boost::hash's extensions are
generally useful, you might want to turn them of in order to check that your
code will work with other implementations of TR1. To do this define the macro
this macro is defined, only the specialisations detailed in TR1 will be declared.
But, if you later undefine the macro and include <
then the non-specialised form will be defined - activating the extensions.
It is strongly recommended that you never undefine the macro - and only define it so that it applies to the complete translation unit, either by defining it at the beginning of the main source file or, preferably, by using a compiler switch or preference. And you really should never define it in header files.
If you are writing a library which has code in the header which requires the extensions, then the best action is to tell users not to define the macro. Their code won't require the macro.
Translation units that are compiled with the macro defined will link with units that were compiled without it. This feature has been designed to avoid ODR violations.