Variadic macros are supported by a number of compilers. They are
macros of the form:
The '...' in the parameter list represents the variadic data when the
macro is invoked and the __VA_ARGS__ in the expansion represents the
variadic data in the expansion of the macro. Variadic data is of the form
of 1 or more preprocessor tokens separated by commas.
The '...' must be the last parameter in the macro definition and there may
be 0 or more non-variadic parameters preceding it.
In the expansion of the macro __VA_ARGS__ may be specified 0 or more times
to represent the variadic data. The variadic data in the expansion is a
comma separated list of preprocessor tokens representing the variadic data
which the invoker of the macro enters as the last arguments to the macro.
#define INITIALIZE_INT_ARRAY(array_name,...) \
static int array_name[] = { __VA_ARGS__ }; \
/**/
INITIALIZE_INT_ARRAY(myname,45,789,33510,9346,2)
The library offers support for variadic macros for those
compilers
which support the feature. The library can automatically detect whether
a compiler supports variadic macros and sets the macro
BOOST_PP_VARIADICS accordingly to 1 if the compiler supports variadic
macros or 0 if the compiler does not support variadic macros.
The end-user can #define BOOST_PP_VARIADICS to 1 or 0 himself in a
translation unit, before including any preprocessor header files, to
prevent the library from attempting to detect whether the compiler
supports variadic macros. This has the effect of manually turning on or
off variadic macro support in the library. Of course if one manually
turns on variadic macro support in the library, and one's compiler does
not support variadic macros, functionality in the library which uses
variadic macros will fail with error messages from the compiler.
When BOOST_PP_VARIADICS is 1, the library offers some extended
functionality
by using variadic macros, and also offers extended support for working
with variadic data.
Visual C++ has a
few quirks related to variadic macros which require the end-user to code
slightly differently. When BOOST_PP_VARIADICS is 1 and Visual C++ is the
compiler BOOST_PP_VARIADICS_MSVC is 1, else when BOOST_PP_VARIADICS is 1
and Visual C++ is not the compiler BOOST_PP_VARIADICS_MSVC is 0. When
BOOST_PP_VARIADICS is 0 then BOOST_PP_VARIADICS_MSVC is not defined.
In this way the end-user, when using variadic macros, can test for the
presence of Visual C++ as the compiler and code accordingly.
Support for working with variadic
data is largely centered on being able to convert variadic data to
other library data types, since the
functionality for working with those Boost preprocessor library data
types is much greater than that for working with variadic data directly.