C++ :: Constructors / Classes For Expression Tree Program

Previously I made a infix postfix calculator with integers only using structures, which had only three files

1.main.cpp,
2.arithmetic_expression.cpp
3.arithmetic expression.h,

I manged eventually to bring it together. now there are 5 files which are using classes, constructors and destructors and there can be letters in the expression that can have assigned values. Newnode, struct node and all that fine and dandy stuff made sense, this doesn't. What needs to be done in those empty functions in arethmetic_expression.cpp? It was going well until I reached createxpressiontree{ in th arithemetic_expression.cpp and those constructors in there. At that point I missed my structures and was totally confused by the constructors. I know constructors are supposed to create objects, but then I dont know what i am creating the object for and what that object is supposed to do.

1.main.cpp
2.arithmetic_expression.cpp
3.tree.cpp
4.arithmetic_expression.h
5.tree.h

Main.cpp

Code:

#include <iostream>
#include <string>
#include <vector>
#include "arithmetic_expression.h"
#include <map>
int main() {
// arithmetic_expression expression1;
// Testing the RPN input

[Code] .....