type_conversion.sgml

GLib是GTK+和GNOME工程的基础底层核心程序库

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Type Conversion Macros

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portably storing integers in pointer variables.

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<para>
Many times GLib, GTK+, and other libraries allow you to pass "user
data" to a callback, in the form of a void pointer. From time to time
you want to pass an integer instead of a pointer. You could allocate
an integer, with something like:
<informalexample><programlisting>
 int *ip = g_new (int, 1);
 *ip = 42;
</programlisting></informalexample>
But this is inconvenient, and it's annoying to have to free the 
memory at some later time.
</para>
<para>
Pointers are always at least 32 bits in size (on all platforms GLib
intends to support). Thus you can store at least 32-bit integer values
in a pointer value. Naively, you might try this, but it's incorrect:
<informalexample><programlisting>
 gpointer p;
 int i;
 p = (void*) 42;
 i = (int) p;
</programlisting></informalexample>
Again, that example was <emphasis>not</emphasis> correct, don't copy it. 
The problem is that on some systems you need to do this:
<informalexample><programlisting>
 gpointer p;
 int i;
 p = (void*) (long) 42;
 i = (int) (long) p;
</programlisting></informalexample>
So GPOINTER_TO_INT(), GINT_TO_POINTER(), etc. do the right thing
on the current platform.
</para>
<para>
<warning>
<para>
YOU MAY NOT STORE POINTERS IN INTEGERS. THIS IS NOT PORTABLE IN ANY
WAY SHAPE OR FORM. These macros <emphasis>ONLY</emphasis> allow
storing integers in pointers, and only preserve 32 bits of the
integer; values outside the range of a 32-bit integer will be mangled.
</para>
</warning>
</para>

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<para>

</para>

<!-- ##### MACRO GINT_TO_POINTER ##### -->
<para>
Stuffs an integer into a pointer type.
</para>
<para>
Remember, YOU MAY NOT STORE POINTERS IN INTEGERS. THIS IS NOT PORTABLE
IN ANY WAY SHAPE OR FORM. These macros <emphasis>ONLY</emphasis> allow
storing integers in pointers, and only preserve 32 bits of the
integer; values outside the range of a 32-bit integer will be mangled.
</para>

@i: integer to stuff into a pointer.


<!-- ##### MACRO GPOINTER_TO_INT ##### -->
<para>
Extracts an integer from a pointer. The integer must have
been stored in the pointer with GINT_TO_POINTER().
</para>
<para>
Remember, YOU MAY NOT STORE POINTERS IN INTEGERS. THIS IS NOT PORTABLE
IN ANY WAY SHAPE OR FORM. These macros <emphasis>ONLY</emphasis> allow
storing integers in pointers, and only preserve 32 bits of the
integer; values outside the range of a 32-bit integer will be mangled.
</para>

@p: pointer containing an integer.


<!-- ##### MACRO GUINT_TO_POINTER ##### -->
<para>
Stuffs an unsigned integer into a pointer type.
</para>

@u: unsigned integer to stuff into the pointer.


<!-- ##### MACRO GPOINTER_TO_UINT ##### -->
<para>
Extracts an unsigned integer from a pointer. The integer must have
been stored in the pointer with GUINT_TO_POINTER().
</para>

@p: pointer to extract an unsigned integer from.


<!-- ##### MACRO GSIZE_TO_POINTER ##### -->
<para>
Stuffs a #gsize into a pointer type.
</para>

@s: #gsize to stuff into the pointer.


<!-- ##### MACRO GPOINTER_TO_SIZE ##### -->
<para>
Extracts a #gsize from a pointer. The #gsize must have
been stored in the pointer with GSIZE_TO_POINTER().
</para>

@p: pointer to extract a #gsize from.