In computer programming, a default argument is an argument to a function that a programmer is not required to specify.

In most programming languages, functions may take one or more arguments. Usually, each argument must be specified in full (this is the case in the C programming language).

Later languages (for example, in C++) allow the programmer to specify default arguments that always have some value, even if the calling program do not write them. For example, in the following function definition:

int MyFunc(int a, int b, int c=12);
this function takes three arguments, of which the last one has a default of twelve. The programmer may call this function in two ways:

result = MyFunc(1, 2, 3);

result = MyFunc(1, 2);
In the first case the value for the argument called c is specified as normal. In the second one, the argument is omitted, and the default 12 value will be used instead.

The called function has no way of knowing if the argument has been specified by the caller or using the default value.