What is a functional object?

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It is interesting to note that, strictly speaking, the C ++ standard does not use the term functional object , that is, the term functional object . There is a term function object .

I do not know whether these terms are equivalent from the point of view of English grammar. :)

I will use the term function object. Simply put, this term means a class object that has a function call operator and can be used where a postfix function call expression is used.

From the standard C ++ (20.9 Function objects)

1 A function object type (3.9) a function call (5.2.2, 13.3.1.1) .231 A function object type. In the whereabouts of the interface, there is a function for the object. This function not only makes it possible.

The most well-known representative of the function object is the lambda expression.

Here is a demo program. In the first loop, the global function ::IsEven , and in the second loop, the function object is called - a local lambda expression with the same name IsEven , which has an implicit function call operator. The declaration lambda expression hides the name of a global function.

 #include <iostream> bool IsEven( int x ) { return x % 2 == 0; } int main() { int a[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 }; for ( int x : a ) { std::cout << x << " is " << ( IsEven( x ) ? "even" : "odd" ) << " number" << std::endl; } std::cout << std::endl; auto IsEven = []( int x ) { return x % 2 == 0; }; for ( int x : a ) { std::cout << x << " is " << ( IsEven( x ) ? "even" : "odd" ) << " number" << std::endl; } }

Output to console:

 0 is even number 1 is odd number 2 is even number 3 is odd number 4 is even number 5 is odd number 6 is even number 7 is odd number 8 is even number 9 is odd number 0 is even number 1 is odd number 2 is even number 3 is odd number 4 is even number 5 is odd number 6 is even number 7 is odd number 8 is even number 9 is odd number

Instead of lambda expressions, you could define your own functional object. For example, you can replace the announcement in the demo program

 auto IsEven = []( int x ) { return x % 2 == 0; };

on

 struct { bool operator ()( int x ) const { return x % 2 == 0; } } IsEven;

and the output of the program would be the same. This last declaration defines an object called IsEven nameless structure that contains the definition of the function call operator.