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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, &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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