Including
 - usage of static_cast instead of c-style cast,
 - single variable declaration per line,
 - removing const from return types
 - omit unnecessary string to char* conversions,

The used clang-tidy checks are:
google-readability-casting,
readability-const-return-type,
readability-container-size-empty,
readability-delete-null-pointer,
readability-deleted-default,
readability-else-after-return,
readability-function-size,
readability-identifier-naming,
readability-inconsistent-declaration-parameter-name,
readability-isolate-declaration,
readability-misleading-indentation,
readability-misplaced-array-index,
readability-named-parameter,
readability-redundant-*,
readability-simplify-*,
readability-static-accessed-through-instance,
readability-static-definition-in-anonymous-namespace,
readability-string-compare,
readability-uniqueptr-delete-release

clang-tidy modernize-use-transparent-functors check.

The `bool uncaught_exception()' is deprecated since c++17
and is replaced by `int uncaught_exceptions()'.

Only in ERR/WARN/INFO/DBUG/OGS_FATAL and similar, not in
formatted output like xml files.

Clang format run on near-the-header lines.

Happy New Year

This potentially saves a second copy compared to using const
reference. See Effective Modern C++, Item 41.

The access now must be done through some non-member functions.

Also checks are made publicly callable

const + mutable enable use as temporary variables
move assignment complements move construction