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referencing a member function with bind1st and mem_fun

Answer 1

A:

I believe it's not working because bind1st() requires a binary function object and returns a unary function object. Since Bar() takes no arguments, mem_fun() is giving you a unary function object. bind1st() doesn't know what to do with that.

How do you actually intend to use the function object produced by mem_fun()?

Fred Larson 2010-08-03 19:08:28

I think the question is, how do you take a unary function object, and by binding it to an argument produce a nullary function object. I.e a version of bind1st but for functions objects of one less arity. In this case the nullary function object when called would call `this->Bar ()` for the value of `this` used at the time of binding.

jon hanson 2010-08-03 19:23:11

Answer 2

A:

What are you trying to accomplish with bind1st?

The bind1st function takes a binary function and adapts it to a unary function by making the first argument implicit (this may not be the best description, sorry). The return value of mem_fun is a unary function.

The mem_fun function returns an adaptor for a member function. The adapted function doesn't take any arguments, though the adaptor returned takes one argument, a pointer to the MyClass object to be used.

So, basically, your mem_fun call is returning a adaptor that takes one argument, but bind1st is expecting an adaptor that takes two arguments. There is a mem_fun1 which returns an adaptor that takes two arguments, the first being the object pointer and the second being an argument for the function, but that isn't what you want here.

In fact, I don't quite understand what you're trying to do; why is the plain mem_fun version not suitable? If you need the object pointer to be 'attached' to the adaptor, I don't think you can do that with the current standard library, unless you use bind.

Of course, you could make a wrapper class that holds the object pointer, and then just define operator() to call the adaptor with that object.

// Quick and dirty example of this.
// You could extend this with a second template parameter for return type, if
// needed. Just be sure to specialize for void if you do that.
template<typename Object>
struct bound_mem_fun_t { 
    bound_mem_fun_t(mem_fun_t<void, Object> f, Object* o) : fun(f), obj(o) { } 
    void operator()() { fun(obj); } 
    mem_fun_t<void, Object> fun; 
    Object* obj; 
};

MyClass a;
bound_mem_fun_t<MyClass> func(mem_fun(&MyClass::Bar), &a);
func();

dauphic 2010-08-03 19:15:10

Answer 3

A:

bind1st requires a binary function, and returns an unary functor. Here, you pass an unary function.

The purpose of "binding" is to use a binary function/functor (like a non-static method with one parameter is) like an unary one, by setting one of its parameters to a given value.

This would work (note the new prototype of Bar):

class MyClass
{
public:
   void Foo()
   {
       using namespace std;

       // this works fine
       bind1st( mem_fun( &MyClass::Bar ), this )(42);
   };

   void Bar(int i)
   {
   };
};

Samuel_xL 2010-08-03 19:16:41

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