[frobaux]

Hi everyone,
I am currently trying to understand the code (precisely the overset library) and follow it up to its roots.



I'm stuck on a runTimeSelection that, even if I read this page, I do not completely get.
The code in cellCellStencil.C (function ::New)

auto cstrIter = meshConstructorTablePtr_->cfind(stencilType);

where stencilType='cellVolumeWeight'

I didnt completly understand what meshConstructor table is but i think this there is a link somewhere to the derived class of cellCellStencil.C.



Then

return autoPtr<cellCellStencil>(cstrIter()(mesh, dict, true))

And this is where I really don't get it. From what I can guess, it returns an autoPtr, pointing to an instance of 'cellVolumeWeight', am I right?


What constructor of this class does he use? Is there a way to print stuffs.


When I do

Info << cstrIter()

I get "1" ...? (->type() of .type() are does not compile)


Thanks for your clues! =)


Have a good day

[raumpolizei]

The function New is basically a base class factory function returning a pointer to an object of the base class, therefore enabling dynamic polymorphism at runtime (so that you can use different type of "baseclass" at runtime without recompiling the code). Most of it is hidden to the user and managed via macros. In OpenFOAM in a class enabling runtime polymorphism, you will find macros as declareRunTimeSelectionTable and defineRunTimeSelectionTable. If you take a look at the definitions of these macros, for instance in OF1812 https://www.openfoam.com/documentati...ce.html#l00273 you will find out that the meshConstructorTable you are referring to is of type HashTable:

     typedef HashTable<argNames##ConstructorPtr, word, string::hash>            \
          argNames##ConstructorTable; // here argNames is "mesh"

The line calling find is returning a function pointer pointing to the constructor in the table refering to the specific input (or key, here stencilType). The constructors are added to the constructorTable at compile time via other macros, like addToRunTimeSelectionTable (these are the classes that are runtime selectable). The Typename("...") line that you are seeing in almost every class definition is basically defining the key to access the constructor of that specific class in the constructorTable of the base class. Anyway, if there is no such key in the hashMap an error is thrown. Otherwise, the constructor gets called and the resulting instantiated object is passed to the autoPtr which is being returned from that function. This is what

return autoPtr<cellCellStencil>(cstrIter()(mesh, dict, true)); //calls constructor, pass mesh dict and true as arguments.

does. I highly recommend you the article series by Tomislav Maric on Sourceflux dedicated to the mechanism of runtime selection (http://sourceflux.de/blog/series/rts-2). It helped me a lot understanding the implementation in OF. As prerequisite, I would read a little bit about runtime or dynamic polymorphism in order to better grasp the concepts.

[frobaux]

Hello,



Thanks for your quick response, it seems more clear now.



If I want to print something to understand, does cstrIter() have an attribute that I can send to Info to help me?



I going to read that article!

[raumpolizei]

Well, what you could do is:

Info << "Selecting cellCellStencil Type " << stencilType << nl;

after stencilType has been initialized. You could also modify the constructor being called (here the cellVolumeWeight constructor). These are just quick modifications to better understand what the code is doing. I would not start to do that everywhere in the code as at some point, you may break it. A better way would be to create a separate library and to create a new class myCellVolumeWeight and do some testing with that.

Good luck!

[frobaux]

Ok,
For the new library that's exactly what I've done, I have created my own library copied from the original to do that kind of small tests.

For the Info on stencilType I have already done this. I also test the type of AutoPtr created.



Ps. The ressource you linked me is really insteresting!
Thanks again!