10.6: Default Arguments

Default Arguments in C++

A default argument is a value provided in a function declaration that is automatically assigned by the compiler if the caller of the function doesn’t provide a value for the argument with a default value.

Following is a simple C++ example to demonstrate the use of default arguments. We don’t have to write 3 sum functions, only one function works by using default values for 3rd and 4th arguments.

#include<iostream> 
using namespace std;

// A function with default arguments, it can be called with 
// 2 arguments or 3 arguments or 4 arguments. 
int sum(int x, int y, int z=0, int w=0) 
{ 
    return (x + y + z + w); 
} 

/* Driver program to test above function*/
int main() 
{ 
    cout << sum(10, 15) << endl; 
    cout << sum(10, 15, 25) << endl; 
    cout << sum(10, 15, 25, 30) << endl; 
    return 0; 
}

All 3 of the function calls work because - in the first instance, where there are only 2 arguments, the defaults set the values of z and w. In the second instance the only defaults value in use is t=for the variable w. The last example - no default values are in use, as all 4 arguments are passed from the calling function.

Output:

25
50
80

When function overloading (we will talk about function overloading in the next sections) done along with default values. Then we need to make sure it will not be ambiguous.
The compiler will throw error if ambiguous. Following is the modified version of above program. 

#include<iostream> 
using namespace std; 

// A function with default arguments, it can be called with 
// 2 arguments or 3 arguments or 4 arguments. 
int sum(int x, int y, int z=0, int w=0) 
{ 
    return (x + y + z + w); 
} 
int sum(int x, int y, float z=0, float w=0) 
{ 
    return (x + y + z + w); 
} 
/* Driver program to test above function*/
int main() 
{ 
    cout << sum(10, 15) << endl; 
    cout << sum(10, 15, 25) << endl; 
    cout << sum(10, 15, 25, 30) << endl; 
    return 0; 
}

In this code we now have an overloaded function, with the

 Error:

prog.cpp: In function 'int main()':
prog.cpp:17:20: error: call of overloaded 
'sum(int, int)' is ambiguous
  cout << sum(10, 15) << endl; 
                    ^
prog.cpp:6:5: note: candidate: 
int sum(int, int, int, int)
 int sum(int x, int y, int z=0, int w=0) 
     ^
prog.cpp:10:5: note: candidate: 
int sum(int, int, float, float)
 int sum(int x, int y, float z=0, float w=0) 
     ^

Key Points:

• Default arguments are different from constant arguments as constant arguments can't be changed whereas default arguments can be overwritten if required.
• Default arguments are overwritten when calling function provides values for them. For example, calling of function sum(10, 15, 25, 30) overwrites the value of z and w to 25 and 30 respectively.
• During calling of function, arguments from calling function to called function are copied from left to right. Therefore, sum(10, 15, 25) will assign 10, 15 and 25 to x, y, and z. Therefore, the default value is used for w only.
• Once default value is used for an argument in function definition, all subsequent arguments to it must have default value. It can also be stated as default arguments are assigned from right to left. For example, the following function definition is invalid as subsequent argument of default variable z is not default.

 

// Invalid because z has default value, but w after it 
// doesn't have default value 
int sum(int x, int y, int z=0, int w) 

Adapted from:
"Default Arguments in C++" by Multiple contributors, Geeks for Geeks is licensed under CC BY-SA 4.0