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std::function, and callbacks, and functors, oh my!


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#1 Servant of the Lord   Crossbones+   -  Reputation: 19564

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Posted 18 September 2012 - 09:23 PM

I've used callback functions alot before, and I understand what a functor is, but I've never created a function that takes a functor as a parameter.

If I go like this:
typedef bool (*CallbackFunc)(int); //Typedefining a function pointer returning a bool and taking one int parameter (right?).
void DoSomething(CallbackFunc callback);

That won't accept functors. So what is the best way to accept functors OR functions?
Looking at how the standard library does it, I see they use templates, which makes sense.
template <class funcType>
void DoSomething(funcType callback);

That, because it is a templated parameter, accepts both functors and functions.
Nice... but is there a better way? What if I have a function that takes more than one callback:
//Two callbacks:
template <class funcTypeA, class funcTypeB>
void DoSomething(funcTypeA callbackA, funcTypeB callbackB);

//Three callbacks:
template <class funcTypeA, class funcTypeB, class funcTypeC>
void DoSomething(funcTypeA callbackA, funcTypeB callbackB, funcTypeC callbackC);

I'm not really liking that too much. I just read up on std::function() (C++11) and am trying to understand it.
Can it really accept functions or functors (or lambdas)?

How does this work in practice?
typedef std::function<bool(int)> CallbackFunc;

//template <class funcType>... Template not needed?
void DoSomething(CallbackFunc callbackA, CallbackFunc callbackB, CallbackFunc callbackC);

Is that the proper usage? Am I understanding it correctly?
And I can pass one function to say, the first parameter, and two different classes as functors to the second and third parameter?
class FunctorA;
FunctorA functorA;

class DifferentFunctorB;
DifferentFunctorB differentFunctorB;

bool myFunction(int value);

DoSomething(myFunction, functorA, differentFunctorB);

This is valid? Without DoSomething() itself being templated? With only one typedef of CallbackFunc taking just the return type and the parameters?
It's perfectly fine to abbreviate my username to 'Servant' rather than copy+pasting it all the time.
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#2 Krohm   Crossbones+   -  Reputation: 3119

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Posted 19 September 2012 - 01:00 AM

  • Can it really accept functions or functors (or lambdas)?
  • How does this work in practice? Is Template not needed?
  • And I can pass one function to say, the first parameter, and two different classes as functors to the second and third parameter?

  • The compiler emits some code mapping lambda to std::function. How it happens, it's unclear to me as lambda is some really odd type. It will map correctly even when using capture lists (which isn't completely unexpected after all). It's important to note those are in fact unrelated types and not just "functions", there will be casts. I'm not quite sure how it's going to work with plain functions, but I suppose it's going to work (see void handler(EventArgs const&) in the first code box or void HandleMouseClick, to the end of the page).
  • I see no problem. No, there's no need to use template.
  • I'm not sure myself but I think as long as operator() is correctly overloaded, it will work.

Edited by Krohm, 19 September 2012 - 01:02 AM.


#3 BitMaster   Crossbones+   -  Reputation: 4088

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Posted 19 September 2012 - 01:08 AM

std::function (or boost::function if C++11 is not available) would indeed be the way to go. Templates are the preferred way, if they are not possible use std/boost::function. The alternative would be handrolling something by yourself. However, unless you have some interesting domain specific restrictions you would be just implementing std/boost::function.

For the record, I have not looked at the code of std/boost::function but I would guess the implementation is as expected: the function object is a facade which contains a pointer to a base class with a pure virtual function to call. The concrete instance created is decided based on the template parameter passed to the std/boost::function constructor. Template magic and/or SFINAE might be required to differentiate all important types. The variable number of arguments might need a lot of black magic depending on your compiler as well. More black magic might be used to avoid a call to new.




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