The BOOST_PP_VA_OPT variadic
macro is a more flexible alternative to the C++20 __VA_OPT__
construct. It expands to either one of two inputs depending on
whether the variadic data is empty or not, whereas the C++20
__VA_OPT__ constructs expands to either its input or nothing
depending on whether the variadic data is empty or not. This macro
only exists when the compilation is at the C++20 level and the
__VA_OPT__ construct is supported.
Usage
BOOST_PP_VA_OPT(x,y,
...)
(v)
Arguments
x
A tuple
whose data is the macro expansion if the variadic data is
not empty
y
A tuple
whose data is the macro expansion if the variadic data is
empty
,,,
The variadic
data to be checked for emptiness
Remarks
When the macro invocation BOOST_PP_VARIADIC_HAS_OPT() expands
to 1, then this macro exists and can be invoked, otherwise this
macro does not exist and attempting to invoke it will lead to a
preprocessor error that the macro can not be found. Because of
this condition the header file for including this macro includes
the header file for the BOOST_PP_VARIADIC_HAS_OPT macro.
The difference between this macro and the __VA_OPT__ construct
illustrates a limitation of the latter construct with a trade off
of simpler syntax. The differences between the __VA_OPT__
construct and this macro are:
- The __VA_OPT__ construct offers a choice as its expansion
only between its input preprocessing tokens or nothing (
called a "single placemarker token" ) depending on whether the
implicit variadic data is empty or not. There is no way using
the __VA_OPT__ construct to specify any alternative but the
"single placemarker token" when the variadic data is empty
whereas any preprocessing tokens can be specified when the
variadic data is not empty. With the BOOST_PP_VA_OPT macro the
user can specify as its expansion preprocessing tokens both
when the variadic data is empty and when the variadic data is
not empty.
- The __VA_OPT__ construct offers a simple syntax whereas this
macro is more verbose. The BOOST_PP_VA_OPT macro's first and
second parameters must be Boost PP tuples of data, in order to
expand to normal or variadic data, and the third parameter
must be the variadic data to check for emptiness, whereas the
__VA_OPT__ construct has an implied variadic data as
__VA_ARGS__ to check for emptiness and can specify its
expansion directly in terms of its input.
- The __VA_OPT__ construct can only be specified in the
replacement list of some macro, whereas the BOOST_PP_VA_OPT
macro can be used both as an alternative to the __VA_OPT__
construct in the replacement list of some macro and anywhere
else a macro can be used.
- It is impossible to have a left parenthesis '(' or a right
parenthesis ')' as preprocessing token data within the
__VA_OPT__ construct whereas both are possible as part of the
expanded data for the BOOST_PP_VA_OPT macro.
The exact BOOST_PP_VA_OPT equivalent to the construct of
'__VA_OPT__ ( pp-tokens )' in the replacement list of a macro is
'BOOST_PP_VA_OPT (( pp-tokens ),(),__VA_ARGS__)'.
See Also
Requirements
Sample Code
#include <boost/preprocessor/facilities/va_opt.hpp>
# if BOOST_PP_VARIADIC_HAS_OPT()
#define DATA
#define OBJECT OBJECT2
#define OBJECT2
#define FUNC(x) FUNC2(x)
#define FUNC2(x)
#define FUNC_GEN(x,y) (1,2,3)
BOOST_PP_VA_OPT((1),(2),DATA) // expands to 2
BOOST_PP_VA_OPT((3),(4),OBJECT) // expands to 4
BOOST_PP_VA_OPT((5),(6),FUNC(1)) // expands to 6
BOOST_PP_VA_OPT((7,8),(9,10),FUNC) // expands to 7,8
BOOST_PP_VA_OPT((1,2,3,4,5),(6,7,8,9,10),FUNC_GEN) // expands to 1,2,3,4,5
#endif
© Copyright Edward Diener 2019