Why static variable needs to be explicitly defined?

Question

In the class:

class foo
{
public:
    static int bar; //declaration of static data member
};

int foo::bar = 0; //definition of data member

W

Answer 1

static is a storage type, when you declare the variable you are telling the compiler "this week be in the data section somewhere" and when you subsequently use it, the compiler emits code that loads a value from a TBD address.

In some contexts, the compiler can drive that a static is really a compile time constant and replace it with such, for example

static const int meaning = 42;

Inside a function that never takes the address of the value.

When dealing with class members, however, the compiler can't guess where this value should be created. It might be in a library you will link against, or a dll, or you might be providing a library where the value must be provided by the library consumer.

Usually, when someone asks this, though, it is because they are misusing static members.

If all you want us a constant value, e.g

static int MaxEntries;
...
int Foo::MaxEntries = 10;

You would be better off with one or other of the following

static const int MaxEntries = 10;
 // or
enum { MaxEntries = 10 };

The static requires no separate definition until something tries to take the address of or form a reference to the variable, the enum version never does.