Multiple definition error on the same line. (C++) - c++

Question

I have a new complicated problem. The compiler complains that I am redefining a function, but it says that the first place I declared it at has the site of re-declaration. The problem began as soon as I included the cpp file in another. In attempt to fix my problem I exported it to a hpp file, but to know avail. Here is my code.
main.cpp:
#include <iostream>
#include <string>
#include "main.hpp"
using namespace std;
int main(int argc, char *argv[])
{
//Deal with arguments and send them to the correct functions
if (argc >= 2){
string op = argv[1];
if (op == "-a" || op == "--automatic"){
if (argc >= 3){
string FName = argv[2];
bool dbgbool;
if (argc == 4){
string dbgstring = argv[3];
if (dbgstring == "debug"){
dbgbool = true;
}
}
Lexer(FName, dbgbool);
}
}
else{
cout << "Invalid Argument\n";
goto help;
}
return 0;
}
//Or, just write help and info
help:
cout << "\n";
cout << "bwc v0.0.1U-(Unstable)\n\n";
cout << "Usage: bwc <operation> [...]\n";
cout << "Operations:\n";
cout << " bwc {-a --automatic} <file(s)>\n";
cout << " bwc {-i --interactive}\n";
cout << " bwc {-c --error-codes}\n";
cout << "\n";
return 0;
}
LA.cpp:
#include<iostream>
#include<fstream>
#include<string>
#include<sstream>
using namespace std;
string Lexer(string FileN, bool dbg){ //This is the line of re-declaration.
//If debugging,this writes out put to the console
if (dbg == true)
cout << "Beginning Lexical Analysis...\n";
//Create new file stream and set it equal to the source file
ifstream Ifile (FileN.c_str());
//Test if the last step failed, if so, write an error to the console, and terminate the compiler
if (!Ifile.is_open()){
cout << "Unable to open file. Path to file may not exist, or the file name could be incorrect.\n";
cout << "Error Code: -1\n";
return NULL;}
//Create new stringstream, and set it equal to the source file
string IFStream;
Ifile >> IFStream;
//Close the source file
Ifile.close();
//If debugging,this writes out put to the console
if (dbg == true)
cout << "Source file sucessfully read.\n";
//Set out stream equal to the modified in stream
string OFStream = IFStream;
return OFStream;
}
and finally,
main.hpp:
#ifndef MAIN_HPP_INCLUDED
#define MAIN_HPP_INCLUDED
#include "LA.cpp"
extern string Lexer(string,bool);
#endif // MAIN_HPP_INCLUDED
Thanks,
Brooks Rady

Answer 1

Your main.cpp is including main.hpp which is including LA.cpp, so the contents of LA.cpp are being compile once for LA.cpp and once for main.cpp.
.hpp files should contain only declarations (string Lexer(string,bool);), while the definitions (string Lexer(string,bool) {... }) should go in the .cpp
You will not see this kind of issue when you are dealing with class methods, because the compiler accepts definitions of methods. But functions should be defined only in the .cpp files.