Macros defined by the library are prefixed with BOOST_METAPARSE_V1_. For example BOOST_METAPARSE_V1_STRING. It is defined in the <boost/metaparse/v1/string.hpp> header file. The library provides the <boost/metaparse/srting.hpp> header file as well, which includes the definition of this macro and provides the following definition:

#define BOOST_METAPARSE_STRING BOOST_METAPARSE_V1_STRING

This makes it possible to use the macro as BOOST_METAPARSE_STRING. Future versions of the library may define other versions of this macro (eg. BOOST_METAPARSE_V2_STRING). BOOST_METAPARSE_STRING will refer to the latest version.

This versioning does not affect macros that may be overridden by the code using the library. (eg. BOOST_METAPARSE_LIMIT_STRING_SIZE)