c++ : multiple definition of 'namespaceName'

kpachopoulos · 03-09-2009, 09:07 AM

Hallo,
the following code hasn't got any meaning, but i'd like to know, what the error "multiple definition of 'Company2" in the given context means.

Main.cpp

Code:

#ifndef PERSON_CPP
#define PERSON_CPP
	#include "Person.cpp"
#endif

int main() {
	company::Person p;	
}

Person.h

Code:

#ifndef STRING_H
#define STRING_H
	#include <string>
#endif

using namespace std;	
	
namespace company {
	struct Person {
		string name;
		int age;
	};
}

Person.cpp

Code:

#ifndef PERSON_H
#define PERSON_H
	#include "Person.h"
#endif
#include <string>

using namespace std;
	
namespace company2 {
	
	string Person2Str(company::Person) {
		return "aperson";
	}
}

dwhitney67 · 03-10-2009, 08:18 AM

Please remove the multiple-inclusion guards from the .cpp files. They are not necessary, nor should they ever be used other than in a header file.

Also, do not include a .cpp file within another .cpp file; that is crude programming.

Lastly, never specify a "using namespace" in a header file.

Here's a better approach:
Person.h:

Code:

#ifndef PERSON_H
#define PERSON_H

#include <string>

namespace company
{
  struct Person
  {
    std::string name;
    int         age;
  };
}

namespace company2
{
  std::string Person2Str(const company::Person& p);
}

#endif

Person.cpp:

Code:

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

namespace company2
{
  std::string Person2Str(const company::Person& p)
  {
    return p.name;
  }
}

Main.cpp:

Code:

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

int main()
{
  using namespace company;

  Person p = { "Fred", 30 };

  std::string name = company2::Person2Str(p);
  ...
}

To compile multiple modules:

Code:

g++ Main.cpp Person.cpp -o myprog

-----------

EDIT: Sorry for the post-edits; your code still confuses me. But hopefully what I have above will clarify things. I have not attempted to compile the code, so you may still have trouble.

graemef · 03-10-2009, 10:50 PM

As dwhitney67 pointed out your header guards are wrong. What I suspect is happening is that you have added Person.cpp to your project so it gets compiled and so company2 exists. Then the compiler comes to Main.cpp and it is asked to include a cpp file, something that it will do but you shouldn't (!) and so once more a company2 is created. Hence the error.

Notice how in dwhitney67 solution the header guard is just in the Person.h file and that it embraces the whole file contents.