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I can't easily describe this question so forgive me.

For context it has to do with GFX: GFX Forever: The Complete Guide to GFX for IoT[^]

I'm trying to reduce the code size for drawing operations.
Currently every *type* of draw target requires the generation of all new drawing code, so if you draw a line to a bitmap and a line to an lcd, there are two copies of much of the line drawing code. The same goes for drawing to large_bitmap objects instead of bitmap objects.

I'd like to be able to share some of this code, but what I need is a virtual base class that itself is probably a template (i know this creates issues but I may have to deal with it)

Essentially, if my draw targets inherit from a particular interface I want it to select the template instantiation for that particular virtual *base class* not the derived one.

For example:

C++
class Interface {
    virtual void foo() const = 0;
};

class Bar : public Interface {
    virtual void foo() const { std::cout << "Bar" << std::endl; }
};
class Baz : public Interface {
    virtual void foo() const { std::cout << "Baz" << std::endl; }
};

template<typename Target> static void do_foo(const Target* target) {
      target->foo();
}


and here

C++
int main() {
    Bar bar;
    Baz baz;
    do_foo(&bar); // prints "Bar"
    do_foo(&baz); // prints "Baz"
    do_foo<Interface>(&bar); // prints "Bar"
    do_foo<Interface>(&baz); // prints "Baz"
}


Here a total of 3 copies of do_foo<>() are instantiated, if I'm correct. There will be one for do_foo<Foo>(...), one for do_foo<Bar>(...) and one for do_foo<Interface>(...)

Here the only one that is reused is the instantiation using Interface, but I'd like *all* of them to use Interface, without explicitly specifying it.

I'm trying to think of way to do that, by hook or crook. I don't care if the code to make it work looks nasty as long as using it doesn't, if that makes sense.

What I have tried:

I haven't tried anything yet.

My first thought is to try to make non-template versions of the template methods to overload them with, but unless I miss my guess the compiler will prefer the most specific overload, which would be the templated one, and that's not what I want.

My other thought is to make non-template versions that take a pointer, while the template versions take a reference. It's kind of ... I don't like it for a number of reasons.

The other option is, I have a draw class that sources all of the template functions i want to recycle like this. I could make a draw_shared:: class that uses the virtual class. It's less than ideal as well.

So this is call for ideas.
Posted
Updated 18-Jun-21 16:50pm
v2

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