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 a value, even if one is not specified when calling the function.
This function takes three arguments, of which the last one has a default of twelve. The programmer may call this function in two ways:
In the first case the value for the argument called c is specified explicitly. In the second case, the argument is omitted, and the default value of 12 will be used instead. For the function called, there is no means to know if the argument has been specified by the caller or if the default value was used.
The above-mentioned method is especially useful when one wants to set default criteria so that the function can be called with or without parameters. Consider the following:
// This outputs a message to the given stream.
stream << "hello world!";
}
The function call:
Because default arguments' values are "filled in" at the call site rather than in the body of the function being called, take their default argument values from the static type of the pointer or reference through which the call is made, rather than from the dynamic type of the object supplying the virtual function's body.
struct Derived : public Base {
int main() {
virtual std::pair
};
std::pair
};
Derived d;
Base& b = d;
assert(d.Foo() == std::make_pair(2, 2));
assert(b.Foo() == std::make_pair(1, 2));
}
However, in addition to several other disadvantages, since the default arguments are not modeled in the type system, the type of a callback (aka higher-order function) can't express that it accepts either of the overloads nor simulate the default arguments with overloaded functions. Whereas, in JavaScript the non-overloaded function definition can substitute the default when the input value is undefined (regardless if it was implicitly undefined via the argument's absence at the call site or an explicitly passed undefined value); which is modeled as an optional argument parameter type ?: in TypeScript. JavaScript's solution is not resolved statically (i.e. not at compile-time, which is why TypeScript models only the optionality and not the default values in the function's type signature) thus incurs additional runtime overhead, although it does provide more flexibility in that callbacks can independently control their defaults instead of centrally dictated by the (callback's type signature in the) type signature of the function which inputs the callback. The TypeScript solution can be simulated in Java with the Optional type except the analogous of an implicit undefined for each absent argument is an explicit Optional.<Integer>absent() at the call site.
Python is a notable language that evaluates expressions in default arguments once, at the time the function declaration is evaluated. If evaluation per function call is desired, it can be replicated by having the default argument be a sentinel value, such as None, and then having the body of the function evaluate the default value's side effects only if the sentinel value was passed in.
For example:
def eager(a=random.random()):
x = eager()
y = eager()
assert x == y
def lazy(a=None):
x = lazy()
y = lazy()
assert x != y
return a
if a is None:
a = random.random()
return a
In other cases a default argument may instead be statically allocated. If the variable is mutable, it will then retain its value across function calls, as with a static variable.
This behavior is found in Python for mutable types, such as lists. As with evaluation, in order to ensure the same extent as a local variable, one can use a sentinel value:
x = eager()
x += 1
assert eager() == 1
def lazy(a=None):
x = lazy()
x += 1
assert lazy() ==
return a
if a is None:
a = []
return a
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