Callable object
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A callable object, in computer programming, is any object that can be called like a function.
In different languages[]
In C++[]
- pointer to function;
- pointer to member function;
- functor;
- lambda expression.
std::function
is a template class that can hold any callable object that matches its signature.
In C++, any class that overloads the function call operator operator()
may be called using function-call syntax.
#include <iostream>
struct Foo
{
void operator()() const
{
std::cout << "Called.";
}
};
int main()
{
Foo foo_instance;
foo_instance(); // This will output "Called." to the screen.
}
In C#[]
- delegate;
- lambda expression.
In PHP[]
PHP 5.3+ has first-class functions that can be used e.g. as parameter to the usort()
function:
$a = array(3, 1, 4);
usort($a, function ($x, $y) { return $x - $y; });
It is also possible in PHP 5.3+ to make objects invokable by adding a magic __invoke()
method to their class:[1]
class Minus
{
public function __invoke($x, $y) { return $x - $y; }
}
$a = array(3, 1, 4);
usort($a, new Minus());
In Python[]
In Python any object with a __call__()
method can be called using function-call syntax.
class Foo:
def __call__(self):
print("Called.")
foo_instance = Foo()
foo_instance() # This will output "Called." to the screen.
Another example:
class Accumulator:
def __init__(self, n):
self.n = n
def __call__(self, x):
self.n += x
return self.n
In Dart[]
To allow your Dart class to be called like a function, implement the call()
method.
class WannabeFunction {
call(String a, String b, String c) => '$a $b $c!';
}
main() {
var wf = new WannabeFunction();
var out = wf("Hi","there,","gang");
print('$out');
}
References[]
- ^ PHP Documentation on Magic Methods
- ^ Bösch, Florian. "What is a "callable" in Python?". StackOverflow.com. Retrieved 24 September 2017.
- ^ "A Tour of the Dart Language". www.dartlang.org. Retrieved 2019-03-25.
External links[]
Categories:
- Object (computer science)
- Subroutines
- Computing stubs