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<title>Multiple return values: Cairo: A Vector Graphics Library</title>
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<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="bindings-return-values"></a>Multiple return values</h2></div></div></div>
<p>
There are a number of functions in the cairo API that have
multiple <em class="firstterm">out parameters</em> or
<em class="firstterm">in-out parameters</em>. In some languages
these can be translated into multiple return values. In Python,
what is:
</p>
<pre class="programlisting">
cairo_user_to_device (cr, &x, &y);</pre>
<p>
can by mapped to:
</p>
<pre class="programlisting">
(x, y) = cr.user_to_device (cr, x, y);</pre>
<p>
but many languages don't have provisions for multiple return
values, so it is necessary to introduce auxiliary types.
Most of the functions that require the auxiliary types
require a type that would, in C, look like
</p>
<pre class="programlisting">
typedef struct _cairo_point cairo_point_t;
struct _cairo_point {
double x;
double y;
}</pre>
<p>
The same type should be used both for functions that use a pair
of coordinates as an absolute position, and functions that use
a pair of coordinates as a displacement. While an argument could
be made that having a separate “distance” type is more correct,
it is more likely just to confuse users.
</p>
<pre class="programlisting">
void
cairo_user_to_device (cairo_t *cr, double *x, double *y);
void
cairo_user_to_device_distance (cairo_t *cr, double *dx, double *dy);
void
cairo_device_to_user (cairo_t *cr, double *x, double *y);
void
cairo_device_to_user_distance (cairo_t *cr, double *dx, double *dy);
void
cairo_matrix_transform_distance (cairo_matrix_t *matrix, double *dx, double *dy);
void
cairo_matrix_transform_point (cairo_matrix_t *matrix, double *x, double *y);
void
cairo_get_current_point (cairo_t *cr, double *x, double *y);
</pre>
<p>
There are also a couple of functions that return four values
representing a rectangle. These should be mapped to a
“rectangle” type that looks like:
</p>
<pre class="programlisting">
typedef struct _cairo_rectangle cairo_rectangle_t;
struct _cairo_rectangle {
double x;
double y;
double width;
double height;
}</pre>
<p>
The C function returns the rectangle as a set of two points to
facilitate rounding to integral extents, but this isn't worth
adding a “box” type to go along with the more obvious
“rectangle” representation.
</p>
<p class="remark"><em><span class="remark">
Q: Would it make sense here to define a standard
<code class="function">cairo_rectangle_round()</code> method
that language bindings should map?
</span></em></p>
<pre class="programlisting">
void
cairo_stroke_extents (cairo_t *cr,
double *x1, double *y1,
double *x2, double *y2);
void
cairo_fill_extents (cairo_t *cr,
double *x1, double *y1,
double *x2, double *y2);
</pre>
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