multiple definition: error at link time

问题

I'm writing a couple of classes for an exercise on copy control members in the C++ primer book.

I put the classes definitions in their respective header files,and implementation in code files. I seem to have done all things like they should be done, but I get an error at link time I can't understand (and I already spent half an hour thinking about it)

here's the message class (headers and code files)

message.h

#include <string>
#include "folder.h"

class message {

  friend void swap(message&,message&);

  public:
    explicit
    message(const std::string& =std::string());
    // copy control
    message(const message&);
    ~message();
    message &operator=(const message&);
    // interface
    void save(folder&);
    void remove(folder&);

  private:
    // data members
    std::string text;
    std::set<folder*> folders;
    // utility functions
    void add_to_folders(const message&);
    void remove_from_folders();

};

void swap(message &m1,message &m2)
{
  swap(m1.text,m2.text);
  swap(m1.folders,m2.folders);
}

message.cpp

#include "message.h"

message::message(const std::string &t) : text(t) {}

message::message(const message &other) : text(other.text),folders(other.folders) 
{
  add_to_folders(other);
}

message::~message() 
{
  remove_from_folders(); 
}

message& message::operator=(const message &other)
{
  remove_from_folders();
  text=other.text;
  folders=other.folders;
  add_to_folders(other);
  return *this;
}

void message::save(folder &f)
{
  folders.insert(&f); 
  f.addMsg(this); 
}

void message::remove(folder &f)
{
  folders.erase(&f);
  f.remMsg(this);
}

void message::add_to_folders(const message& m)
{
  for (auto i:m.folders)
    i->addMsg(this);
}

void message::remove_from_folders()
{
  for (auto i:folders)
    i->remMsg(this);
  folders.clear();  
}

and here's the folder class

folder.h

#include <set>

class message;

class folder {

  public:
    /*folder();
    folder(const folder&);
    ~folder();
    folder &operator=(const folder&);*/
    void addMsg(message*);
    void remMsg(message*);

  private:

    std::set<message*> messages;

};

folder.cpp

#include "folder.h"

void folder::addMsg(message *m)
{
  messages.insert(m);
}

void folder::remMsg(message *m)
{
  messages.erase(m);
}

Every message holds a collection of pointer to folders and vice versa. Each file compiles fine separately, but when I link the three of them I get an error saying that the swap function (that I defined in the message class header) is redefined.

main.cpp

#include "message.h"

int main()
{
  folder f1;
  message m1;
}

What am I doing wrong?

回答1:

You are including the header file message.h in both message.cpp and main.cpp. Since you have:

void swap(message &m1,message &m2)
{
  swap(m1.text,m2.text);
  swap(m1.folders,m2.folders);
}

In message.h you now have the swap function declared in both the main.cpp translation unit and the message.cpp translation unit. This will cause a multiple definition error. You need to move swap() into message.cpp

回答2:

Did you remember to put include guards in your header files?

#ifndef __MY_HEADER__
#define __MY_HEADER__

...

#endif

or if you're comfortable using features outside the C++ standard:

#pragma once

...

回答3:

The solution to your problem is very simple: you have declared void swap( data1, data2 ) as a friend function in your message class then you define it at the bottom of your *.h file. Take the implementation out of the *.h file and place it into your message.cpp file before any of your classes functions. Make sure you have the definition(implementation) before your classe's constructor in the *.cpp file since your class object will rely on this swap function. This should resolve your issue on the build process. Yes each of the 3 files will compile successfully but the solution will not build because it can not resolve this method since you have it defined in your *.h file. Also you are using the class prototype message in your folder.h file you should also have the #include "message.h" in your folder.cpp file, but this will require you to have your *.h files with the #ifndef ... #define... #endif directives. This should help you!

来源：https://stackoverflow.com/questions/32504570/multiple-definition-error-at-link-time