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scylladb/utils
Nadav Har'El e0c13518ae config: suppress named_value<T> instantiation in every source file
config.hh is included by a large fraction of the codebase. It pulls in
utils/config_file.hh, whose named_value<T> template has its method
bodies defined in config_file_impl.hh. Those bodies depend on three of
the heaviest Boost headers – boost/program_options.hpp,
boost/lexical_cast.hpp, and boost/regex.hpp – as well as yaml-cpp.
Because the method bodies are reachable from config.hh, every
translation unit that includes config.hh was silently instantiating all
of named_value<T>'s methods (for each distinct T) and compiling that
Boost/yaml-cpp machinery from scratch.

Fix this by adding extern template struct declarations for all 32
distinct named_value<T> specialisations used by db::config:
- the 14 primitive/stdlib types go into utils/config_file.hh
- the 18 db-specific types (enum_option<…>, seed_provider_type, etc.)
  go into db/config.hh

Matching explicit template struct instantiation definitions are added in
db/config.cc, which is already the only translation unit that includes
config_file_impl.hh.  As a result the Boost/yaml-cpp template machinery
is compiled exactly once (in config.o) instead of being re-instantiated
in every including TU.

One subtlety: named_value<seed_provider_type> has an explicit member
specialisation of add_command_line_option.  Per [temp.expl.spec], such
a specialisation must be declared before any extern template declaration
of the enclosing class template, so a forward declaration of the
specialisation is added to config.hh ahead of the extern template line.

Also, for some of the types we explicitly instantiated in db/config.cc,
the named_value<T> constructor calls config_type_for<T>(), which we
also need to provide explicit specializations - some of them we already
had but some were missing.

Signed-off-by: Nadav Har'El <nyh@scylladb.com>
2026-03-11 11:30:39 +02:00
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