Pass an unary predicate to a function in C++

arguments, c++, predicate, stl

Solution

Turn `SetDisplayPolicy` into a function template:

template<typename Pred>
void SetDisplayPolicy(Pred &pred)
{
   // Depending on what you want exactly, you may want to set a pointer to pred,
   // or copy it, etc.  You may need to templetize the appropriate field for
   // this.
}

Then to use, do:

struct MyPredClass
{
   bool operator()(myType a) { /* your code here */ }
};

SetDisplayPolicy(MyPredClass());

In the display code you would then d someting like:

if(myPred(/* whatever */)
   Display();

Of course, your functor may need to have a state and you may want its constructor to do stuff, etc. The point is that `SetDisplayPolicy` doesn't care what you give it (including a function pointer), provided that you can stick a function call onto it and get back a `bool`.

Edit: And, as csj said, you could inherit from STL's `unary_function` which does the same thing and will also buy you the two `typedef`s `argument_type` and `result_type`.

Problem

I need a function which establishes a policy for my class for displaying items. e.g: ``` SetDisplayPolicy(BOOLEAN_PRED_T f) ``` This is assuming BOOLEAN_PRED_T is a function-pointer to some boolean predicate type like: ``` typedef bool (*BOOLEAN_PRED_T) (int); ``` I'm interested only on e.g: display something when the passed predicate is TRUE, do not display when it's false. The above example works for functions returning bool and taking an int, but I need a very generic pointer for the SetDisplayPolicy argument, so I thought of UnaryPredicate, but it's boost related. How I can pass a unary predicate to a function in STL/C++? `unary_function< bool,T >` won't work because I need a bool as return value, but I want to ask the user just for "unary function that returns bool", in the most generic approach. I thought of deriving my own type as: ``` template<typename T> class MyOwnPredicate : public std::unary_function<bool, T>{}; ``` Could that be a good approach?

Original source