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2025-01-12 20:41:24 +08:00
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<h4>Passing nothing</h4>
<div> Although rarely desirable it has always been legal in C++ to
pass nothing, aka no preprocessor tokens, as an argument when
invoking a macro, whether the equivalent parameter be a regular
parameter or a variadic one. </div>
<div class="code">
<pre> #define SOME_MACRO(Parameter1,Parameter2) macro expansion using Parameter1 and Parameter2
#define SOME_VARIADIC_MACRO(Parameter1,...) macro expansion using Parameter1 and __VA_ARGS__
SOME_MACRO(a,b) // Normal
SOME_MACRO(a,) // Legal, second argument is empty
SOME_MACRO(,b) // Legal, first argument is empty
SOME_MACRO(a) // Preprocessor error, passing the wrong number of arguments
SOME_VARIADIC_MACRO(a,b,c,d) // Normal
SOME_VARIADIC_MACRO(a,) // Legal, variadic argument is empty
SOME_VARIADIC_MACRO(,b,c,d) // Legal, first argument is empty
SOME_VARIADIC_MACRO(a) /* Preprocessor error in standard below C++20 level,
but in C++20 exactly equivalent to SOME_VARIADIC_MACRO(a,) */</pre>
</div>
<h4>Expanding to nothing</h4>
<div> Given certain arguments a macro might expand to nothing, aka
no preprocessor tokens. This may happen more than in the previous
case of an argument to a macro being nothing because the expansion
of a macro is often used to initialize some C++ construct, and C++
has some places where a part of a compile-time construct can be
empty. However a macro which expands to nothing rarely occurs when
that macro's expansion is used as an argument to another macro
because we would again have a macro where we are passing nothing
as an argument. </div>
<div class="code">
<pre> #define ANOTHER_MACRO(Parameter1,Parameter2) /* expands to nothing when Parameter1 and Parameter2
are numbers, otherwise expands to some preprocessing
token, such as '1' */
int another_int = { ANOTHER_MACRO(x,y) }; // ANOTHER_MACRO Expands to 1
int some_int = { ANOTHER_MACRO(1,2) }; // ANOTHER_MACRO Expands to nothing
SOME_MACRO(ANOTHER_MACRO(x,y),z) // Normal, ANOTHER_MACRO Expands to 1
SOME_MACRO(ANOTHER_MACRO(1,2),z) // Legal, first argument is empty as ANOTHER_MACRO Expands to nothing</pre>
</div>
<h4>Emptiness defined</h4>
<div> Passing nothing as a macro argument or a macro expanding to
nothing I term as 'emptiness', as 'nothing' is too amorphous a
term which can be used in too many other contexts for my liking.
In the vast majority of cases when designing a macro for use
emptiness is not a part of such a design, and passing emptiness as
an argument or expanding to emptiness is not anything that someone
writing a macro takes into account when he explains to other
programmers how a macro should be used.<br>
<br>
Other than the fact that macros are generally created so that some
actual preprocessor data of a particular kind needs to be passed
as arguments or gets generated as part of macro expansion when a
macro is invoked, there is another very good reason why working
with emptiness is not part of a macro's design: there has been no
perfectly fail-safe way to test for emptiness during macro
expansion, whether it be in creating macros using just the
facilities of the C++ standard or using a 3rd party library, such
as this Boost preprocessor library. When I say 'fail-safe' I mean
that there has always been some argument input, no matter how
small the number of potential cases, where a macro designed to
test whether or not the preprocessor data passed to it as an
argument when the macro is invoked is actually empty fails in some
way, with the failure normally occurring as a preprocessor error.<br>
<br>
Of course this does not mean that the best macro designed to test
for emptiness will not work correctly the vast majority of the
time. It only means that there has been no guarantee that such a
macro will work correctly all 100% of the time. Nonetheless there
have been uses of testing for emptiness, when a macro documents
what a particular argument should generally consist of, even if
the test is not guaranteed to work 100% of the time if particular
unexpected argument data does get passed. </div>
<h4>A C++20 solution for testing for emptiness</h4>
<div> The C++ standard committee recognized, in the upcoming
specification for the C++20 standard, that a way of testing
whether variadic data is empty or not in the expansion of a
variadic macro would be very useful when designing certain types
of macros. Because of this the C++20 standard added a preprocessor
construct which could do this in a certain way for variadic data
in the expansion of a variadic macro. The construct is called
__VA_OPT__, as in '__VA_OPT__ ( prepocessing tokens )' specified
in the replacement list of a variadic macro. <br>
<br>
The way that the __VA_OPT__ constructs works is that if the
variadic arguments to the variadic macro are empty or expand to
emptiness then the __VA_OPT__ construct and its enclosed
preprocessing token data expands to nothing, or in C++ terms "a
single placemarker preprocessing token". Otherwise the __VA_OPT__
construct expands to its enclosed preprocessing tokens. A further,
possibly unintended, upshot of adding the __VA_OPT__ construct to
C++20 is that it is now possible to create a variadic macro which
is 100% reliable in testing for emptiness whenever a compiler
supports the __VA_OPT__ construct in its compilation of
preprocessor code.<br>
<br>
For such a macro to always work which tests for emptiness the code
