Refactor .extend() and .remove() to use FExpr

[samukweku]

WIP for #2562

[oleksiyskononenko]

Instead of a class attribute, we should use a template parameter bool EXTEND like we just did for SUM. This way compiler will create two classes: one for EXTEND == true and another for EXTEND == false, streamlining all the if (EXTEND) ... conditions within those classes.

[samukweku]

@oleksiyskononenko could you kindly have a look at my latest commit. where am I getting it wrong with my knowledge of templates

[oleksiyskononenko]

Sure, the way this template should be implemented is exactly the same as for many other classes we have recently created. Could you please explain me what is the issue you are facing?

[samukweku]

this is the error I am getting:

src/core/expr/fexpr_extend_remove.cc:28:1: error: invalid use of template-name ‘dt::expr::FExpr_Extend_Remove’ without an argument list
   28 | FExpr_Extend_Remove::FExpr_Extend_Remove(ptrExpr&& arg, ptrExpr&& other) :
      | ^~~~~~~~~~~~~~~~~~~

my google fu hasnt fetched me anything helpful

[samukweku]

i basically add the template<bool EXTEND> to fexpr_extend_remove.h and fexpr_extend_remove.cc

[oleksiyskononenko]

Since you keep template declarations in the .h file and all the definitions in .cc, you need to explicitly specify for which EXTEND values, i.e. true and false, you're creating implementations outside of the header file. Also, definitions of the class methods should all include the template parameter. Take a look at the changes I've just pushed.

[samukweku]

Thank you @oleksiyskononenko ... It was hard for me to decipher this solution based on the error message. Thanks agaon

[oleksiyskononenko]

Yeah, error messages for templates are not really helpful. The standard way to implement it is to put everything in the header file. Once you split the declarations/definitions, some issues may arise. If you need more info on this, google for “C++ template instantiation”.

[oleksiyskononenko]

@samukweku I pushed some changes to this PR. See if they look good to you.

[samukweku]

Thanks for the review and update @oleksiyskononenko. Looks great