#include <unordered_map>

#define LIBRARY
#if defined(LIBRARY)
#  define EXPORT __declspec(dllexport)
#else
#  define EXPORT __declspec(dllimport)
#endif


struct Base {
    double d1;
    double d2;
    double d3;
    /* It can be built if I uncommon the following code. */ 
    //bool operator==(const Base& rhs) const {
    //    return true;
    //}
};

struct EXPORT Derived : public std::unordered_map<int, Base> {};

int main() {
    Derived d;

    return 0;
}

It can be built if I defined operator== in Base or I remove the EXPORT sign in front of Derived.
I mainly confused how does the EXPORT sign affect the building process?