$ cat foo.hpp 
#ifndef QWERTY123
#define QWERTY123


namespace XYZ {
    template<class T> class Foo {
        public:
            T x;
        virtual void bar() = 0;
    };

    template<class T> class Bar: public Foo<T> {
        virtual void bar() final;
    };
}
#endif

$ cat foo.cpp 
#include <iostream>
#include "foo.hpp"

namespace XYZ {
    template<class T>
    void Foo<T>::bar(){ std::cout << "bar print: " << x << std::endl; }

    template<> class Foo<float>;
    template<> class Foo<int>;
}

$ cat main.cpp
#include <iostream>
#include "foo.hpp"

int main()
{
    XYZ::Bar<int> f;
    f.x=25;
    std::cout << f.x ;
}

$ g++ -std=c++11 -c foo.cpp
$ ar crs libfoo.a ./foo.o
$ g++ -std=c++11 main.cpp -L./ -lfoo && ./a.out
undefined reference to `XYZ::Bar<int>::bar()'
collect2: error: ld returned 1 exit status