must know when the __VA_OPT__ construct is available. It is not
enough to know that a compiler is working at the C++20 level,
since as all C++ programmers know an adherence to a C++ standard
level never guarantees that a particular compiler supports every
aspect of that level. Happily there is a way to test whether a
compiler supports the __VA_OPT__ construct as long as the compiler
supports variadic macros, and that way has been openly published
on the Internet, although the actual macro code would not have
been hard to create even if it had not publicly appeared. This
library uses that code to test for __VA_OPT__ as a necessary
prelude for creating a variadic macro which is 100% reliable in
testing for emptiness.<br>
<br>
The Boost Preprocessor macro for testing whether the __VA_OPT__
construct is supported during compilation is called
BOOST_PP_VARIADIC_HAS_OPT, which is a function-like macro taking
no parameters and returning 1 if the __VA_OPT__ construct is
supported and 0 if it is not. The macro only returns 1 when
the compiler is at the C++20 level, and when the __VA_OPT__ construct
can be used according to the C++20 standard. In particular the macro
needs the compiler to be working at the C++20 level despite the fact
that at least one major compiler supports the __VA_OPT__ construct in
some of its latest releases even when the compiler is being used at a
C++ standard level below that of C++20. The reason this Boost
preprocessor library requires the C++20 level is because that same
major compiler can produce a warning, or even an error, when it
even sees a macro using the __VA_OPT__ construct at a level below
C++20, even though it supports it, if other compiler options
requiring strict adherence to the level of the C++ standard being
used are passed on the command line. So taking a conservative
approach the BOOST_PP_VARIADIC_HAS_OPT macros requires compilation
at the C++20 level, along with the testing code expanding to 1,
in order to specify that __VA_OPT__ is supported.<br>
<br>
The actual Boost Preprocessor library for testing for emptiness in
C++20 mode is called BOOST_PP_CHECK_EMPTY. The macro is a variadic
macro with a single variadic parameter. The macro only exists if
our previous macro for testing for __VA_OPT__, called
BOOST_PP_VARIADIC_HAS_OPT, expands to 1 when invoked as
BOOST_PP_VARIADIC_HAS_OPT(). If BOOST_PP_VARIADIC_HAS_OPT()
expands to 0 the BOOST_PP_CHECK_EMPTY macro does not exist at all
in this library. The input to the BOOST_PP_CHECK_EMPTY macro can
be any variadic data. If the data passed to the macro is empty, or
if the data passed to the macro is not empty but when the data
itself is expanded it is empty, the macro returns 1, otherwise it
returns 0. The macro works 100% of the time and is completely
reliable no matter what preprocessor data is passed to it. But of
course it only works when compiling at the C++20 level with the
__VA_OPT__ construct supported by the compiler. It solves an old
problem that it has never been possible, prior to C++20, to
provide a 100% reliable implementation of a macro which tests for
emptiness in C++.<br>
<br>
Along with the valuable BOOST_PP_CHECK_EMPTY macro the Boost
Preprocessor library has also added a more flexible, if slightly
verbose, alternative to the __VA_OPT__ construct, which works by
using the ability of BOOST_PP_CHECK_EMPTY to reliably test for
emptiness. This macro is called BOOST_PP_VA_OPT and allows the
programmer to specify preprocessing tokens for expansion both when
the variadic data is <b>not</b> empty and when the variadic data
is empty. This improves on the __VA_OPT__ construct's ability to
specify preprocessing tokens for expansion only when the variadic
data is not empty. Like BOOST_PP_CHECK_EMPTY, which it uses, the
BOOST_PP_VA_OPT macro only exists when BOOST_PP_VARIADIC_HAS_OPT()
expands to 1. You can read further about how this macro works as
an alternative to the C++20 __VA_OPT__ construct in the
documentation for the macro itself.<br>
<br>
Eventually more C++ compilers will support C++20 and the
__VA_OPT__ construct and more programmers will use compilers at
the C++20 level. At that point the macro BOOST_PP_CHECK_EMPTY can
be used reliably for testing emptiness in preprocessor data in
macro code by all those programmers. The BOOST_PP_VA_OPT macro
serves as a useful example of such use. This does not mean that
designing macros with emptiness in mind needs to be done, much
less considered, but that the possibility of doing so with
complete reliability will be there if needed by the macro
programmer. Along with the __VA_OPT__ construct as mandated by the
C++20 standard the BOOST_PP_CHECK_EMPTY and BOOST_PP_VA_OPT macros
add three more tools in the arsenal of macro programming, which is
a good thing, while programmers who wanted to ignore any dealing
with emptiness in macro code can continue to do so. </div>
<b>See</b> <b>Also</b><br>
<ul>
<li><a href="../ref/variadic_has_opt.html">BOOST_PP_VARIADIC_HAS_OPT</a></li>
<li><a href="../ref/check_empty.html">BOOST_PP_CHECK_EMPTY</a></li>
<li><a href="../ref/va_opt.html">BOOST_PP_VA_OPT</a><br>
</li>
</ul>
<hr size="1">
<div style="margin-left: 0px;"> <i><EFBFBD> Copyright Edward Diener 2019</i>
</div>
<div style="margin-left: 0px;">
<p><small>Distributed under the Boost Software License, Version
1.0. (See accompanying file <a
href="../../../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
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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