8.13 — Default arguments

A default argument is a default value provided for a function parameter. For example:

void print(int x, int y=10) // 10 is the default argument
{
    std::cout << "x: " << x << '\n';
    std::cout << "y: " << y << '\n';
}

When making a function call, the caller can optionally provide an argument for any function parameter that has a default argument. If the caller provides an argument, the value of the argument in the function call is used. If the caller does not provide an argument, the value of the default argument is used.

Consider the following program:

#include <iostream>

void print(int x, int y=4) // 4 is the default argument
{
    std::cout << "x: " << x << '\n';
    std::cout << "y: " << y << '\n';
}

int main()
{
    print(1, 2); // y will use user-supplied argument 2
    print(3); // y will use default argument 4, as if we had called print(3, 4)

}

This program produces the following output:

x: 1
y: 2
x: 3
y: 4

In the first function call, the caller supplied explicit arguments for both parameters, so those argument values are used. In the second function call, the caller omitted the second argument, so the default value of 4 was used.

Note that you must use the equals sign to specify a default argument. Using parenthesis or brace initialization won’t work:

void foo(int x = 5);   // ok
void goo(int x ( 5 )); // compile error
void boo(int x { 5 }); // compile error

Perhaps surprisingly, default arguments are handled by the compiler at the call site. In the above example, when the compiler sees print(3), it will rewrite this function call as print(3, 4), so that the number of arguments matches the number of parameters. The rewritten function call then works as per usual.

When to use default arguments

Default arguments are an excellent option when a function needs a value that has a reasonable default value, but for which you want to let the caller override if they wish.

For example, here are a couple of function prototypes for which default arguments might be commonly used:

int rollDie(int sides=6);
void openLogFile(std::string filename="default.log");

Multiple default arguments

A function can have multiple parameters with default arguments:

#include <iostream>

void print(int x=10, int y=20, int z=30)
{
    std::cout << "Values: " << x << " " << y << " " << z << '\n';
}

int main()
{
    print(1, 2, 3); // all explicit arguments
    print(1, 2); // rightmost argument defaulted
    print(1); // two rightmost arguments defaulted
    print(); // all arguments defaulted

    return 0;
}

The following output is produced:

Values: 1 2 3
Values: 1 2 30
Values: 1 20 30
Values: 10 20 30

C++ does not (as of C++20) support a function call syntax such as print(,,3) (as a way to provide an explicit value for z while using the default arguments for x and y. This has two major consequences:

1. Default arguments can only be supplied for the rightmost parameters. The following is not allowed:

void print(int x=10, int y); // not allowed

2. If more than one default argument exists, the leftmost default argument should be the one most likely to be explicitly set by the user.

Default arguments can not be redeclared

Once declared, a default argument can not be redeclared (in the same file). That means for a function with a forward declaration and a function definition, the default argument can be declared in either the forward declaration or the function definition, but not both.

#include <iostream>

void print(int x, int y=4); // forward declaration

void print(int x, int y=4) // error: redefinition of default argument
{
    std::cout << "x: " << x << '\n';
    std::cout << "y: " << y << '\n';
}

Best practice is to declare the default argument in the forward declaration and not in the function definition, as the forward declaration is more likely to be seen by other files (particularly if it’s in a header file).

in foo.h:

#ifndef FOO_H
#define FOO_H
void print(int x, int y=4);
#endif

in main.cpp:

#include "foo.h"
#include <iostream>

void print(int x, int y)
{
    std::cout << "x: " << x << '\n';
    std::cout << "y: " << y << '\n';
}

int main()
{
    print(5);

    return 0;
}

Note that in the above example, we’re able to use the default argument for function print() because main.cpp #includes foo.h, which has the forward declaration that defines the default argument.

Default arguments and function overloading

Functions with default arguments may be overloaded. For example, the following is allowed:

void print(std::string string)
{
}

void print(char ch=' ')
{
}

int main()
{
    print("Hello, world"); // resolves to print(std::string)
    print('a'); // resolves to print(char)
    print(); // resolves to print(char)

    return 0;
}

The function call to print() acts as if the user had explicitly called print(' '), which resolves to print(char).

Now consider this case:

void print(int x);
void print(int x, int y = 10);
void print(int x, double y = 20.5);

Parameters with default values will differentiate a function overload (meaning the above will compile).
However, such functions can lead to potentially ambiguous function calls. For example:

print(1, 2); // will resolve to print(int, int)
print(1, 2.5); // will resolve to print(int, double) 
print(1); // ambiguous function call

In the last case, the compiler is unable to tell whether print(1) should resolve to print(int) or one of the two functions where the second parameter has a default value. The result is an ambiguous function call.

Summary

Default arguments provide a useful mechanism to specify values for parameters that the user may or may not want to override. They are frequently used in C++, and you’ll see them a lot in future lessons.