << " years old\n";

    std::cout << "Calling f...\n";
    f(myObject);
    std::cout << "myObject is " << myObject.GetAge() << " years old\n";
    return 0;
}

const MyClass & f (const MyClass & obj)
{
    std::cout << "Function Two. Returning...\n";
    std::cout << "myObject is now " << obj.GetAge()<< " years old \n";
    return obj;
}</source>

Simple Cat Constructor...
myObject is 1 years old
myObject is 5 years old
Calling f...
Function Two. Returning...
myObject is now 5 years old
myObject is 5 years old
Simple Cat Destructor...

Returning multiple values from a function using references

<source lang="cpp">#include <iostream>

void f(int, int&, int&);

int main()
{
    int number = 2, squared, cubed;
    f(number, squared, cubed);

    std::cout << "number: " << number << "\n";
    std::cout << "square: " << squared << "\n";
    std::cout << "cubed: "  << cubed   << "\n";
    return 0;
}

void f(int n, int &rSquared, int &rCubed)
{
     rSquared = n*n;
     rCubed = n*n*n;
}</source>

number: 2
square: 4
cubed: 8

Use reference parameters to create the swap() function.

<source lang="cpp">#include <iostream> using namespace std;

void swap(int &x, int &y);

int main() {

 int i, j; 

 i = 10; 
 j = 20; 

 cout << "Initial values of i and j: "; 
 cout << i << " " << j << "\n"; 

 swap(j, i); 

 cout << "Swapped values of i and j: "; 
 cout << i << " " << j << "\n"; 

 return 0; 

}

void swap(int &x, int &y) {

 int temp; 

 temp = x;  
 x = y;     
 y = temp;  

}</source>

Initial values of i and j: 10 20
Swapped values of i and j: 20 10

Using a reference parameter.

<source lang="cpp">#include <iostream> using namespace std;

void f(int &i); // i is a reference parameter

int main() {

 int val = 1; 

 cout << "Old value for val: " << val << "\n"; 

 f(val); 

 cout << "New value for val: " << val << "\n"; 

 return 0; 

}

void f(int &i) {

 i = 10; 

}</source>

Old value for val: 1
New value for val: 10