Syntax of the functor definition in c++?

Question

I have an example functor deceleration as;

#include <iostream>

class myFunctorClass
{
    public:
        myFunctorClass (int x) : _x( x ) {}
        int operator() (int y) { return _x + y; }
    private:
        int _x;
};

int main()
{
    myFunctorClass addFive( 5 );
    std::cout << addFive( 6 );

    return 0;
}

My "What" is about the line just after the public:. I do not understand the syntax of the line myFunctorClass (int x) : _x( x ) {}. What is doing and what another instance of the use of such syntax rule?

It's a constructor, get [a good book](http://tinyurl.com/so-cxxbooks). — Xeo, May 24 '13 at 13:39
And the functor is not slowing down, so "functor decelaration" is not a very meaningful term — Andy Prowl, May 24 '13 at 13:40
It initializes the private member _x to the value specified when calling the constructor. So _x is initialized to 5 or 6 in your case even before the constructor actually runs. — hetepeperfan, May 24 '13 at 13:41
possible duplicate of [What is this weird colon-member syntax in the constructor?](http://stackoverflow.com/questions/1711990/what-is-this-weird-colon-member-syntax-in-the-constructor) — hmjd, May 24 '13 at 13:46

score 1 · Accepted Answer · edited May 24 '13 at 14:01

1

myFunctorClass (int x) : _x( x ) {} is the constructor. It's a function that is executed whenever an object of the class is instantiated.

The : _x(x) part is an initialization list. That means it initializes a member with a value (in this case the value x is used to initialize _x), or intializes a parent class.

The code would in this case be functionally equal to

myFunctorClass (int x) {
    _x = x; 
}

Note this equivalence is not always true. There are slight differences between initialization and assignment and copy/move-construction.

edited May 24 '13 at 14:01

rubenvb

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answered May 24 '13 at 13:44

Appleshell

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is it just for constructor or might be used for other function definitions. – erogol May 24 '13 at 13:53
@Erogol the initializer list is only for constructors. It would make no sense anywhere else. – rubenvb May 24 '13 at 14:02
Intialization lists can only be used in constructors. As the name implies, it's just there to _initialize_ the members or parents for the first time. This is especially useful for `const` members, as they can not be re-assigned, or parent classes that take constructor parameters themselves. – Appleshell May 24 '13 at 14:03

score 0 · Answer 2 · answered May 24 '13 at 13:39

It's simply the definition of myFunctorClass's constructor. Perhaps it is just the : _x( x ) part that is confusing you. Would myFunctorClass (int x) {} make sense to you? The : _x( x ) part is a member initialization list and says that the member _x should be initialised with the value of the argument x.

score 0 · Answer 3 · answered May 24 '13 at 13:40

0

That's the class' constructor. It's initializing the data members (after the :). And the function body of the constructor us empty.

answered May 24 '13 at 13:40

Anon Mail

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score 0 · Answer 4 · answered May 24 '13 at 13:40

0

You're using the class constructor member initializer list to initialize your member _x to x that was passed as 5 to the constructor in myFunctorClass addFive( 5 );

answered May 24 '13 at 13:40

Johann Gerell

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Syntax of the functor definition in c++?

4 Answers4