Cannot Pass Lambda Function into Templated Class C++ [duplicate]

This question already has an answer here:

• Why can templates only be implemented in the header file? 14 answers

I have the following code:

template <class T>
BinaryHeap<T>::BinaryHeap(std::function<bool(T, T)> isHigherPriority)
{
    m_isHigherPriority = isHigherPriority;
}

...

std::function<bool(int, int)> isHigherPriority = [](int a, int b) { return a <= b;  };
BinaryHeap<int> heap(isHigherPriority);

The code seems to compile fine, but I get linker a linker error: LNK2019 unresolved external symbol "public: __thiscall BinaryHeap::BinaryHeap(class std::function)" (??0?$BinaryHeap@H@@QAE@V?$function@$$A6A_NHH@Z@std@@@Z)

Is there any reason why the linker would get confused?