C++ templates declare in .h, define in .hpp

Question

I saw some code in which the developer defined a class template in a .h file, and defined its methods in a .hpp file. This caught me a bit by surprise.

Are there are

Answer 1

That sounds unusual to me. A template's definition and all specializations must be compiled along with its declaration, with the exception of export templates, a feature which effectively doesn't exist.

C++0x does introduce extern template declarations, which allow you to define explicit specializations in a different source file (translation unit). This exists already as an extension in GCC and probably other platforms.

Splitting into two files could help "hide" the implementation a little, or allow some kind of laziness with doxygen, maybe.

Aha! It's also possibly to improve compile time with precompiled headers. The compiler may cache headers on a per-file basis. A separate "implementation" header could then be modified without touching the "interface" header. But not vice versa, the implementation header would still take the bulk of the compile time, and the gain would be very fragile and dependent on the platform and specific changes made. In the end, PCH improves time over several source files, and optimizing header dependencies is pointless.