Templates & unresolved external symbol

Question

let's say I am trying to do something like this:

header.h:

template <typename T>
class foo { 
private: 
    // members...
public:
    foo();
    ~foo();
}

source.cpp:

template <typename T>
foo::foo() {
    // do something... 
}

template <typename T>
foo::~foo() {
    // do something... 
}

but I keep getting error:

error LNK2019: unresolved external symbol ...

I am working on Microsoft Visual Studio 2015;