How to make the first invocation of a macro different from all the next ones ?

Posted by LB on Stack Overflow See other posts from Stack Overflow or by LB
Published on 2010-03-15T14:31:54Z Indexed on 2010/03/15 15:19 UTC
Read the original article Hit count: 129

Filed under:

c

|

preprocessor

|

macros

Hi,

that may be really simple but i'm unable to find a good answer. How can I make a macro representing first a certain value and then a different one ?

I know that's nasty but i need it to implicitly declare a variable the first time and then do nothing. This variable is required by other macros that i'm implementing.

Should i leverage "argument prescan" ?

thanks for the answers.

EDIT To make things clearer. Suppose i have a macro FOO, and I do something like

FOO
FOO
FOO

I would like the result to be

foo
bar
bar

I don't want the actual code to be cluttered by ifndef. The programmer should only have to write macro invocations.

Related posts about preprocessor

Any other preprocessor directives in VBScript/Classic ASP?

as seen on Stack Overflow - Search for 'Stack Overflow'
The only pre-process directive that I know about in VBScript / Classic ASP is the #include. I don't know if that is the official name but I'm basically looking for code that can execute code or other instructions before the general VBScript. Are there any other such directives in VBScript? Such as… >>> More
Visual Studio 2008 Preprocessor wierdness

as seen on Stack Overflow - Search for 'Stack Overflow'
We have set-up a simple versioning system for our builds to ensure the built files always indicate whether they are Beta Debug or Beta Release builds I moved the file version info to to myapp.rc2 and created version.h // version.h // _DEBUG is defined by VS #define _BETA #ifdef… >>> More
Writing preprocessor directives to get string

as seen on Stack Overflow - Search for 'Stack Overflow'
Can you write preprocessor directives to return you a std::string or char*? For example: In case of integers: #define square(x) (x*x) int main() { int x = square(5); } I'm looking to do the same but with strings like a switch-case pattern. if pass 1 it should return "One" and 2 for "Two" so… >>> More
Are preprocessors obsolete in modern languages?

as seen on Stack Overflow - Search for 'Stack Overflow'
Hello, I'm making a simple compiler for a simple pet language I'm creating and coming from a C background(though I'm writing it in Ruby) I wondered if a preprocessor is necessary. What do you think? Is a "dumb" preprocessor still necessary in modern languages? Would C#'s conditional compilation capabilities… >>> More
The Bizarre Hidden Powers of the Preprocessor? [closed]

as seen on Stack Overflow - Search for 'Stack Overflow'
The preprocessor in C and C++ deserves an entire essay on its own to explore its rich possibilities for obfuscation. It is true that the C++ (and C) preprocessor can be used for a lot of powerful stuff. #ifdefs and #defines are often used to determine platforms, compilers and backends. Manipulating… >>> More

Developer IT