Determine object and method in a functor using boost::function and boost::bind

Question

I'd like to obtain the pointer to the object and an indication of which method the functor will call from a functor constructed using boost::function and boost::bind. This will allow me to automatically determine the order in a which bunch of functors must be executed.

The following (pseudo) code (see POINTER_OF & METHOD_OF) shows what I'm trying to do:

class myClassA
{
  public:
    DoIt(int i) { return i+i; }
};

class myClassB
{
  public:
    DoItToo(double d) { return d*d; }
};

typedef boost::function0<int> Functor;

myClassA classA;
myClassB classB;

Functor funcA = boost::bind( &myClassA::DoIt, &classA, 10 );
Functor funcB = boost::bind( &myClassB::DoItToo, &classB, 12.34 );

// Create a vector containing some functors and try to determine the objects
// they are called upon and the methods they invoke
std::vector<Functor> vec;
vec.push_back( funcA );
vec.push_back( funcB );

for (int i = 0; i < vec.size();i++)
{
  if (POINTER_OF(vec[i]) == &classA)
  {
    // This functor acts on classA
    if (METHOD_OF(vec[i]) == &myClassA::DoIt)
    {
      // This functor calls the 'DoIt' method.
    }
    else if (METHOD_OF(vec[i]) == &myClassB::DoItToo)
    {
      // This functor calls the 'DoItToo' method.
    }
  }
  // etc...
}

Thanks in advance!

code what you wnat write looks ugly. sorry. please descirbe addtional information, what you wnat to do with information about pointer and method? — bayda, Mar 23 '09 at 15:43
some style advice, rename "classA" -- it's not a class, it's an object that is an instance of a particular class. "instanceA" would be fine; "instanceOfClassA" is particularly verbose, but would at least be accurate. — Aaron, Mar 23 '09 at 17:13

score 4 · Answer 1 · answered Mar 23 '09 at 15:12

4

I know that the following is not a strict answer to your question but.

Don't do this. Use polymorphism instead. It is one of the strangest things I saw in my current project code: if function pointer points to "someFunction" - do some extra acctions.

You can add extra behavior without changing your classes much with Decorator design pattern. That will extend your myClassA::DoIt with Decorator::DoIt.

http://en.wikipedia.org/wiki/Decorator_pattern

answered Mar 23 '09 at 15:12

Mykola Golubyev

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Maybe my example isn't clear ;-) I only need to determine the object and method called on it for a large list of functors to determine the execution order of the methods as they all have side-effects and depend on eachother (used in machine control - objects represent physical systems) – Mar 23 '09 at 15:25
@lasser - that doesn't change the good sense of Mykola's answer. You need a list of objects which have a priority and a function. – Daniel Earwicker Mar 23 '09 at 15:35

score 0 · Answer 2 · edited May 23 '17 at 11:48

0

No, I don't think you can get the target of a boost function that is set to a boost bind (ie METHOD_OF). According to this post on the boost mailing list this is because the return type of bind is not formally specified.

Edit: Link to an earlier question that is some-what related to this question: demote-boostfunction-to-a-plain-function-pointer

edited May 23 '17 at 11:48

Community

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answered Mar 23 '09 at 19:29

Mic

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score 0 · Answer 3 · answered Mar 24 '09 at 14:01

0

boost::function objects are equality comparable, so you should be able to do stuff like

if (vec[i] == boost::bind( &myClassA::DoIt, &classA, 10 ))
{
   // ... this Functor calls DoIt on classA with argument 10
}

I suspect you're looking for something more general though. Digging into the implementation details of boost/function_equal.hpp (ie boost::bind's function_equal_impl) might give you some idea how to selectively test a subset of the boost::bind arguments.

But I really think youd be better off with a polymorphism-based solution, or just aggregating the function objects with some metadata.

answered Mar 24 '09 at 14:01

timday

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Thanks for your comment, timday. If I understand right, when I use equal_impl I will still face the problem of getting the object and method out of boost::bind... Comparing boost::bind instances also won't work as they are different when different parameters are passed. – Mar 26 '09 at 08:04
The equality test implementation itself must have access to the object and method (to compare them), which is why I suggest looking at how those are implemented. – timday Mar 26 '09 at 08:18

Determine object and method in a functor using boost::function and boost::bind

3 Answers3