[cairo] const usage in API

[Carl Worth]

On Fri, 09 Nov 2007 23:59:16 +0200, Hakki Dogusan wrote:
> Some functions use const for parameters, but some not. Is there a rule?

Yes.

> cairo_public void
> cairo_scaled_font_extents (cairo_scaled_font_t  *scaled_font,
>       	cairo_font_extents_t *extents);
>
> cairo_public void
> cairo_set_matrix (cairo_t *cr,
> 	const cairo_matrix_t *matrix);

In the case of cairo_scaled_font_extents, that function is *writing*
to the extents structure you pass in, (definitely not const).

In the case of cairo_set_matrix, that function is only *reading* from
the matrix you pass in---the const guarantees that it will not be
modified.

For things like the cairo_scaled_font_t* and cairo_t*, those are
opaque data types. The user of the API can't look inside them so can't
care if the structures will be modified or not.

-Carl
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