What requirements does the standard impose on the Value type of unordered_set in terms of definition visibility

I'm creating a class that contains an unordered_set with its own hash and predicate as follows:

//SetHolder.h
#include <unordered_set>

struct SetHolder
{
    SetHolder(); //Defined in SetHolder.cpp
    ~SetHolder(); //Defined in SetHolder.cpp

    struct ArtifactImpl; //Defined in SetHolder.cpp
    struct ArtifactSetKeyOps
    {
      std::size_t operator()(
        const ArtifactImpl& artifact) const noexcept;
      bool operator()(
        const ArtifactImpl& lhs, const ArtifactImpl& rhs) const;
    };

    std::unordered_set<ArtifactImpl,
      ArtifactSetKeyOps,ArtifactSetKeyOps> artifactSet_;
};

I'm using the gcc 4.8.2 compiler under ubuntu linux (stdlibc++), and I've observed that it compiles when only declaring ArtifactImpl if I only use unordered_set from the cpp file. Yet, I need to provide the definition of ArtifactSetKeyOps (why, I don't understand).

• What requirements does the standard impose on the Value type of unordered_set in terms of definition visibility at the time of declaring unordered_set?
• Is this code supposed to work on all platforms? Also, if so, why does the Hash and the Predicate need to be visible during the declaration of unordered_set?