Polymorphic functors in std::for_each

Question

I'm trying to use stl algorithm for_each without proliferating templates throughout my code. std::for_each wants to instantiate MyFunctor class by value, but it can't since its abstract. I've created a functor adapter class which passes a pointer around and then derefernces it when appropriate.

My Question:

Does the STL or Boost already have such an adapter class? I don't want to have to reinvent the wheel!

 struct MyFunctor  {
     virtual ~MyFunctor() {}
     virtual void operator()(int a) = 0; 
 }

 namespace {
     template<typename FunctorType, typename OperandType> struct
 FunctorAdapter
     {
         FunctorAdapter(FunctorType* functor) : mFunctor(functor) {}
         void operator()(OperandType& subject)
         {
             (*mFunctor)(subject);
         }

         FunctorType* mFunctor;
     }; }

 void applyToAll(MyFunctor &f) {
     FunctorHelper<MyFunctor, int> tmp(&f);
     std::for_each(myvector.begin(), myvector.end(), tmp); }

Cheers,

Dave

Answer 1

tr1::ref may help you here --- it's meant to be a reference wrapper so that you can pass normal objects by reference to bind or function objects (even abstract ones) by reference to standard algorithms.

// requires TR1 support from your compiler / standard library implementation
#include <functional>

void applyToAll(MyFunctor &f) {
    std::for_each(
        myvector.begin(), 
        myvector.end(), 
        std::tr1::ref(f) 
    ); 
}

However, NOTE that compilers without decltype support MAY reject passing a reference to an abstract type... so this code may not compile until you get C++0x support.

Answer 2

You could use the function adapters (and their shims) from functional.

#include <functional>

using namespace std;
for_each( vec.begin(), vec.end(), :mem_fun_ptr( &MyClass::f ) );

If your container contains pointers-to-objects, use mem_fun_ptr, else use mem_fun. Next to these, there are wrappers for member functions that take 1 argument: mem_fun1_ptr and mem_fun1.

@Evan: indeed, you could call the member function with the same argument for each object. The first argument of the mem_fun1 wrappers is the this pointer, the second is the member function argument:

for_each( vec.begin(), vec.end(), bind2nd( mem_fun_ptr( &MyClass::f ), 1 ) );

With more arguments, it becomes more readable to create a loop yourself, or create a custom functor that has const member variables representing the arguments.

Answer 3

why not use BOOST_FOREACH?

http://www.boost.org/doc/libs/1_35_0/doc/html/foreach.html

Answer 4

Sounds like you could benefit from Boost::Function.

If I remember correctly it's a header only library too, so it's easy to get it going with it.

Answer 5

What about forgetting all the wrapping of the functor pointer, and instead use bind(functor_pointer,mem_fun1(&MyFunctor::operator()); as the functor? that way, you don't have to worry about managing the copy in any way shape or form.

Answer 6

Building on @xtofl's answer, since the array contains int's and not "this" pointers, I think the correct incantation is

class MyClass
{
  virtual void process(int number) = 0;
};
MyClass *instance = ...;

for_each( vec.begin(), vec.end(), binder1st(instance, mem_fun_ptr(&MyClass::process) );

The only difference versus @xtofl's code is binder1st rather than binder2nd. binder2nd allows you to pass teh same number to various "this" pointers. binder1st allows you to pass various numbers to one "this" pointer.