[cairo] What is the closure in stream output to file based surfaces for?

[David Weiß]

Hey Uli,

> 
> Hi,
> 
> On 29.07.2014 11:03, David Weiß wrote:
> > Hey,
> >
> > I want to implement file output for a rendering application and want to use
> C++ file streams using the following callback write function:
> >
> >     cairo_status_t cairo_write_func (void *closure, const unsigned char
> > [...]
> > I just can't figure out what the closure is for.
> > Can anyone explain the concept to me?
> 
> You pass in a closure argument to cairo. Cairo doesn't do anything with it, it
> just passes it on to your callback function. So it is meant to be used to pass
> some state to the callback.
> 
> > Is it comparable to variable capturing in C++11 closures?
> 
> I guess so, yeah.

After Your explanation above, yes it seems like that's the C-ish way of capturing state, which is what closures in C++11 do (basically lambdas extended with state).

> > Could I just pass nullptr as I do not use it anyway?
> [...]
> 
> Sure, if that works for you, you can do so.
> 
> The "C approach" for your above code would be to use the closure argument
> to pass a pointer to fout to the callback function via cairo and use a
> reinterpret_cast to use it in there.

Thanx, as I'm using cairo in a C++ project, I'll stick to the object abstraction and hold/pass state using members :)

> 
> Cheers,
> Uli

Thank you!
David