Specifying default values to function parameters in C++

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For every parameter you declare in a function prototype and definition, the calling function must pass in a value. The value passed in must be of the declared type. Thus, if you have a function declared as

long myFunction(int);

the function must in fact take an integer variable. If the function definition differs, or if you fail to pass in an integer, you will get a compiler error.

The one exception to this rule is if the function prototype declares a default value for the parameter. A default value is a value to use if none is supplied. The preceding declaration could be rewritten as

long myFunction (int x = 50);

This prototype says, "myFunction() returns a long and takes an integer parameter. If an argument is not supplied, use the default value of 50." Because parameter names are not required in function prototypes, this declaration could have been written as

long myFunction (int = 50);

The function definition is not changed by declaring a default parameter. The function definition header for this function would be

long myFunction (int x)

If the calling function did not include a parameter, the compiler would fill x with the default value of 50. The name of the default parameter in the prototype need not be the same as the name in the function header; the default value is assigned by position, not name.

Any or all of the function's parameters can be assigned default values. The one restriction is this: If any of the parameters does not have a default value, no previous parameter may have a default value.

If the function prototype looks like

long myFunction (int Param1, int Param2, int Param3);

you can assign a default value to Param2 only if you have assigned a default value to Param3. You can assign a default value to Param1 only if you've assigned default values to both Param2 and Param3. Program below demonstrates the use of default values.

A demonstration of default parameter values.

1:   // Demonstrates use
2:   // of default parameter values
4:   #include <iostream.h>
6:   int AreaCube(int length, int width = 25, int height = 1);
8:   int main()
9:   {
10:       int length = 100;
11:       int width = 50;
12:       int height = 2;
13:       int area;
15:       area = AreaCube(length, width, height);
16:       cout << "First area equals: " << area << "\n";
18:       area = AreaCube(length, width);
19:       cout << "Second time area equals: " << area << "\n";
21:       area = AreaCube(length);
22:       cout << "Third time area equals: " << area << "\n";
23:        return 0;
24:   }
26:   AreaCube(int length, int width, int height)
27:   {
29:       return (length * width * height);
30: }

Output: First area equals: 10000
Second time area equals: 5000
Third time area equals: 2500

Analysis: On line 6, the AreaCube() prototype specifies that the AreaCube() function takes three integer parameters. The last two have default values.
This function computes the area of the cube whose dimensions are passed in. If no width is passed in, a width of 25 is used and a height of 1 is used. If the width but not the height is passed in, a height of 1 is used. It is not possible to pass in the height without passing in a width.

On lines 10-12, the dimensions length, height, and width are initialized, and they are passed to the AreaCube() function on line 15. The values are computed, and the result is printed on line 16.

Execution returns to line 18, where AreaCube() is called again, but with no value for height. The default value is used, and again the dimensions are computed and printed.

Execution returns to line 21, and this time neither the width nor the height is passed in. Execution branches for a third time to line 27. The default values are used. The area is computed and then printed.


DO remember that function parameters act as local variables within the function. DON'T try to create a default value for a first parameter if there is no default value for the second. DON'T forget that arguments passed by value can not affect the variables in the calling function. DON'T forget that changes to a global variable in one function change that variable for all functions.

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