In Xcode developer Returning Undefined symbols for architecture x86_64:

Question

I am using Xcode 10.3 developer for a C++ project and after fixing my declaration, when I go to run it says build failed with the four errors at the bottom. The file with the main is under the two other files. I have attached the screenshot of the errors as well.

#include <strstream>
#include <vector>
using namespace std;
#include "expression.h"
#include "operand.h"
#include "variable.h"
#include "symboltable.h"
//create an object of SymbolTable
extern SymbolTable symbolTable;
//definition of the function evaluate()
int Variable::evaluate()
{
    //return the name from the symbolTable
    return symbolTable.lookUp(name);    
}

The header file variable.h is:

#include <strstream>
#include <vector>
using namespace std;
//define the class Variable subclass of the Operand
class Variable : public Operand
{
public:
    //define the constructor
    Variable(string name)
    {
        this->name = name;
    }    
    //define the function evaluate()    
    int evaluate();
private:
    string name;
};

module.cpp

#include <iostream>
#include <string>
#include <vector>
#include <fstream>
#include <sstream>
using namespace std;
#include "expression.h"
#include "subexpression.h"
#include "symboltable.h"
#include "parse.h"
//create an object of SymbolTable
SymbolTable symbolTable;
//prototype of the function
void parseAssignments(stringstream& in);
//define main function
int main(){
    //Variables
    Expression* expression;
    char paren, comma;
    string line;
    //create input file stream
    ifstream fin("input.txt");
    //check if the file is open
    //if file is not open error message
    if(!fin.is_open())
        perror("error with opening file!");
    //create loop to read from file content
    while(getline(fin, line)){
        symbolTable.init();
        if(!fin)
            break;
        stringstream in(line, ios_base::in);
        in >> paren;
        cout << line << " ";
        expression = SubExpression::parse(in);
        in >> comma;
        //call the function 
        parseAssignments(in);
        //display results
        int result = expression->evaluate();
        cout << "value = " << result << endl;
    }
    system("pause");
    return 0;
}
//define parseAssignments()
void parseAssignments(stringstream& in){
    char assignop, delimiter;
    string variable;
    int value;
    symbolTable.init();
    do{
        variable = parseName(in);
        in >> ws >> assignop >> value >> delimiter;
        symbolTable.insert(variable, value);        
    } 
    while (delimiter == ','); 

}

errors: Showing Recent Issues :-1: Undefined symbol: parseName(std::__1::basic_stringstream, std::__1::allocator >&) Showing Recent Issues :-1: Undefined symbol: SymbolTable::init() Showing Recent Issues :-1: Undefined symbol: SymbolTable::insert(std::__1::basic_string, std::__1::allocator >, int) Showing Recent Issues :-1: Undefined symbol: SymbolTable::lookUp(std::__1::basic_string, std::__1::allocator >) const