Thread: Pure virtual function error

You haven't declared BITMAP, and that confuses the compiler (it seems - I can't explain why it's not producing a more appropriate error message), but this compiles fine:

Code:

struct BITMAP;

class Drawable
{
	public:
	virtual void draw(BITMAP* b) =0;
};

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Mats

Why not?

Originally Posted by pandu

How would you use a forward declaration here? You can't just say "struct BITMAP" like he did, can you?

Yes, you can - you do need a semicolon on the end. As long as you don't need to know the content of the struct/class, it's fine to forward declare it by just saying "struct aname;" or "class somename;".

--
Mats

Originally Posted by pandu

What? Even without the braces? Never read of it in any tutorial. Lol, so many surprises in this language.

Yes, definitely without braces.

And it's a way to tell the compiler "there is a class[1] of this name" without actually defining what it contains. It has (at least) two potential uses:
1. You want to return a pointer or reference to a class, but you don't actually want to tell the code receiving this class what is in the class - a HANDLE in windows is a typical example where the application is given a pointer to something without actually knowing what the data it points at is.
2. You need one class to hold a reference or pointer to another, where the other class also holds a reference or pointer to the first class (e.g. a parent/child relationship).

[1] When I say class, it is, for the purposes of this discussion, interchangeable with struct.

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Mats