Why put dllexport after class declaration will eliminate C4251 warning in C++

Question

In MSVC, When I write a class with a std::unordered_map member like below,

class DLLEXPORT TTT
{
    std::unordered_map<int, int> _map;
};

a C4251 warning is given:

warning C4251: 'TTT::_map' : class 'std::unordered_map<_Kty,_Ty>' needs to have dll-interface to be used by clients of class 'TTT'  ...

where DLLEXPORT macro is in normal pattern, which is expand to __declspec(dllexport) or __declspec(dllimport) in the right place.

My question is: if put DLLEXPORT after this class declaration, the C4251 warning is gone, WHY?

class TTT
{
    unordered_map<int, int> _map;
};
class DLLEXPORT TTT;