variadic_macros.html

#### Variadic Macros

Variadic macros are supported by nearly all compilers. Variadic macros 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 requires support for variadic macros for all compilers using the library. This usually means that compilation at the C level is C99 or higher and that compilation at the C++ level is C++11 or higher. Many C++ compilers support variadic macros at the C++98/C++03 level as long as strict compliance is not turned on for those compilers at that level. In particular both the gcc and clang compilers will support variadic macros at the C++98/C++03 levels as long as strict ANSI compliance is not turned on at that level. For those compilers this largely means that the `-pedantic` or `-pedantic-errors` option is not used at the C++98/C++03 level of compilation. Boost C++ is deprecating compiling Boost libraries at the C++98/C++03 level, so if you must still use this library at that level be aware of these aforementioned caveats.
Visual C++'s default preprocessor has a few quirks related to variadic macros which require the end-user to code slightly differently. When Visual C++'s default preprocessor is being used BOOST_PP_VARIADICS_MSVC is 1, otherwise it is 0. In this way the end-user can test for the presence of Visual C++'s default preprocessor 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.

#### Extended Functionality Using Variadic Macros

Some macros in the library offer extended functionality through the use of variadic macros.
The variadic macro version offers extended functionality because of the ability of the variadic parameters to encompass a variable number of arguments. The library has functionality which can know the number of variadic arguments passed when invoking a variadic macro. This allows the same variadic macro to work with different numbers of parameters, therefore providing more than one syntactical equivalent for 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 syntactical versions of each of the tuple macros 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.
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.
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 use variadic macros. These functions are BOOST_PP_ARRAY_ENUM, BOOST_PP_LIST_ENUM, BOOST_PP_SEQ_ENUM, and BOOST_PP_TUPLE_ENUM. You can use this variadic data reliably as arguments to other macros using variadic macro support.

#### C++20 Support For Variadic Macros

In the C++20 specification there is a new construct which can be used in the expansion of a variadic macro, called __VA_OPT__. This construct when used in the expansion of a variadic macro is followed by an opening paranthesis '(', preprocessor data, and a closing parenthesis ')'. When the variadic data passed by the invocation of a variadic macro is empty, this new construct expands to nothing. When the variadic data passed by the invocation of a variadic macro is not empty, this new construct expands to the preprocessor data between its opening and closing parentheses.
This library offers support for this new C++20 construct by automatically detecting whether this new construct is supported by the compiler's preprocessor when using the library. The library macro which detects support for the __VA_OPT__ construct is called BOOST_PP_VARIADIC_HAS_OPT. This is a function-like macro which takes no parameters and returns 1 if the compiler is working in C++20 mode and supports the __VA_OPT__ construct, while otherwise it returns 0.
When the __VA_OPT__ construct is supported in C++20 mode the variadic data passed to the variadic macros and to BOOST_PP_OVERLOAD can be empty, otherwise when not in this mode variadic data passed to the variadic macros should never be empty. In this C+++20 mode invoking BOOST_PP_VARIADIC_SIZE with empty data expands to 0, invoking BOOST_PP_VARIADIC_TO_ARRAY with empty data expands to the empty array '(0,())', invoking BOOST_PP_VARIADIC_TO_LIST with empty data expands to the empty list 'BOOST_PP_NIL', and invoking BOOST_PP_OVERLOAD with empty data creates an overload name with 0 appended. Similarly in this C++20 mode passing an empty array '(0,())' to BOOST_PP_ARRAY_ENUM expands to empty variadic data and passing an empty list 'BOOST_PP_NIL' to BOOST_PP_LIST_ENUM also expands to empty variadic data. Neither a seq or a tuple can be empty so passing empty variadic data to either BOOST_PP_VARIADIC_TO_SEQ or BOOST_PP_VARIADIC_TO_TUPLE is erroneous. Likewise passing empty data to BOOST_PP_VARIADIC_ELEM is always erroneous since there are no tokens of variadic data to access.
Using a Tuple Instead of an Array
An array as a preprocessor data type is a two-element tuple where the first element is the array size and the second element is a tuple which constitutes the array data. Because a tuple knows its own size because of compiler support for variadic macros, there is no reason to use the array preprocessor data type as opposed to the tuple preprocessor data type; the tuple data type now has all of the functionality which the array data type has and is syntactically easier to use. The preprocessor array data type is essentially obsolete for modern C++ compilers.
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++'s default preprocessor. 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++'s default preprocessor 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

- Tuple Macros
- Variadic Macros
- BOOST_PP_ARRAY_ENUM
- BOOST_PP_LIST_ENUM
- BOOST_PP_SEQ_ENUM
- BOOST_PP_TUPLE_ENUM
- BOOST_PP_OVERLOAD

© Copyright Edward Diener 2011,2013,2016

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)