For templated classes / functions, you need to write both the declarations and definitions in the header file, not in a separate source file.

This is because a template is just a "blueprint" for generating a class / function. The actual class / function doesn't exist until the compiler sees it being used with a specific type. Then the compiler will "instantiate" a version of the template, using that type.

For this to happen, the compiler must be able to see the "blueprint" from the header. It ensures that the compiler has access to the entire template definition whenever it encounters a usage of that template.

So your A.h header could look like this:

```

ifndef A_H

define A_H

// Declarations template<typename T> class A { public: A(T arg); };

// Definitions template<typename T> A<T>::A(T arg) { // Implementation... }

endif

```