📄 bindings-return-values.html
字号:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head><meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"><title>Multiple return values</title><meta name="generator" content="DocBook XSL Stylesheets V1.68.1"><link rel="start" href="index.html" title="Cairo: A Vector Graphics Library"><link rel="up" href="language-bindings.html" title="Appendix A. Creating a language binding for cairo"><link rel="prev" href="bindings-memory.html" title="Memory Management"><link rel="next" href="bindings-overloading.html" title="Overloading and optional arguments"><meta name="generator" content="GTK-Doc V1.6 (XML mode)"><link rel="stylesheet" href="style.css" type="text/css"><link rel="part" href="pt01.html" title="Part I. Tutorial"><link rel="part" href="pt02.html" title="Part II. Reference"><link rel="chapter" href="Drawing.html" title="Drawing"><link rel="chapter" href="Fonts.html" title="Fonts"><link rel="chapter" href="Surfaces.html" title="Surfaces"><link rel="chapter" href="Support.html" title="Utilities"><link rel="index" href="ix01.html" title="Index"><link rel="appendix" href="language-bindings.html" title="Appendix A. Creating a language binding for cairo"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><table class="navigation" id="top" width="100%" summary="Navigation header" cellpadding="2" cellspacing="2"><tr valign="middle"><td><a accesskey="p" href="bindings-memory.html"><img src="left.png" width="24" height="24" border="0" alt="Prev"></a></td><td><a accesskey="u" href="language-bindings.html"><img src="up.png" width="24" height="24" border="0" alt="Up"></a></td><td><a accesskey="h" href="index.html"><img src="home.png" width="24" height="24" border="0" alt="Home"></a></td><th width="100%" align="center">Cairo: A Vector Graphics Library</th><td><a accesskey="n" href="bindings-overloading.html"><img src="right.png" width="24" height="24" border="0" alt="Next"></a></td></tr></table><div class="sect1" lang="en"><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">voidcairo_user_to_device (cairo_t *cr, double *x, double *y);voidcairo_user_to_device_distance (cairo_t *cr, double *dx, double *dy);voidcairo_device_to_user (cairo_t *cr, double *x, double *y);voidcairo_device_to_user_distance (cairo_t *cr, double *dx, double *dy);voidcairo_matrix_transform_distance (cairo_matrix_t *matrix, double *dx, double *dy);voidcairo_matrix_transform_point (cairo_matrix_t *matrix, double *x, double *y);voidcairo_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"><i><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></i></p><pre class="programlisting">voidcairo_stroke_extents (cairo_t *cr, double *x1, double *y1, double *x2, double *y2);voidcairo_fill_extents (cairo_t *cr, double *x1, double *y1, double *x2, double *y2); </pre></div></body></html>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -