How to make G++ preprocessor output a newline in a macro?

Question

Is there a way in gcc/g++ 4.* to write a macro that expands into several lines?

The following code:

#define A X \\ Y

Expands into

Answer 1

Putting a side the fact that not being able to put newlines in macros create unreadable code, making it harder to debug preprocessor outputs. It is true that C and C++ might not care about newlines, but the C preprocessor does.

I would really like to make a macro ConditionalDefine(x,y) that outputs the following.

    #ifdef x
    #define y
    #endif

The following defines do something close:

    #define hash #
    #define nl
    #define _def_p8(A,B) A ifdef _P8_K60_BOARD_ A define B  A endif
    #define X_def_p8(A,B) _def_p8(A,B)
    #define def_p8(A) X_def_p8(nl hash,A)

expanding the following:

    def_p8(PTPD_DBGA 0)

results in:

    # ifdef _P8_K60_BOARD_ # define PTPD_DBGA 0    # endif

But without being able to put new lines in before the hashes it would not work as intended. It is also annoying the hoops you have to jump through just to get that close.

Answer 2

Why does the spacing matter?

The imake program used in (older?) builds of X11 used the C pre-processor to generate makefiles, but imake program used a special technique of indicating line endings with @@ symbols at the ends of lines, and then post-processed the output of the pre-processor to replace the @@ symbols with newlines.

From this design, I conclude that there is no reliable way to obtain newlines from expanded macros in C (or C++). Indeed, for C, there is no need since:

• C does not care about newlines compared with white space after the C pre-processor is run, and
• You cannot generate pre-processor directives from macros etc.