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<h4>Variadic Macros</h4>
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<div> Variadic macros are supported by a number of compilers. They
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are macros of the form: </div>
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<div class="code">
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<pre>#define SOME_MACRO(ZeroOrMoreParameters,...) macro expansion possible specifying __VA_ARGS__</pre>
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</div>
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<div> The '...' in the parameter list represents the variadic data
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when the macro is invoked and the __VA_ARGS__ in the expansion
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represents the variadic data in the expansion of the macro.
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Variadic data is of the form of 1 or more preprocessor tokens
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separated by commas.<br>
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<br>
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The '...' must be the last parameter in the macro definition and
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there may be 0 or more non-variadic parameters preceding it.<br>
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<br>
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In the expansion of the macro __VA_ARGS__ may be specified 0 or
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more times to represent the variadic data. The variadic data in
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the expansion is a comma separated list of preprocessor tokens
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representing the variadic data which the invoker of the macro
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enters as the last arguments to the macro.<br>
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</div>
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<h4>Example<u> - Creating and invoking a variadic macro.</u></h4>
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<div class="code">
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<pre>#define INITIALIZE_INT_ARRAY(array_name,...) \ <br> static int array_name[] = { __VA_ARGS__ }; \ <br> /**/<br><br> INITIALIZE_INT_ARRAY(myname,45,789,33510,9346,2)</pre>
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</div>
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<u> <span style="font-weight: bold;">Preprocessor Library Support<br>
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</span></u>
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<div>The library offers support for variadic macros for those
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compilers which support the feature. The library can automatically
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detect whether a compiler supports variadic macros and sets the
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macro BOOST_PP_VARIADICS accordingly to 1 if the compiler supports
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variadic macros or 0 if the compiler does not support variadic
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macros.<br>
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<br>
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The end-user can #define BOOST_PP_VARIADICS to 1 or 0 himself in a
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translation unit, before including any preprocessor header files,
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to prevent the library from attempting to detect whether the
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compiler supports variadic macros. This has the effect of manually
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turning on or off variadic macro support in the library. Of course
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if one manually turns on variadic macro support in the library,
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and one's compiler does not support variadic macros, functionality
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in the library which uses variadic macros will fail with error
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messages from the compiler.<br>
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<br>
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When BOOST_PP_VARIADICS is 1, the library offers some extended
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functionality by using variadic macros, and also offers extended
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support for working with variadic data.<br>
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<br>
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<a name="vmvcquirk"></a>Visual C++ has a few quirks related to
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variadic macros which require the end-user to code slightly
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differently. When BOOST_PP_VARIADICS is 1 and Visual C++ is the
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compiler BOOST_PP_VARIADICS_MSVC is 1, else when
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BOOST_PP_VARIADICS is 1 and Visual C++ is not the compiler
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BOOST_PP_VARIADICS_MSVC is 0. When BOOST_PP_VARIADICS is 0 then
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BOOST_PP_VARIADICS_MSVC is not defined. In this way the end-user,
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when using variadic macros, can test for the presence of Visual
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C++ as the compiler and code accordingly.<br>
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<br>
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Support for working with variadic data is largely centered on
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being able to convert variadic data to other library data types,
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since the functionality for working with those Boost preprocessor
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library data types is much greater than that for working with
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variadic data directly.<br>
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</div>
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<a name="VNotation"></a>
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<h4>Notation For Variadic Macros<br>
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</h4>
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<div>In the documentation, headers which have variadic macros, and
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variadic macros themselves, have a notation of '(v)' appended to
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them. For the variadic macros themselves this signifies that
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BOOST_PP_VARIADICS must be 1 for those variadic macros to be
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usable. For variadic macros which have a non-variadic equivalent,
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the non-variadic equivalent will be used if BOOST_PP_VARIADICS is
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set to 0. </div>
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<h4>Extended Functionality Using Variadic Macros<br>
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</h4>
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<div>Some macros in the library offer extended functionality through
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the use of variadic macros.<br>
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<br>
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The variadic macro version offers the same functionality as the
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non-variadic version, but because of the ability of the variadic
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parameters to encompass a variable number of arguments, it also
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offers an enhanced syntax using the same macro name.<br>
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<br>
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The macros in the library which offer this enhanced functionality
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are all centered on <i>tuple</i> manipulation. With variadic
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macros it is possible to manipulate tuples without having to know
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the size of the tuple. So while the invoker can still specify the
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size when using tuple macro functionality, there are variadic
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versions of each of the tuple macros, with the exact same name as
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the non-variadic macro, where the size need not be specified.<br>
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</div>
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<h4>Extended Support For Variadic Data</h4>
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<div>The library offers extended support for working with variadic
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data which goes beyond the functionality offered by the C++
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specification for variadic macros. It does this through
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preprocessor programming and by using some of the other
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functionality in the library itself. Header and macro names in the
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library which offer extended support for working with variadic
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data, and need the compiler to support variadic macros, are marked
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with a (v)<sup> </sup>to indicate a variadic macro.<br>
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<br>
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The form of the functionality which the library offers is centered
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on two macros which work with variadic data itself, and a set of
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macros which convert between variadic data and other library data
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types.<br>
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<br>
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The two macros are BOOST_PP_VARIADIC_ELEM and
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BOOST_PP_VARIADIC_SIZE, which respectively return a particular
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token of variadic data and the number of tokens of variadic data.<br>
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<br>
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The macros for converting variadic data to the library's data
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types are BOOST_PP_VARIADIC_TO_ARRAY, BOOST_PP_VARIADIC_TO_LIST,
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BOOST_PP_VARIADIC_TO_SEQ, and BOOST_PP_VARIADIC_TO_TUPLE.<br>
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<br>
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All of these macros need compiler support for variadic data and
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only exist if BOOST_PP_VARIADICS is 1. <br>
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<br>
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The remaining four macros, which convert from a library data type
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to comma-separated preprocessor tokens, which is the form of
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variadic data, do not need compiler support for variadic macros.
