Question

I am currently writing a C++ library which will be required to compile with both GCC for linux and Sun CC for Solaris. In the interest of performance I am creating some classes which select different headers based on compiler; GCC with c++0x or TR1 or with niether and Sun CC RogueWave or STLPort. I’m sturggling to work out the best means of #ifdef’ing the typedefs, for example:

namespace project {

    #if defined(__GNUG__)
        #if defined(HAVE_CXXOX)
            #include <unorderd_map>
            typedef srd::unordered_map map;
        #elif defined(HAVE_TR1)
            #include <tr1/unordered_map>
            typedef std::tr1::unordered_map map;
        #else
            #include <map>
            typedef std::map map;
        #endif
    #elif defined(__SUNPROC_CC)
        #include <map>
        typedef std::map map;
    #endif

} //namespaces

Answer 1

This won’t work for two reasons:

1. Headers must be included outside of the scope of namespace project { ... }. (If the header contains nothing but templates and inline functions, it might work anyway, but I wouldn’t count on it.)
2. typedef doesn’t work on templates. There’s a workaround where you define an empty derived class.

So perhaps something like this:

#if defined(__GNUG__)
    #if defined(HAVE_CXXOX)
        #include <unordered_map>
        #define MAP std::unordered_map
    #elif defined(HAVE_TR1)
        #include <tr1/unordered_map>
        #define MAP std::tr1::unordered_map
    #else
        #include <map>
        #define MAP std::map
    #endif
#elif defined(__SUNPROC_CC)
    #include <map>
    #define MAP std::map
#endif

namespace myproject {
    template <class K, class V>
    class map : public MAP<K, V> {};
}

#undef MAP