Preprocessor Precedence

This is a discussion on Preprocessor Precedence within the C++ Programming forums, part of the General Programming Boards category; Here's what I'm trying to do.... to make a macro that will give it an error as a c-string, and ...

  1. #1
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    Preprocessor Precedence

    Here's what I'm trying to do.... to make a macro that will give it an error as a c-string, and it will give me a c-string with the line and file and the error, all put together. Then I can use that string to display, or whatever. Here's what I have:

    Code:
    #include <iostream>
    using namespace std;
    
    #define FlagF(Line,File,Error) ( "Line " #Line ", File " File ", Error " Error )
    #define Flag(Error) ( FlagF( __LINE__, __FILE__, Error ) )
    
    int main()
    {
    	cout << Flag( "Blah!" ) << endl;
    }
    ...and for output I get this:
    "Line __LINE__, File c:\documents and settings\evan ovadia\my documents\visual studio projects\combination\main.cpp, Error Blah!"

    Why is it saying __LINE__? I stringized it, but it was supposed to stringize the line number AFTER __LINE__ did its job. Is there a way to make the precedence of __LINE__ higher than the stringizer operator? If there isn't, is there any way to make a macro like I tried to make?

    Thanks!

  2. #2
    and the hat of int overfl Salem's Avatar
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    The preprocessor is a strange animal.

    Code:
    #define str(x)  #x
    #define FlagF(Line,File,Error) ( "Line " str(Line) ", File " File ", Error " Error )
    #define Flag(Error) ( FlagF( __LINE__, __FILE__, Error ) )
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  3. #3
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    It is indeed a strange animal...

    That code snippet produces:
    "Line (__LINE__Var+1), File c:\documents and settings\evan ovadia\my documents\visual studio projects\combination\main.cpp, Error Blah!"

    Where's Var+1 come from...?

  4. #4
    Yes, my avatar is stolen anonytmouse's Avatar
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    This is explained here. You need to treat __LINE__ as an integer expression rather than a numeric constant and do a run-time conversion.

  5. #5
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    Why don't you make FlagF an inline function instead?

  6. #6
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    because I put a new file into the system root, called Verd.h. I put macros and definitions and stuff in there, but I can't put any code, because all the files in all my project include it. Since they all include it, if I put any function code in there, it'll be copied and waste space.

  7. #7
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    Salem's advice works well on gcc/g++
    You're using a MS compiler which doesn't do what you want.
    You can use precompiler directives to use a dinamic calculation when not using gcc.
    Code:
    #ifdef 	__GNUC__
    #define str(x)  #x
    #define FlagF(Line,File,Error) ( "Line " str(Line) ", File " File ", Error " Error )
    #define Flag(Error) ( FlagF( __LINE__, __FILE__, Error ) )
    #else
    #define Flag(Error) (( std::string("Line")+__LINE+" File "+ __FILE__+" Error "+ Error ).c_str())
    #endif

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