OpenCV - Ptr syntax and class definition / declaration - confusion?

Question

I'm attempting to understand the OpenCV implementation of the Generalized Hough Transform based on Ballard 1981 . I'm reading through generalized_hough.cpp . At this line is where the confusion is beginning:

namespace
{
    class GeneralizedHoughBallardImpl : public GeneralizedHoughBallard, private GeneralizedHoughBase
    {

Clearly this is defining a namespace, then a class named GeneralizedHoughBallardImpl, which I gather is short for Generalized Hough Ballard Implementation. The ":" character clearly means "inherits from" as usual, and the "private GeneralizedHoughBase" is clear since that class is declared and its functions are defined earlier on in the same file:

class GeneralizedHoughBase
    {
    protected:
        GeneralizedHoughBase();
        virtual ~GeneralizedHoughBase() {}

        void setTemplateImpl(InputArray templ, Point templCenter);
        void setTemplateImpl(InputArray edges, InputArray dx, InputArray dy, Point templCenter);
(much more code for this class omitted) . . .

Now where I'm lost, I can find where "GeneralizedHoughBallard" is declared or defined. In the same file (generalized_hough.cpp) there is this code:

Ptr<GeneralizedHoughBallard> cv::createGeneralizedHoughBallard()
{
    return makePtr<GeneralizedHoughBallardImpl>();
}

If I'm understanding this correctly, this is specifying that "Ptr GeneralizedHoughBallard" is the return type of function createGeneralizedHoughBallard(), but is not in any way defining class GeneralizedHoughBallard. Moreover, if I search the entire OpenCV repo for "GeneralizedHoughBallard" , I'm finding 11 instances of the text GeneralizedHoughBallard in 7 different files, but they all seem to be uses similar to the above, rather than declarations or definitions.

What am I missing here? Is there something special about the Ptr syntax in OpenCV that a class does not have to be defined/declared? Is GeneralizedHoughBallard defined/declared somewhere that I'm not finding or not noticing? If anybody could provide some clarification it would be greatly appreciated.

Answer 1

So, to review what you have listed above, there is the GeneralizedHoughBallardImpl class, which is defined within an anonymous class*.

Then, in the same file, there is the createGeneralizedHoughBallard function, defined in the cv namespace. This function creates an instance of GeneralizedHoughBallardImpl using the templated makePtr function, then C++ type-casts the pointer instance to GeneralizedHoughBallard, since GeneralizedHoughBallardImpl derives from GeneralizedHoughBallard.

The createGeneralizedHoughBallard function is declared as CV_EXPORTS Ptr createGeneralizedHoughBallard() in 2 different header files (at the moment). So, since createGeneralizedHoughBallard is declared in a header file that defines the GeneralizedHoughBallard class, the function will compile without issue.

Note, the GeneralizedHoughBallard class is defined in https://github.com/opencv/opencv/blob/e93aa158cf39b88836797a6e74b06c7dbe8199b7/modules/imgproc/include/opencv2/imgproc.hpp

*Specifying items within an anonymous class is a good way to make sure the items declared within the file do not clash with potentially same named items in other files.