Learn C++ - C++ Function Default Parameters

A default argument is a value that's used automatically if you omit the corresponding actual argument from a function call.

For example, if in my_func(int n) n has a default value of 1, the function call my_func() is the same as my_func(1).

To establish a default value, use the function prototype.

The compiler looks at the prototype to see how many arguments a function uses.

The following prototype creates a function with default parameter.

char * left(const char * str, int n = 1);

The function above returns a char pointer.

To leave the original string unaltered, the const qualifier for the first argument is used.

You want n to have a default value of 1, so you assign that value to n.

A default argument value is an initialization value.

If you leave n alone, it has the value 1, but if you pass an argument, the new value overwrites the 1.

When you use a function with an argument list, you must add defaults from right to left.

int my_func(int n, int m = 4, int j = 5);         // VALID 
int my_func(int n, int m = 6, int j);             // INVALID 
int my_func(int k = 1, int m = 2, int n = 3);     // VALID

Example


// Using default arguments. 
#include <iostream> 
using namespace std; 
/*www.ja va 2  s.  com*/
// function prototype that specifies default arguments 
int boxVolume( int length = 1, int width = 1, int height = 1 ); 

int main() 
{ 
    // no arguments--use default values for all dimensions 
    cout << "The default box volume is: " << boxVolume(); 

    // specify length; default width and height 
    cout << "\n\nThe volume of a box with length 10,\n" 
        << "width 1 and height 1 is: " << boxVolume( 10 ); 

    // specify length and width; default height 
    cout << "\n\nThe volume of a box with length 10,\n" 
        << "width 5 and height 1 is: " << boxVolume( 10, 5 ); 

    // specify all arguments 
    cout << "\n\nThe volume of a box with length 10,\n" 
        << "width 5 and height 2 is: " << boxVolume( 10, 5, 2 ) 
        << endl; 
} // end main 

// function boxVolume calculates the volume of a box 
int boxVolume( int length, int width, int height ) 
{ 
    return length * width * height; 
}

The code above generates the following result.

Example 2

The following code creates a string function with a default argument.


#include <iostream>
using namespace std;
const int SIZE = 80;
char * left(const char * str, int n = 1);
/*from   www  .j  a v  a 2  s.c  om*/
int main(){    
    char sample[SIZE];
    cout << "Enter a string:\n";
    cin.get(sample,SIZE);
    char *ps = left(sample, 4);
    cout << ps << endl;
    delete [] ps;       // free old string
    ps = left(sample);
    cout << ps << endl;
    delete [] ps;       // free new string
    return 0;
}

char * left(const char * str, int n){
    if(n < 0)
        n = 0;
    char * p = new char[n+1];
    int i;
    for (i = 0; i < n && str[i]; i++)
        p[i] = str[i];  // copy characters
    while (i <= n)
        p[i++] = '\0';  // set rest of string to '\0'
    return p; 
}

The code above generates the following result.