Question

I declare a class in matlab and here is the constructor and a function.

function Ri = RGBimages(datadir)
    if(isempty(dir(datadir)))
        warning([datadir, ' is empty']);
        return;
    end
    Ri.dataSrc=datadir;
    Ri.twoEnds = load([datadir,'\seIndex.txt']);
    Ri.startFrame = Ri.twoEnds(1);
    Ri.endFrame = Ri.twoEnds(2);
    Ri.numberOfFrames=Ri.twoEnds(2)-Ri.twoEnds(1)+1;
    Ri.faceLmks = zeros(68,2,Ri.numberOfFrames);
end

function obtainFaceLmks(Ri)
    indx=1;
    for i = Ri.startFrame;%:Ri.endFrame
        imstr = [Ri.dataSrc,'\rgb_',num2str(i),'.png'];
        [status,result] = system(['fitD -m src\my.amf -h src\haarcascade_frontalface_alt2.xml -i ',imstr,' -n 30']);
        if(status==-1)
            warning(result);
        else
            temp=flHelper(result(1:size(result,2)-59));
            Ri.faceLmks(:,:,indx)=temp;
        end
        indx=indx+1;
    end
    return;
end

then I do this:

ims = RGBimages('data\a01_s01_e01');

ims.obtainFaceLmks();

and it seems ims’s attributes (ims.faceLmks) didn’t change, why?

Thanks for anyhelp

Answer 1

Value classes are passed by value. Consequently, a method should return the (updated) class instance, and you need to capture that. In other words, the method definition should be

function Ri = obtainFaceLmks(Ri)

and you’d call the method

ims = ims.obtainFaceLmks();

Handle classes are passed by reference. Consequently, a method doesn’t need to return the updated class instance, and you don’t need to capture it. However, you need to inherit from handle, and you need to implement a copy method to make a copy of a class instance; assigning to another variable won’t work.

See the documentation for further info.