c++ template and header files

So, I heard that C++ templates shouldn't be separated into a header (.h) and source (.cpp) files.

For instance, a template like this:

template 
class J
{   
   T something;
};

Is this true? Why is it so?

If because of that I'm gonna have to put both declaration and implementation in the same file, should I put it in a .h file or a .cpp file?

Answer

Headers.

It's because templates are instantiated at compile-time, not link-time, and different translation units (roughly equivalent to your .cpp files) only "know about" each other at link-time. Headers tend to be widely "known about" at compile-time because you #include them in any translation unit that needs them.

Read https://isocpp.org/wiki/faq/templates for more.