How to organize the Template Files in C++

Normally you put class definitions in a header file and method definitions in a source file. Code that creates or uses objects of the class #includes the header file and obtains access to the method
code via the linker.

But the Template files can not organized like the non-template files. If you define the template class by organizing the declaration file (.h) and the definition file (.cpp) Normally. You will
get some linking errors after building the project.

But why templates don't work this way? 

The reason is clear:

When the compiler encounters template method definitions, it performs syntax checking, but doesn't actually compile the templates. 
When the compiler encounters an instantiation of the template, such as Ruler<int> myRuler, it writes code for an int version of the  Ruler template by replacing each T in the template class definition
with int. 
So, if you don't instantiate a class template for any types in your program, then the class method definitions are never compiled.

Then how to organize the Template Files? There are 3 methods listed below can be used.
1.
You can place the method definitions directly in the same header file where you define the class itself. When you #include this file in a source file where you use the template, the compiler
will have access to all the code it needs.

2.
You can place the template method definitions in a separate header file that you #include in the header file with the class definitions. Make
sure the #include for the method definitions follows the class definition; otherwise the code won't compile.
But method implementations look strange in header files. 

template <typename T>
class Ruler
{
    // Class definition omitted for brevity
};
#include " Ruler_Def.h"

3.
You can do by #include the method implementation source file(.cpp) in the template class definition header file, must follows the class definition.

template <typename T>
class Ruler
{
    // Class definition omitted for brevity
};
#include " Ruler.cpp"

When using this technique, make sure you don't add the  Ruler.cpp file to your project, because it is not supposed to be, and cannot be compiled separately; it should only be #included in a header file.

You can actually call your file with method implementations anything you want. Some programmers like to give source files that are included an .inl extension.