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These functions are BOOST_PP_ARRAY_ENUM, BOOST_PP_LIST_ENUM,
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BOOST_PP_SEQ_ENUM, and BOOST_PP_TUPLE_ENUM. However if one wishes
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to use this variadic data reliably as arguments to other macros,
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one needs variadic macro support.<br>
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</div>
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<h4><a name="C20_Support_For_Variadic_Macros"></a>C++20 Support For
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Variadic Macros</h4>
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<div> In the C++20 specification there is a new construct which can
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be used in the expansion of a variadic macro, called __VA_OPT__.
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This construct when used in the expansion of a variadic macro is
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followed by an opening paranthesis '(', preprocessor data, and a
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closing parenthesis ')'. When the variadic data passed by the
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invocation of a variadic macro is empty, this new construct
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expands to nothing. When the variadic data passed by the
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invocation of a variadic macro is not empty, this new construct
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expands to the preprocessor data between its opening and closing
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parentheses. <br>
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<br>
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This library offers support for this new C++20 construct by
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automatically detecting whether this new construct is supported by
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the compiler's preprocessor when using the library. The library
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macro which detects support for the __VA_OPT__ construct is called
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BOOST_PP_VARIADIC_HAS_OPT. This is a function-like macro which
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takes no parameters and returns 1 if the compiler is working in
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C++20 mode and supports the __VA_OPT__ construct, while otherwise
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it returns 0. <br>
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<br>
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When the __VA_OPT__ construct is supported in C++20 mode the
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variadic data passed to the variadic macros and to
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BOOST_PP_OVERLOAD can be empty, otherwise when not in this mode
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variadic data passed to the variadic macros should never be empty.
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In this C+++20 mode invoking BOOST_PP_VARIADIC_SIZE with empty
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data expands to 0, invoking BOOST_PP_VARIADIC_TO_ARRAY with empty
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data expands to the empty array '(0,())', invoking
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BOOST_PP_VARIADIC_TO_LIST with empty data expands to the empty
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list 'BOOST_PP_NIL', and invoking BOOST_PP_OVERLOAD with empty
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data creates an overload name with 0 appended. Similarly in this
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C++20 mode passing an empty array '(0,())' to BOOST_PP_ARRAY_ENUM
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expands to empty variadic data and passing an empty list
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'BOOST_PP_NIL' to BOOST_PP_LIST_ENUM also expands to empty
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variadic data. Neither a seq or a tuple can be empty so passing
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empty variadic data to either BOOST_PP_VARIADIC_TO_SEQ or
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BOOST_PP_VARIADIC_TO_TUPLE is erroneous. Likewise passing empty
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data to BOOST_PP_VARIADIC_ELEM is always erroneous since there are
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no tokens of variadic data to access.<br>
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</div>
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<u style="font-weight: bold;"> Using a Tuple Instead of an Array<br>
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</u>
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<div>An array as a preprocessor data type is a two-element tuple
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where the first element is the array size and the second element
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is a tuple which constitutes the array data. Because a tuple knows
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its own size when the compiler supports variadic macros, there is
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no reason to use the array preprocessor data type as opposed to
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the tuple preprocessor data type; the tuple data type now has all
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of the functionality which the array data type has and is
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syntactically easier to use. With variadic macro support, which is
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now officially part of the latest C++ standard, the preprocessor
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array data type is essentially obsolete for conforming C++
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compilers. Only if your compiler does not support variadic macros
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is the preprocessor array data type still useful.</div>
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<u style="font-weight: bold;">Using Variadic Data</u>
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<div>Variadic data exists in the form of comma-separated
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preprocessor tokens. This is the case whether the variadic data
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comes from the __VA_ARGS__ of a variadic macro, from the
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conversion of a library's data type to variadic data, or the
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manual construction of comma-separated preprocessing tokens by the
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programmer writing a macro.<br>
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<br>
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The easiest way to work with variadic data internally is to
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convert it to a library data type. Library data types, whether an
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<i>array</i>, <i>list</i>, <i>sequence</i>, or <i>tuple</i>,
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have a rich set of functionality for manipulating data whereas
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variadic data functionality in the library only allows one to
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access the variadic data as a whole or to access a single token of
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the variadic data at a time.<br>
