Question

I am new to C++.

So I am creating a program which gathers then displays information about book etc. First, I want to learn little more about code I am doing but I am unfamiliar with c++ error codes.

I have created 2 classes Book and Publisher where each contains its own constructors and methods. After I tried to make my program including all classes and methods that I am given in my class diagram I came to halt when I received error saying:

1>Publisher.obj : error LNK2005: "class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __cdecl getPublisherInfo(void)" (?getPublisherInfo@@YA?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@XZ) already defined in Book.obj
1>Publisher.obj : error LNK2005: "void __cdecl setAddress(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >)" (?setAddress@@YAXV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@Z) already defined in Book.obj
1>Publisher.obj : error LNK2005: "void __cdecl setCity(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >)" (?setCity@@YAXV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@Z) already defined in Book.obj
1>Publisher.obj : error LNK2005: "void __cdecl setName(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >)" (?setName@@YAXV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@Z) already defined in Book.obj
1>C:\Users\pc\Desktop\School\ITS 340\Labs\Lab 1\Lab 1\Debug\Lab 1.exe : fatal error LNK1169: one or more multiply defined symbols found

This is my Publisher.cpp file:

#include <iostream>
using namespace std;

class Publisher
{
    public: 
        Publisher();
        Publisher(string name, string address, string city);
        string getPublisherInfo();
        void setName(string name);
        void setAddress(string address);
        void setCity(string city);

    private:
        string name;
        string address;
        string city;
};

Publisher::Publisher()
{
}

Publisher::Publisher(string name, string address, string city)
{
}

string getPublisherInfo()
{
    return 0;
}

void setName(string name)
{
}

void setAddress(string address)
{
}

void setCity(string city)
{
}

How can I avoid this error?

Answer 1

You are breaking the one definition rule. You need to declare classes and functions in header files, protect them with include guards , and only put implementations or definitions of functions and class member functions in your .cpp files.

For example:

Foo.h:

#ifndef FOO_H_
#define FOO_H_
class Foo {
  Foo();
  void foo() const;
};
#endif

Foo.cpp:

#include "Foo.h"
Foo::Foo() { .... }
void Foo::foo() const { .... }

main.cpp

#include "Foo.h"
int main()
{
  Foo f;
  f.foo();
}

This is completely independent of OOP. You would have to do the same with free functions.