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114 lines
5.4 KiB
Text
114 lines
5.4 KiB
Text
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/* _______ ____ __ ___ ___
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* \ _ \ \ / \ / \ \ / / ' ' '
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* | | \ \ | | || | \/ | . .
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* | | | | | | || ||\ /| |
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* | | | | | | || || \/ | | ' ' '
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* | | | | | | || || | | . .
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* | |_/ / \ \__// || | |
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* /_______/ynamic \____/niversal /__\ /____\usic /| . . ibliotheque
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* / \
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* / . \
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* fnptr.txt - Function pointer explanation. / / \ \
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* | < / \_
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* | \/ /\ /
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* \_ / > /
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* | \ / /
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* | ' /
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* \__/
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*/
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C allows you to create and use function pointers. A function pointer is a
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variable that points to a function, and you can use it to call that function.
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Why is this useful?
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Function pointers can be passed as parameters. As an example, here's a
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function from Allegro:
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void create_light_table(COLOR_MAP *table, const PALETTE pal, int r, g, b,
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void (*callback)(int pos));
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Don't worry about the syntax just yet, but the last parameter, 'callback', is
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a pointer to a function that takes an int parameter. create_light_table() can
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take some time to complete its work, and you may want to display a progress
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indicator. So you write a function to draw the progress indicator, and then,
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for 'callback', you specify a pointer to your function. This will enable
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create_light_table() to call your function at intervals during its
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processing. (If you don't want to use the callback, you can pass NULL, but
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this only works because create_light_table() checks actively for NULL. You
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can't always specify NULL when you want nothing to happen.)
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There are many other uses. In addition to using function pointers as
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parameters, Allegro has some global function pointers you can set to point to
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your functions. Function pointers can also be used in structs, and this is
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where DUMB makes the most use of them.
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So how are they used?
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void bar(void) { ... } /* Here's a function */
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void (*foo)(void) = &bar; /* Take a pointer */
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(*foo)(); /* Call the function */
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char *baz(float a) { ... } /* Here's another function */
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char *(*foobarbaz)(float a) = &baz; /* Take a pointer */
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char *rv = (*foobarbaz)(0.1); /* Call the function */
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In both these cases, note how the statement for calling the pointed-to
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function (third line) resembles the definition of the function pointer
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(second line). This is true of any variable in C, and can lead to some truly
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obfuscated definitions if you are that way inclined. Such definitions can be
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clarified with typedefs, but before you use those, it is important you
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understand how the above statements work. I speak from experience: function
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pointer notation looks random and scary, until you understand why it's the
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way it is; then it makes perfect sense.
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(It is actually permissible to omit the & when taking a pointer and to write
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e.g. foobarbaz(0.1) instead of (*foobarbaz)(0.1). However, I recommend not
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doing this, since the syntax for using the pointer no longer resembles the
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definition. Writing e.g. (*foobarbaz)(0.1) also makes a clear distinction
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between function pointer calls and ordinary function calls, which makes code
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more readable.)
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Note that function pointers have the return value and parameter list
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specified. A function pointer can only point to a function with a matching
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return value and matching parameters. (You can break this rule by casting the
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pointer explicitly, but there is no situation where doing so is portable to
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all computers, and I strongly advise against it unless you're writing system
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code. If you're not sure whether you're writing system code or not, then
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you're not.)
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The parameter names need not match (although the types must). If you wish to
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rename a parameter in your function, you do not have to change the function
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pointer accordingly. In fact, when you define a function pointer, you don't
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even have to specify the names of parameters if you don't want to. I normally
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do so for clarity.
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It is possible to typedef a function pointer. In order to typedef a function
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pointer, you start by declaring the pointer as a variable:
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void (*myfunc)(void);
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Then you write 'typedef' before it and replace the variable name, which is
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myfunc, with the type name (this rule can be applied to any variable when you
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want to use typedef):
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typedef void (*MYTYPE)(void);
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Now 'MYTYPE' represents a pointer to a function with no parameters and no
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return value. The following two lines are completely equivalent:
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MYTYPE myfunc;
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void (*myfunc)(void);
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Note that we use MYTYPE without an asterisk (*), since it is already a
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pointer.
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That's it. If you feel anything should be explained better here, or if you
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feel something should be added, please don't hesitate to let me know!
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Ben Davis
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entheh@users.sf.net
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IRC EFnet #dumb
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See readme.txt for details on using IRC.
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