exprtk | C++ Mathematical Expression Parsing And Evaluation Library | Math library

I'm trying to learn how to use the parsing library exprtk with objects of a custom class - I'm still learning C++, tbh but I can write functional non-OO code.

I am trying to follow an example from the repo, but with objects that I'd have created.

Here's my code:

Class definitions:

I've been trying to get into C++ again, and I have a linker error when I try to compile :

main.cpp:(.text+0x92): undefined reference to `eval(std::string, double, double, double)'

I've already tried compiling each file separatly and then linking by hand but it never works.

I have tried numerous solutions provided by other similar posts but none of these work and I don't have any idea on how to fix my problem (which doesn't seem to be that hard)

main.cpp :

I am having trouble getting my function to return a variable properly.

I print the variable that I want to return above the return statement, and it looks fine. Once I attempt to return the value and print it onto the console though it instead prints -nan(ind). I do not understand why this is happening.

I am programming in C++, using Visual Studio. I am using this library to parse strings into expressions: http://www.partow.net/programming/exprtk/index.html

Here is the function and the statement that prints its results:

I'm trying to make a derivative calculator (I have to use linked list).

The structure of every node should be as follows:

I'm creating an expression with exprtk using variables which change constantly.

Do I have to reset and recompile the exprtk::expression using an updated exprtk::symbol_table everytime I change the value of a variable?

Or are the updated values evaluated directly by the existing, compiled expression?