3

I think is an old question but I don't get it.

I have a header routines.h, its functions file routines.cpp and the main file main.cpp.

In the header there is:

class myclass{
public:
static const double a;
void mymethod();
};

const double myclass::a=0.0;

The routines.cpp contains: #include"routines.h" and then define the methods.

main.cpp also has #include"routines.h".

This setup gives a link error: a it's already defined. 

public: static double const myclass::a" (?a148@myclass@@2NB) already defined in DBFLOWPAR2.obj

DBFLOWPAR2 is my main file.

If I define the methods in routines.h it works fine, but I don't like that. What else it's possible? I don't care how the variables are defined, I just want to be able to access myclass.a and find the right value in it.

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4 Answers 4

Reset to default
4

You should define the static variable in a cpp file.
Move,

 const double myclass::a=0.0;

to your cpp file.

Defining a in header file creates a copy of the variable in each Translation Unit where the header is included.

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7
  • All right, thanks. First to answer so I'll accept this one. Just curious: isn't the macro #ifndef Blala #define Blala #endif supposed to avoid that?
    – MarcoS
    Commented Mar 1, 2012 at 16:34
  • @MarcoS: no. Include guards prevent the definition from appearing multiple times in the same translation unit. They don't stop different translation units (.cpp files) from each including the header file and hence the definition.
    – Steve Jessop
    Commented Mar 1, 2012 at 16:39
  • @MarcoS Include guards prevent a header from being included multiple times in a single translation unit. Your problem is that const double myclass::a=0.0; is occurring multiple times in a single program, which is made up of multiple translation units. I think a good exercise for understanding C++'s build model is to write some programs without using header files. Just manually 'include' the declarations and definitions you need. See what happens when you; duplicate a class definition in a single TU, put a function def in more than one TU, put a variable def in more than one TU.
    – bames53
    Commented Mar 1, 2012 at 16:51
  • Just FMI is the =0.0 part necessary?
    – Luchian Grigore
    Commented Mar 1, 2012 at 18:57
  • @LuchianGrigore: No, it is not, static variables are Zero Initialized.
    – Alok Save
    Commented Mar 2, 2012 at 2:42
3

You have violated the One Definition Rule. Because you #include "routines.h" in multiple files, the definition of myclass::a appears in multiple files.

You must define a variable once, and only once.

Choose a convenient .cpp file, and move your definition to that .cpp.

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1

MyClass.h

class myclass{
public:
   static const double a;
   void mymethod();
};

MyClass.cpp

const double myclass::a;

The way you have it now, you're redefining myclass::a in every translation unit that includes the header.

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0

Don't put const double myclass::a=0.0; in the header file, otherwise each compilation unit including this file will have it's own separate variable. put this to routines.cpp.

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