Static variables and header files

I was wondering if there's a way around having to declare static variables only once when writing classes which are just in header files. I didn't think it could be done, but looking around the boost libraries, I've noticed that there are some libs which are entirely contained in header files but still seem to have static members. You can't just declare it outside of the class can you? Including the file in different translation units means a multiply defined static object, no?

This may not be what you want, but you can use the keyword "extern" in this case

header file

Code:

---

class Foo
{
public:
  Foo();
  ~Foo();

  static int s_value;
};

extern Foo fo; // object

---

.cpp file

Code:

---

#include "foo.h"

Foo fo; // declares this class object can be used elsewhere

int Foo::s_value = 5;

---

Please ignore this if it is not what you want. I was trying to understand what you were
looking for. In general, delcare static members outside the class in the .cpp header refering to the header class defintion file.

Thanks for the reply, but what you're saying to do is the only way I know to do that. The thing I'm wondering is how you implement the declaration of, in your example, Foo::s_value without having a .cpp file. It must be possible but I can't figure out how. Any ideas?

You can do this:

Code:

---

int Foo::s_value = 5

---

in the header file which contains that class. Just inialize the data member with the value
under the class. I personally think it goes against the idea of seperating declaration from
implementation. But that is just my view, it is by no means an absolute

>> looking around the boost libraries, I've noticed that there are some libs which are entirely contained in header files but still seem to have static members.

Do you have an example? Do you have your own small example of what you are trying to do?

> I've noticed that there are some libs which are entirely contained in header files but still seem to have static members.

Are you sure you aren't looking at const static members?

Sorry about the late reply.

The idea does go against seperation of code, but I prefer the idea of having a library that doesn't need to be linked with to work. It's more just for learning though. The example which made me think I could do this was Boost.Asio. If you look here, the class has static member functions which seem like they have to be non-const (I could be wrong though). I don't see how you can declare a static function/variable without having it in an isolated file. Can you?

Well, to start with static member functions cannot be const. Since they are a part of the class and not of the object, they cannot promise to not alter the object. It wouldn't make sense... they aren't aware of the object. They aren't passed the This pointer as non-static member functions are.

Those static member functions on asio's address class are special. The key to understanding them is to look at the return type of those functions.

They are declared as members of address and return an address object. They are made static in order to be able to be used without the need to instantiate an object. From the return type (and hinted from the documentation), these seem to be functions that create instances of the address class.

Lets take static from_string (const char *str) as an example. From the documentation it promises to "Create an address from an IPv4 address string in dotted decimal form, or from an IPv6 address in hexadecimal notation."

How would you do this without using a static member function? Well, you could make a non member function and declare it inside some address namespace. That function would then basically do the same as the current one do. Create an instance of address class and return it.

By making it a member function, the function is automatically defined in the scope of the address class. Meawnhile, despite being declared static, there are no static data members involved.

>> The example which made me think I could do this was Boost.Asio.
That uses static member functions. Static member functions are different than static member variables. You can implement a static member function inline. Static member variables are more of a problem, because there is no "inline" for variables.

Do you have your own small example of what you are trying to do?