boost/system/error_code.hpp defines "errc" as a collection of standard error codes (based on posix). It also defines a constructor for error_code and error_condition that allows them to accept a single parameter of an enum type when is_error_code_enum is defined to true for that enum type. It does not, however, actually do this for its own errc enum. As a result, code such as this fails to compile: boost::system::error_code ec(boost::system::errc::operation_canceled); Is there some good reason for this, or is it an oversight? It's certainly more annoying to have to explicitly specify the generic_category().