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<br>
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The user of the library still may choose to pass variadic data
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back into internal macros rather than convert it to other library
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data types. There is no problem passing variadic data as a whole
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to variadic macros as the last parameter of the macro. However: <br>
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<br>
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<span style="font-weight: bold;">Attempting to pass variadic data
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as a whole directly into a non-variadic macro is not guaranteed
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to work and may fail.<br>
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</span><br>
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This occurs because of a preprocessor weakness in a number of
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compilers, currently most notably Visual C++. Even passing
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variadic data as arguments to a non-variadic macro, when it is not
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represented in the form of __VA_ARGS__, may fail with
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certain compilers.<br>
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<br>
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What follows are very simple examples, showing how variadic data
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can be passed to a non-variadic macro.<br>
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<br>
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First an example of what NOT to do.<br>
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</div>
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<h4>Example<u> - Passing variadic data as a whole to a non-variadic
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macro. DO NOT DO.</u></h4>
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<div class="code">
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<pre>#define MACRO_ARG_2(x,y) BOOST_PP_ADD(x,y)<br>#define VAR_MACRO(...) __VA_ARGS__<br><br>/* The following should not be done and is not guaranteed to work with compilers. */<br><br><span style="font-weight: bold;"><span style="font-family: monospace;"></span></span>int xx = MACRO_ARG_2(VAR_MACRO(2,3));</pre>
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</div>
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<div> There are two ways to pass variadic data to a non-variadic
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macro. The first of these is to pass the individual tokens of the
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variadic data separately to the non-variadic macro using the
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BOOST_PP_VARIADIC_ELEM macro in the library.<br>
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</div>
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<h4>Example<u> - Passing individual variadic data tokens to a
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non-variadic macro.<br>
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</u></h4>
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<div class="code">
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<pre>#define MACRO_ARG_2(x,y) BOOST_PP_ADD(x,y)<br>#define VAR_MACRO(...) __VA_ARGS__<br><br>/* The following will work correctly */<br><br>int xx = MACRO_ARG_2<br> (<br> BOOST_PP_VARIADIC_ELEM(0,VAR_MACRO(2,3)),<br> BOOST_PP_VARIADIC_ELEM(1,VAR_MACRO(2,3))<br> );</pre>
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</div>
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<div>The second way is to use a macro in the library called
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BOOST_PP_OVERLOAD. This macro allows one to "overload" a variadic
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macro to non-variadic macros of different numbers of parameters,
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using a common prefix. </div>
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<h4>Example<u> - Passing variadic data as a whole to
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BOOST_PP_OVERLOAD and on to a non-variadic macro.<br>
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</u></h4>
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<div class="code">
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<pre>#define MACRO_ARG_2(x,y) BOOST_PP_ADD(x,y)<br>#define VAR_MACRO(...) __VA_ARGS__<br><br>/* The following will work correctly */<br><br>int xx = BOOST_PP_OVERLOAD(MACRO_ARG_,VAR_MACRO(2,3))(VAR_MACRO(2,3));<br><br>/* For Visual C++ it is necessary to do this */<br><br>int xx = <br>BOOST_PP_CAT(BOOST_PP_OVERLOAD(MACRO_ARG_,VAR_MACRO(2,3))(VAR_MACRO(2,3)),BOOST_PP_EMPTY());</pre>
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</div>
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<br>
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<div>Although these techniques will work when passing variadic data
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to non-variadic macros, it is much better and less problematical
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to work internally with the existing library data types and to
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only use variadic macros as an interface for end-users when there
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is a need to have a macro which takes a variable number of
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parameters.<br>
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</div>
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<b>See</b> <b>Also</b><br>
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<ul>
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<li><a href="../ref/variadics.html">BOOST_PP_VARIADICS</a></li>
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<li><a href="../headers/tuple.html">Tuple Macros</a><br>
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</li>
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<li><a href="../headers/variadic.html">Variadic Macros<br>
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</a></li>
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<li><a href="../ref/array_enum.html">BOOST_PP_ARRAY_ENUM</a></li>
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<li><a href="../ref/list_enum_r.html">BOOST_PP_LIST_ENUM</a></li>
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<li><a href="../ref/seq_enum.html">BOOST_PP_SEQ_ENUM</a></li>
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<li><a href="../ref/tuple_enum.html">BOOST_PP_TUPLE_ENUM</a></li>
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<li><a href="../ref/overload.html">BOOST_PP_OVERLOAD</a></li>
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</ul>
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<hr size="1">
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<div style="margin-left: 0px;"> <i><EFBFBD> Copyright Edward Diener
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2011,2013,2016</i> </div>
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<div style="margin-left: 0px;">
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<p><small>Distributed under the Boost Software License, Version
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1.0. (See accompanying file <a
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href="../../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
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copy at <a href="http://www.boost.org/LICENSE_1_0.txt">www.boost.org/LICENSE_1_0.txt</a>)</small></p>
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</div>
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