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+<title>Cairo: A Vector Graphics Library: Multiple return values</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, &amp;x, &amp;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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