ive been trying to do this for a while, but i havent had any luck
how (if you can) do you have other preprosessor commands within a #define? and is there a way to have a #define within a #define?
like can i do something like this (using code that doesnt work)
#define BLAH
#ifdef Blah2
1
#else
2
#endif
and then could i have something like this?
#define BLAH
#define Blah2 0
#ifdef Blah2
1
#else
2
#endif
#undef Blah2
I'm not sure what you're asking, but you can do this -
#ifdef Blah2
#define BLAH 1
#else
#define BLAH 2
#endif
zen
no, i mean can i do something like this
sorry, it removed the spaces the last time
#define macro(x)
_#if x==1
__dostuff();
_#elif x==2
__dootherstuff();
_#else
__domorestuff();
_#endif
_#define Blah 2
_do_stuff_that_uses_Blah();
I think I understand what you're asking and I think the answers no. Macros are just used for text substitution by the pre-processor at compile time and you'd be asking it to evaluate runtime variables. To get your second example to work you'd have to write the macro like a C function rather than a series of #if/#else.
zen
ok, then how about this?
#define macro(x)
_#if 1==1
__dostuff();
_#elif 2==2
__dootherstuff();
_#else
__domorestuff();
_#endif
_#define Blah 2
_do_stuff_that_uses_Blah();
the reason why i used the first example was because i was able use simlar code in an assembler
and it wouldnt be anything done at run time, it'd still be able to be completed by the preprocessor, if you look, it's comparing x, a constant declared by the macro, to a value, which you can do if you declare the constant using a #define. it would have this value at preprossessor time
i wasnt planning on doing it like that anyway, although it does make things a lot easier. i was planning on using an #ifdef and have it result in a different substitution depending upon whether or not something was defined
Do you mean something like this -
Code:#include <iostream> using namespace std; #define x 2 #if x==1 #define macro() cout << "1" #elif x==2 #define macro() cout << "2" #else #define macro() cout << "X" #endif int main() { macro(); return 0; }
zen
yeah, that would work in most cases
but I still need to know if it's possible to use have a define inside of a macro
what i want to do is something like this
#define macro(text)
_#define Blah array
_text
_#undefine Blah
_#define Blah array2
_text
that way i can do something like this
int x;
macro({
for(x=1; x<Blah[0]; x++) Blah[x] *=2;
somefunc(Blah[0]);
}) //yes, this does work, ive tried it already
without the macro I'd have to type it twice or put an extra load on the processor to loop it and change a pointer
what im actually doing is trying to optimize the code for an rpg battle system that im writing, and i found that i was writing most code twice
also, what do i do to specify a 2nd argument for a macro? would the above code still work with a 2nd argument if i had a , in there somewhere for some reason?
actually, i probably dont need the {}s
actually, it doesnt work for what i want, i just tried itOriginally posted by zen
Do you mean something like this -
Code:#include <iostream> using namespace std; #define x 2 #if x==1 #define macro() cout << "1" #elif x==2 #define macro() cout << "2" #else #define macro() cout << "X" #endif int main() { macro(); return 0; }
what i want is to be able to do something like that, but then if i do
#undef x
#define x 1
it'll change which one is used
i know that what i want is possible with more advanced macro processors.
if it's not possible, i'll probably end up writing my own preprocessor to run before the C++ preprocessor
Yes. But not exactly the way you think:no, i mean can i do something like this
sorry, it removed the spaces the last time
#define macro(x)
_#if x==1
__dostuff();
_#elif x==2
__dootherstuff();
_#else
__domorestuff();
_#endif
_#define Blah 2
_do_stuff_that_uses_Blah();
That will do it. You can use \ in macros to tell the macro that it spans multiple lines. Remember though, that you'll have to make sure you call it correctly. You could modify that macro to work like this:Code:#define myMacro(x) \ if((x)==1) \ doStuff( ); \ else \ if((x)==2) \ doOtherStuff( );
#define myMacro(x) ((x)==1?doStuff( ):doOtherStuff( ))
Or perhaps:
#define myMacro(x) ((x)==1?doStuff( )x)==2?doOtherStuff( ):someDefaultElseResult( ))
This way, the macro is easy to call:
if( myMacro(x) ) .... and the return value will be the return value of either function, based on whatever x evaluates to in your macro.
So to answer your question, "Yes, you can."
Quzah.
Last edited by quzah; 12-18-2001 at 08:14 PM.
Hope is the first step on the road to disappointment.
but would that still be handled by the preprocessor? if not, i dont think that would even work because x isnt a memory address, it's a defined constant
Yes. Macros are handled by the preprocessor. All the preprocessor does is replace every instance of 'myMacro' with the code as defined. It will work. Trust me on this one. Those macros are valid C/C++ code.
Quzah.
Hope is the first step on the road to disappointment.
no, i meant is the x==1 being handled by the preprocessor?
also, do you know anything about having #define's in a macro?
Let me clarify how macros work:
#deifne myMacro(x) (someFunction( ) == (x) ) ? "true" : "false"
If you wrote this macro, as above, and you called it like so:
cout << myMacro( myVariable );
Then the preprocessor would expand the code to actually be:
cout << (someFunction( ) == (myVariable)) ? "true" : "false";
All it does is basicly a text replacement for your definition to the body of the macro.
[edit] Regarding macros in macros:
#define MYMACRO 20
#define SOMEMACRO "hello there"
#define aMacro(x) (x)==4? MYMACRO : MYMACRO * MYMACRO
#define bMacro(y,z) aMacro(y) == MYMACRO ? "invalid" : aMacro(z) > MYMACRO ? "valid" : SOMEMACRO
Consider the above.
cout << bMacro( 15, 30 );
Expands to:
cout << aMacro( 15 ) == MYMACRO ? "invalid" : aMacro( 30 ) > MYMACRO ? "valid" : SOMEMACRO ;
Expands to:
cout << aMacro( 15 ) == 20 ? "invalid" : aMacro( 30 ) > 20 ? "valid" : "hello there" ;
Expands to:
cout << (15 == 4 ? 20 : 400) == 20 ? "invalid" : (30 == 4 ? 20 : 400 ) > 20 ? "valid" "hello there";
Or something like that. As you can see, you can compact, from the reader's point of view, some fairly complicated code into much simpler end results with macros.
Some people think macro usage is bad, but in reality they can be useful when used sparingly or when appropriate.
Quzah.
Last edited by quzah; 12-18-2001 at 08:32 PM.
Hope is the first step on the road to disappointment.
ah, ok, got it
that's still not quite what i wanted though, i cant have it output the name of a variable or something with that
