C++ multiple definition of struct functions

Question

Sorry for the noob question..

devf.h

#ifndef DEVF_H
#define DEVF_H

#include <string>

struct A {
    int ax();
    std::string ay(std::string jj);
};

struct B {
    struct A* f;            

    int bx() {
        return f->ax();
    }

    std::string by(std::string jj){
        return f->ay(jj);
    }        
};

#endif // DEVF_H

devf.cpp

#include "devf.h"

int A::ax() {
    return 5;
}

std::string A::ay(std::string jj){
    return jj;
}

I get this error:

multiple definition of `A::ax()'

multiple definition of `A::ay(std::string)'

How can I solve this problem? I want the definitions of ax and ay functions in header file and the implementions in .cpp

Have you used include guards? Something like `#ifndef DEVF_H / #define DEVFH / ... your code ... #endif` — alter_igel, Jan 16 '18 at 03:49
In your example of `devf.cpp` you have `#include "cni.h"` but where is `#include "devf.h"`? — Francis Cugler, Jan 16 '18 at 03:53
I also noticed that in `devf.h` you are including `` that doesn't appear to be needed and that you are not including ``... — Francis Cugler, Jan 16 '18 at 03:56
OHH! Sorry i will update the question, yes the code has #ifndef DEVF_H / #define DEVFH / — LeAdErQ, Jan 16 '18 at 03:58
What you show is correct, so the problem is in something you didn't show. What's the command you use to compile this? — Mark Ransom, Jan 16 '18 at 03:59
I believe the OP is a victim of `Circular Includes` which will result in multiple definitions. — Francis Cugler, Jan 16 '18 at 04:48

Francis Cugler · Accepted Answer · 2018-01-16T05:56:45.113

Try doing this with your code:

devf.h

#ifndef DEVF_H  // Add this line
#define DEVF_H  // Add this line

// #include <iostream> // removed this as not needed from what is shown
#include <string> // added this as is needed

struct A {    
    int ax() const; // added const as it doesn't change
    std::string ay( const std::string& jj ) const; // changed string to const ref & added const  
};

struct B {
    /*struct*/ A* f; // keyword struct not needed here.

    int bx() const { // added const
        return f->ax();
    }

    std::string by( const std::string& jj ) const { // changed to const ref & added const
        return f->ay( jj );
    }        
};

#endif // !DEVF_H  // Add this line

devf.cpp

#include "devf.h"

int A::ax() const {
    return 5;
}

std::string A::ay( const std::string& jj ) const {
    return jj;
}

Then you asked this or made this statement:

How can I solve this problem? I want the definitions of ax and ay functions in header file and the implementions in .cpp

devf.h

#ifndef DEVF_H 
#define DEVF_H

#include <string>

struct A {    
    int ax() const;
    std::string ay( const std::string& jj ) const;    
};

struct B {
   A* f;

   int bx() const;     
   std::string by( const std::string& jj ) const; 
};

#endif // !DEVF_H

devf.cpp

#include "devf.h"

int A::ax() const {
    return 5;
}

std::string A::ay( const std::string& jj ) const {
    return jj;
}

int B::bx() const {
    return f->ax();
}

std::string B::by( const std::string& ) const { 
    return f->ay( jj );
}

This should help from what you have shown.

I've tried it with this:

main.cpp

#include <iostream>
#include "devf.h"

int main() {

    B b;
    std::cout << b.bx() << std::endl;
    std::cout << b.by( "hello world" ) << std::endl;

    std::cout << "\nPress any key and enter to quit." << std::endl;
    char q;
    std::cin >> q;
    return 0;
}

output

5
hello world

Press any key and enter to quit.

- EDIT

I asked the OP: Are you including other header files?

The OP answered with:

Yes, the header "devf.h" is included in other headers

And I believe this is where the OP's problem resides. I believe that the OP is a victim of Circular Includes which will result in multiple definitions.

Example:

A.h

#ifndef A_H
#define A_H

#include "B.h" // circular include

struct A {
    int a;
    B b;
};

#endif  // !A_H

B.h

 #ifndef B_H
 #define B_H

 #include "A.h" // circular include

 struct B {
     int b;
 };

 #endif // !B_H

To resolve this circular include problem...

You need to use class or struct prototypes - forward declarations in the headers; works with pointer & reference types.
Only include other header files within a header file if the definition not the declaration is needed.
Once you have the class or struct's forward declaration in the header file remove that class's include directive and put it within its cpp file

fixing the above example:

A.h

#ifndef A_H
#define A_H

// Since A has a pointer to B we remove the include from here
// and replace it with a forward declaration or class prototype...
// #include "b.h" // cicular include
class B;
struct A {
    int a;
    B b;
};

#endif // !A_H

A.cpp

#include "A.h"
#include "B.h" // moved the include to here - prevents circular includes

B.h

 #ifndef B_H
 #define B_H

 // Since B does not require A we can omit this all together.
 //#include "A.h" // circular include

 struct B {
     int b;
 };

 #endif // !B_H

B.cpp

#include "B.h"

You can refer to this Q & A Resolve build errors due to circular dependency amongst classes on stack for a more detailed explanation.

Once you go through the rest of your code base and fix all of the circular includes you should have no error as the above set of classes do compile and run without error.

@LeAdErQ I've updated my answer to reflect on your answer to my last question in the comments about the includes... check out the updated answer to see if it helps you to resolve your issue(s). — Francis Cugler, Jan 16 '18 at 04:42
The other files just include the devf.h header. I can't understand why this is circular problem. — LeAdErQ, Jan 16 '18 at 04:50
@ Yes; and when you include header files in multiple locations that happen to be in other header files you end up getting the run-around which can and probably will result in `multiple definitions`. — Francis Cugler, Jan 16 '18 at 04:54
@LeAdErQ I edited again with a link to another `Q & A` from here on stack that has more detailed information about circular dependencies. — Francis Cugler, Jan 16 '18 at 04:56

C++ multiple definition of struct functions

1 Answers1