静态多态问题

Question

I am on a micro controller (which means I can only have static memory allocation) and I am trying to work with inheritance.....

Suppose I have a abstract class Image and an abstract class Font . An instance of Font can return an Image based off of the char given in a function like so:

Font* mf;
Image* image = mf->GetImage("a");

Now the real issue is I have no idea what to do for the GetImage Function.

The problem is this: in c++ for you to have a member of an abstract class you have to use it as a pointer. So my dilemma is that I have a Font which wants to create a new Image and then return it.

If it returns a pointer to its newly created Image you are returning a reference to a temporary object:

Image* FontImpl::GetImage(char c){
  return &ImageImpl(c);  //This object is destroyed once this function exits
}

And then if I return I try to return an actual type like this:

Image FontImpl::GetImage(char c){
  return ImageImpl(c);   //Cannot cast from ImageImpl to Image
}

So is there an idiom or something for this kind of static memory problem?

Answer 1

Using dynamic allocation would be easiest but if that's not possible, you have to store the instance somewhere outside the function, eg:

static ImageImpl image;

Image& FontImpl::GetImage(char c) {
  image = ImageImpl(c);
  return image;
}