constant tables code
[Date Prev][
Date Next][Thread Prev][
Thread Next]
[
Date Index]
[
Thread Index]
- Subject: constant tables code
- From: "Ashley Fryer" <lua@...>
- Date: Sat, 1 Apr 2000 03:15:37 -0800
I've put together a few helper macros for creating tables of constant values
in Lua. They are useful in situations where you would like scripts to have
access to a set of values without allowing the scripts to modify the values
or the set.
For example, I have used this code to generate a table of the VK_xxx and
DIK_xxx key constants for Windows programming. It could also be useful for
generating a table of error strings, mathematical constants, etc.
Here's an example...
First, define the table in C. I find it convenient to group the CONST_TABLE
definitions together with my lua_register() calls.
int num_var = 42;
char str_var[] = "this is str_var";
CONST_TABLE_BEGIN( "MY_TABLE" )
CONST_NUM_VAR( num_var )
CONST_STR_VAR( str_var )
CONST_NUM( "foo", 7 )
CONST_STR( "bar", "this is bar")
CONST_TABLE_END
Then, once the table is initialized, you can do this in Lua:
print( MY_TABLE.num_var )
42
print( MY_TABLE.str_var )
this is str_var
print( MY_TABLE.foo )
7
print( MY_TABLE.bar )
this is bar
The table is "constant" because all of these things generate an error:
MY_TABLE = 9
MY_TABLE.str_var = 5
MY_TABLE.new_var = "new variable"
The error message is:
Cannot modify constant table: MY_TABLE
One unfortunate limitation of the macros is that you can't directly use
#defined values. This is because the C pre-processor isn't reentrant. So,
for example, this fails:
#define PI 3.14
CONST_TABLE_BEGIN( "MATH_TABLE" )
CONST_NUM( "PI", PI )
CONST_TABLE_END
My workaround is:
#define PI 3.14
CONST_TABLE_BEGIN( "MATH_TABLE" )
double pi = PI;
CONST_NUM( "PI", pi )
CONST_TABLE_END
Perhaps someone can think of a better solution.
The code follows. Anyone is free to use it any way, and I hope that someone
DOES find it useful. :-)
ashley
--- cut here ---
#define CONST_TABLE_BEGIN( TABLE_NAME ) \
{\
lua_Object table = lua_createtable();\
lua_pushobject( table );\
lua_setglobal( TABLE_NAME );\
lua_dostring( "function tmp() error(\
\"Cannot modify constant table: " TABLE_NAME "\" ) end" );
#define CONST_NUM( NAME, VALUE) \
lua_pushobject( table );\
lua_pushstring( ( char * ) NAME );\
lua_pushnumber( VALUE );\
lua_settable();
#define CONST_NUM_VAR( NAME ) \
lua_pushobject( table );\
lua_pushstring( ( char * ) #NAME );\
lua_pushnumber( NAME );\
lua_settable();
#define CONST_STR( NAME, VALUE ) \
lua_pushobject( table );\
lua_pushstring( ( char * ) NAME );\
lua_pushstring( VALUE );\
lua_settable();
#define CONST_STR_VAR( NAME ) \
lua_pushobject( table );\
lua_pushstring( ( char * ) #NAME );\
lua_pushstring( ( char * ) NAME );\
lua_settable();
#define CONST_TABLE_END \
int tag = lua_newtag();\
lua_pushobject( table );\
lua_settag( tag );\
lua_pushobject( table );\
lua_pushobject( lua_getglobal( "tmp" ) );\
lua_settagmethod( tag, "settable" );\
lua_pushobject( table );\
lua_pushobject( lua_getglobal( "tmp" ) );\
lua_settagmethod( tag, "setglobal" );\
}
--- cut here ---