How do I separate test types with the template extern keywords?

Question

Suppose I have some type X which is templated on T :

// hpp file
template <typename T>
struct X
{
  int Get();
};

extern template X<TraitsType>;

// cpp file
template <typename T>
int X<T>::Get() { return T::Get(); }

template X<TraitsType>;

The type T might be a random number generator in a production environment ( TraitsType ), but for the sake of unit tests it might be a type that will generate predictable or fudgible values (say, TestTraitsType ). If I want to use this TestTraitsType I now have to scatter extern template and template declarations for X<TestTraitsType> throughout my hpp and cpp files, which is undesirable because I am polluting my production code with unit test code.

How can I avoid this while keeping these type instantiations in separate translation units?

Answer 1

Move method definitions in (private) header .inl (or .hxx )

•  // inl file template <typename T> int X<T>::Get() { return T::Get(); } 

•  // cpp file #include "X.inl" template X<TraitsType>; 

•  // test file #include "X.inl" template X<TestTraitsType>;