What's the difference between declaring static variables inside a class, in the .h file or in the class definition .cpp file?

Example:

```
//File.h
class File
{
private:
    const static int number = 10;    
public:
    File();
    ~File();
};

```

or

```
//File.cpp

#include "File.h"

const static int number = 10;

File::File()
{
}

File::~File()
{
}

```

The idea is to use those variables for every instance of `File`.