diff options
Diffstat (limited to 'gcc/doc')
| -rw-r--r-- | gcc/doc/cpp.texi | 46 |
1 files changed, 36 insertions, 10 deletions
diff --git a/gcc/doc/cpp.texi b/gcc/doc/cpp.texi index 8cafb65..94437d5 100644 --- a/gcc/doc/cpp.texi +++ b/gcc/doc/cpp.texi @@ -1675,20 +1675,27 @@ macro. We could define @code{eprintf} like this, instead: @end smallexample @noindent -This formulation looks more descriptive, but unfortunately it is less -flexible: you must now supply at least one argument after the format -string. In standard C, you cannot omit the comma separating the named -argument from the variable arguments. Furthermore, if you leave the -variable argument empty, you will get a syntax error, because -there will be an extra comma after the format string. +This formulation looks more descriptive, but historically it was less +flexible: you had to supply at least one argument after the format +string. In standard C, you could not omit the comma separating the +named argument from the variable arguments. (Note that this +restriction has been lifted in C++2a, and never existed in GNU C; see +below.) + +Furthermore, if you left the variable argument empty, you would have +gotten a syntax error, because there would have been an extra comma +after the format string. @smallexample eprintf("success!\n", ); @expansion{} fprintf(stderr, "success!\n", ); @end smallexample -GNU CPP has a pair of extensions which deal with this problem. First, -you are allowed to leave the variable argument out entirely: +This has been fixed in C++2a, and GNU CPP also has a pair of +extensions which deal with this problem. + +First, in GNU CPP, and in C++ beginning in C++2a, you are allowed to +leave the variable argument out entirely: @smallexample eprintf ("success!\n") @@ -1696,8 +1703,24 @@ eprintf ("success!\n") @end smallexample @noindent -Second, the @samp{##} token paste operator has a special meaning when -placed between a comma and a variable argument. If you write +Second, C++2a introduces the @code{@w{__VA_OPT__}} function macro. +This macro may only appear in the definition of a variadic macro. If +the variable argument has any tokens, then a @code{@w{__VA_OPT__}} +invocation expands to its argument; but if the variable argument does +not have any tokens, the @code{@w{__VA_OPT__}} expands to nothing: + +@smallexample +#define eprintf(format, @dots{}) \\ + fprintf (stderr, format __VA_OPT__(,) __VA_ARGS__) +@end smallexample + +@code{@w{__VA_OPT__}} is also available in GNU C and GNU C++. + +Historically, GNU CPP has also had another extension to handle the +trailing comma: the @samp{##} token paste operator has a special +meaning when placed between a comma and a variable argument. Despite +the introduction of @code{@w{__VA_OPT__}}, this extension remains +supported in GNU CPP, for backward compatibility. If you write @smallexample #define eprintf(format, @dots{}) fprintf (stderr, format, ##__VA_ARGS__) @@ -1730,6 +1753,9 @@ of macro. It may also be forbidden in open text; the standard is ambiguous. We recommend you avoid using it except for its defined purpose. +Likewise, C++ forbids @code{@w{__VA_OPT__}} anywhere outside the +replacement list of a variadic macro. + Variadic macros became a standard part of the C language with C99. GNU CPP previously supported them with a named variable argument |