Member Avatar for Sudo Bash

I have two questions relating to classes. I have run into a problem in using classes in multi-file programs. Here are three example files which illustrate the problem I am having and the errors I am getting.

file1.cpp

//file1.cpp
#include<iostream>
#include"file3.h"

using std::cout;
using std::cin;

namespace baz
{
    extern MyClass foobar;
}

int main()
{
    baz::foobar.setFOO();
    
    baz::foobar.bar();
    cout << *baz::foobar.FOO << "\n";
    
    cin.ignore();
    
    return 0;
}

/*errors:
         
         In function `main':
                     [Linker error] undefined refference to `baz::foobar'
                     [Linker error] undefined refference to `MyClass::setFOO()'
                     [Linker error] undefined refference to `baz::foobar'
                     [Linker error] undefined refference to `MyClass::bar()'
                     [Linker error] undefined refference to `baz::foobar'
         Id returned 1 exit status
*/

file2.c

//file2.c

#include"file3.h"

namespace baz
{
    MyClass foobar;
}

void baz::foobar.bar(void)
{
     foo = 42;
}

void baz::foobar.setFOO(void)
{
     FOO = &foo;     
}

file3.h

//file3.h

class MyClass
{
      private:
      int foo;
      
      public:      
      void bar(void);
      void setFOO(void);
      int * FOO;
};

What I was wanting this program to do was display 42 and wait for [Enter] to be pressed; however, it will not compile. As far as I can tell the main program needs to see a function prototype, but if I place

void baz::foobar.bar(void);
void baz::foobar.setFOO(void);

in file3.h then file3.h gets an error! How should I make a prototype?

My other question is how I would be able to get rid of the setFOO() function and say something like

const int & FOO = foo;

inside the class definition. My compiler says this is not allowed. I want FOO to be visible globaly and to reflect the contents of foo but be unmodifiable.

Thanks in advance. :)

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  • Latest Post Latest Post by programmersbook

Dani AI

Generated

Good catch by : your undefined reference errors came from declaring baz::foobar without providing a single definition the linker could see, and from not linking all translation units into one project. One more gotcha: naming a C++ source file file2.c often makes IDEs compile it as C, which will not understand C++ namespaces or member definitions. Rename it to .cpp and ensure all three files are in the same project and being linked.

For a clean multifile layout, declare the object in the header and define it once in a .cpp. Also prefer read-only accessors over exposing raw pointers.

Header (add this to your existing header):

namespace baz {
  class MyClass;          // if needed
  extern MyClass foobar;  // declaration only (no storage)
}

Implementation (in exactly one .cpp):

#include "file3.h"

namespace baz {
  MyClass foobar; // single definition with storage
}

Instead of a public int* FOO plus a setFOO(), make the member private and expose a const view. For example, add a const accessor to your class (keeps callers from modifying the value):

// inside class MyClass
public:
  const int& FOO() const { return foo; }  // read-only reference to the private member

Use it like this:

std::cout << baz::foobar.FOO() << '\n';

If you truly want a global read-only alias, declare it in the header and define it in the same .cpp that defines foobar so initialization order is well-defined:

// file3.h
namespace baz {
  extern const int& FOO;  // global, read-only view
}

// file2.cpp
#include "file3.h"
namespace baz {
  MyClass foobar;                // object lives here
  const int& FOO = foobar.FOO(); // bind after foobar exists
}

Note: you cannot write const int& FOO = foo; inside the class body; reference data members must be bound in a constructor initializer list, which is why an accessor or the TU-level alias above is safer.

Recommended Answers

All 9 Replies

Member Avatar for alexchen

Foo is an undefined variable. Use other words instead of foo or FOO.

Edited by alexchen because: n/a
Member Avatar for Sudo Bash

Are you saying that in the baz::foobar.bar() function I should say

baz::foobar.foo = 42;

I did say

int foo;
...
const int * FOO;

inside MyClass, so foo and FOO should exist.

Member Avatar for programmersbook

Some things got messed up it should be like this:

main.cpp

//file1.cpp
#include<iostream>
#include"file3.h"

using std::cout;
using std::cin;

namespace baz
{
    extern MyClass foobar;
}

int main()
{
    baz::foobar.setFOO();
    
    baz::foobar.bar();
    cout << *baz::foobar.FOO << "\n";
    
    cin.ignore();
    
    return 0;
}
//file3.h

namespace baz{

class MyClass
{
      private:
      int foo;
      
      public:      
      void bar(void);
      void setFOO(void);
      int * FOO;
};

}
//file2.c

#include"file3.h"

namespace baz
{
    MyClass foobar;

    void MyClass::bar(void)
    {
         foo = 42;
    }

    void MyClass::setFOO(void)
    {
         FOO = &foo;     
    }

}
Member Avatar for Sudo Bash

:icon_redface: oops! The changes you made to file2.cpp made it so file 2.cpp compiles with no errors, but file1.cpp still has the same errors. (This is my first attempt at classes so I am sorry about the simple errors.)

Member Avatar for programmersbook

hmm, it compiles without problems with gcc.

How do you compile it?

Member Avatar for Sudo Bash

It works! :icon_biggrin: I am using Dev-C++. I would have compiled it with g++ on my linux computer but, it bit the dust a while back. I just got a win XP computer and put Dev-C++ on it. The problem was with my lack of knowledge of Dev-C++. I didn't have all three files properly put into a project file. Thank you for your help! :)
I just read about classes yesterday and I am really eager to try them out. Can anyone tell me why this code doesn't work?

class baz
{
  private:
  int foo;

  public:
  const int & FOO = foo;
}

I want to make it so that everyone can see what is in foo via FOO but, only foo is modifiable.

Member Avatar for programmersbook

You have to initilialize the FOO in constructor, FOO is a reference and has to initialize in contructor

class baz
{
  private:
  int foo;

  public:
  const int & FOO;
  baz() : FOO(foo) {} // constructor
 ~baz(){ } // destructor

};
Edited by programmersbook because: n/a
Member Avatar for Sudo Bash

Thanks for all of your help!

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