[Insight-developers] Best method for a do-nothing return-value function

[M Stauffer (V)]

So if I create an abstract functor base class that has a pure virtual
function call operator, it seems a derived class can have an inlined
overloaded function call operator and the compiler will still be able to
inline it since it knows the base class is abstract?

Or do we eschew the idea of a functor base class altogether?

Cheers,
Michael 

>-----Original Message-----
>From: Luis Ibanez [mailto:luis.ibanez at kitware.com] 
>Sent: Wednesday, December 22, 2010 4:02 PM
>To: M Stauffer (V)
>Cc: ITK-dev-list
>Subject: Re: [Insight-developers] Best method for a do-nothing 
>return-value function
>
>Hi Michael,
>
>Yeap,
>essentially the two typical options are
>
>
>A) Functors:
>
>     Class X is templated over the Functor type
>     and everything gets resolved at compilation time.
>     The code here gets inlined in the instantiation of
>     Class X.
>
>or
>
>B)  Polymorphism and virtual methods.
>
>      Class X knows about the base class of the
>      Function classes, holds a pointer to one of such
>      instances and call a virtual method of the Function class.
>      Then you create a family of function classes and
>      implement different behaviors in their virtual methods.
>
>      It is flexible at run time,
>      but don't get the benefit of inlining.
>
>
>--
>
>Something that can help in to choose between (A) and (B) is to 
>profile the time that one of the typical virtual methods will 
>take to execute at run time.  It might be that the overhead of 
>the virtual call is not significant when compared to the time 
>that it takes to execute the body of the method.
>
>
>     Luis
>
>
>-----------------------------------------------------------------
>On Wed, Dec 22, 2010 at 1:20 PM, M Stauffer (V) 
><mstauff at verizon.net> wrote:
>> Hi Luis,
>>
>> After looking into this more, it seems that to be able to inline a 
>> functor's function operator, I would have to template a containing 
>> class over the functor class.
>>
>> Otherwise, if I were to not template and simply require the user to 
>> assign a derived functor to a containing class' member variable of 
>> type MyFunctorBaseClass, then it seems the compiler wouldn't know 
>> ahead of time the particular functor implementation that's 
>being used, 
>> and likely couldn't inline. This then seems like the same issue with 
>> inlining a virtual method in a regular class.
>>
>> Does this sound right?
>>
>> Cheers,
>> Michael
>>
>> ________________________________
>> From: Luis Ibanez [mailto:luis.ibanez at kitware.com]
>> Sent: Saturday, December 18, 2010 6:22 PM
>> To: M Stauffer (V)
>> Cc: ITK-dev-list
>> Subject: Re: [Insight-developers] Best method for a do-nothing 
>> return-value function
>>
>> Hi Michael,
>>
>> This call looks reasonable, note however that if you declare the 
>> function to be "virtual" as you probably need to do in this case in 
>> order to achieve polymorphism, then the call to this 
>function will not 
>> be inlined.
>>
>> The typical option here is to use Functors, if you want to get both 
>> the flexibility of polymorphism, and the efficiency of inlining.
>>
>>
>>        Luis
>>
>>
>> On Fri, Dec 17, 2010 at 6:09 PM, M Stauffer (V) 
><mstauff at verizon.net> wrote:
>>>
>>> Hi,
>>>
>>> I'm not sure how to phrase the subject. I'm creating a 
>function base 
>>> class whose "Evaluate", i.e. do-its-thing, method needs to simply 
>>> return the input value. Derived classes will modify the 
>value in some 
>>> way, not surprisingly.
>>>
>>> Is there a more efficient way to implement this than what I 
>have below?
>>> I figure most compilers will actually make this inline, but 
>maybe not 
>>> all?
>>>
>>> < typename TValue >
>>> class FunctionBase {
>>>
>>> ...
>>>   inline TValue Evaluate( TValue value ) { return value; } ...
>>>
>>> };
>>>
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>>