Variadic Macros

Variadic macros are supported by a number of compilers. They are macros of the form:
#define SOME_MACRO(ZeroOrMoreParameters,...) macro expansion possible specifying __VA_ARGS__
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.

Example - Creating and invoking a variadic macro.

#define INITIALIZE_INT_ARRAY(array_name,...) \ 
static int array_name[] = { __VA_ARGS__ }; \
/**/

INITIALIZE_INT_ARRAY(myname,45,789,33510,9346,2)
Preprocessor Library Support
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.

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.

Notation For Variadic Macros

In the documentation, headers which have variadic macros, and variadic macros themselves, have a notation of '(v)' appended to them. For the variadic macros themselves this signifies that BOOST_PP_VARIADICS must be 1 for those variadic macros to be usable. For variadic macros which have a non-variadic equivalent, the non-variadic equivalent will be used if BOOST_PP_VARIADICS is set to 0 and the non-variadic version of the macro does not have a '(v)' appended to its name in the documentation.

Extended Functionality Using Variadic Macros

Some macros in the library offer extended functionality through the use of variadic macros.

The variadic macro version offers the same functionality as the non-variadic version, but because of the ability of the variadic parameters to encompass a variable number of arguments, it also offers an enhanced syntax using the same macro name.

The macros in the library which offer this enhanced functionality are all centered on tuple manipulation. With variadic macros it is possible to manipulate tuples without having to know the size of the tuple. So while the invoker can still specify the size when using tuple macro functionality, there are variadic versions of each of the tuple macros, with the exact same name as the non-variadic macro, where the size need not be specified.

Extended Support For Variadic Data

The library offers extended support for working with variadic data which goes beyond the functionality offered by the C++ specification for variadic macros. It does this through preprocessor programming and by using some of the other functionality in the library itself. Header and macro names in the library which offer extended support for working with variadic data, and need the compiler to support variadic macros, are marked with a (v) to indicate a variadic macro.

The form of the functionality which the library offers is centered on two macros which work with variadic data itself, and a set of macros which convert between variadic data and other library data types.

The two macros are BOOST_PP_VARIADIC_ELEM and BOOST_PP_VARIADIC_SIZE, which respectively return a particular token of variadic data and the number of tokens of variadic data.

The macros for converting variadic data to the library's data types are BOOST_PP_VARIADIC_TO_ARRAY, BOOST_PP_VARIADIC_TO_LIST, BOOST_PP_VARIADIC_TO_SEQ, and BOOST_PP_VARIADIC_TO_TUPLE.

All of these macros need compiler support for variadic data and only exist if BOOST_PP_VARIADICS is 1.

The remaining four macros, which convert from a library data type to comma-separated preprocessor tokens, which is the form of variadic data, do not need compiler support for variadic macros. These functions are BOOST_PP_ARRAY_ENUM, BOOST_PP_LIST_ENUM, BOOST_PP_SEQ_ENUM, and BOOST_PP_TUPLE_ENUM. However if one wishes to use this variadic data reliably as arguments to other macros, one needs variadic macro support.
Using Variadic Data
Variadic data exists in the form of comma-separated preprocessor tokens. This is the case whether the variadic data comes from the __VA_ARGS__ of a variadic macro, from the conversion of a library's data type to variadic data, or the manual construction of comma-separated preprocessing tokens by the programmer writing a macro.

The easiest way to work with variadic data internally is to convert it to a library data type. Library data types, whether an array, list, sequence, or tuple, have a rich set of functionality for manipulating data whereas variadic data functionality in the library only allows one to access the variadic data as a whole or to access a single token of the variadic data at a time.

The user of the library still may choose to pass variadic data back into internal macros rather than convert it to other library data types. There is no problem passing variadic data as a whole to variadic macros as the last parameter of the macro. However:

Attempting to pass variadic data as a whole directly into a non-variadic macro is not guaranteed to work and may fail.

This occurs because of a preprocessor weakness in a number of compilers, currently most notably Visual C++. Even passing variadic data as arguments to a non-variadic macro, when it is not represented in the form of  __VA_ARGS__, may fail with certain compilers.

What follows are very simple examples, showing how variadic data can be passed to a non-variadic macro.

First an example of what NOT to do.

Example - Passing variadic data as a whole to a non-variadic macro. DO NOT DO.

#define MACRO_ARG_2(x,y) BOOST_PP_ADD(x,y)
#define VAR_MACRO(...) __VA_ARGS__

/* The following should not be done and is not guaranteed to work with compilers. */

int xx = MACRO_ARG_2(VAR_MACRO(2,3));
There are two ways to pass variadic data to a non-variadic macro. The first of these is to pass the individual tokens of the variadic data separately to the non-variadic macro using the BOOST_PP_VARIADIC_ELEM macro in the library.

Example - Passing individual variadic data tokens to a non-variadic macro.

#define MACRO_ARG_2(x,y) BOOST_PP_ADD(x,y)
#define VAR_MACRO(...) __VA_ARGS__

/* The following will work correctly */

int xx = MACRO_ARG_2
(
BOOST_PP_VARIADIC_ELEM(0,VAR_MACRO(2,3)),
BOOST_PP_VARIADIC_ELEM(1,VAR_MACRO(2,3))
);
The second way is to use a macro in the library called BOOST_PP_OVERLOAD. This macro allows one to "overload" a variadic macro to non-variadic macros of different numbers of parameters, using a common prefix.

Example - Passing variadic data as a whole to BOOST_PP_OVERLOAD and on to a non-variadic macro.

#define MACRO_ARG_2(x,y) BOOST_PP_ADD(x,y)
#define VAR_MACRO(...) __VA_ARGS__

/* The following will work correctly */

int xx = BOOST_PP_OVERLOAD(MACRO_ARG_,VAR_MACRO(2,3))(VAR_MACRO(2,3));

/* For Visual C++ it is necessary to do this */

int xx =
BOOST_PP_CAT(BOOST_PP_OVERLOAD(MACRO_ARG_,VAR_MACRO(2,3))(VAR_MACRO(2,3)),BOOST_PP_EMPTY());

Although these techniques will work when passing variadic data to non-variadic macros, it is much better and less problematical to work internally with the existing library data types and to only use variadic macros as an interface for end-users when there is a need to have a macro which takes a variable number of parameters.
See Also

Copyright Edward Diener 2011

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at www.boost.org/LICENSE_1_0.txt)