Starting in GCC 12, the compiler emits a warning when std::unary_function/std::binary_function is used (they haven't gone as far as removing these for C++ standard versions beyond 14 as mandated). This affects boost::hash as it conditionally uses std::unary_function based on BOOST_NO_CXX98_FUNCTION_BASE, which, correctly, is still undefined for GCC 12.

There are various alternatives to deal with this:

• Stop using std::unary_function altogether (unlikely, it may break C++03 user code).
• Stop using it based on a combination of BOOST_NO_CXX98_FUNCTION_BASE and __cplusplus.
• Wrap in warning-stopping pragma.

Thank you,