The BOOST_PP_IF macro chooses between to values based on a logical condition.
BOOST_PP_IF(cond, t, f)
The condition that determines if the result is expr or nothing.
Valid values range from 0 to BOOST_PP_LIMIT_MAG.
The result of the expansion if cond is non-zero.
The result of the expansion if cond is 0.
This macro performs a boolean conversion on its first argument.
If that conversion is unnecessary, use BOOST_PP_IIF instead.