长江三峡风景:C++中::的用法的问题

Second, it's often convenient to define class-specific constants, and that calls for a slightly different tack. To limit the scope of a constant to a class, you must make it a member, and to ensure there's at most one copy of the constant, you must make it a static member: ¤ Item E1, P8

class GamePlayer {
private:
static const int NUM_TURNS = 5; // constant declaration
int scores[NUM_TURNS]; // use of constant
...
};
There's a minor wrinkle, however, which is that what you see above is a declaration for NUM_TURNS, not a definition. You must still define static class members in an implementation file: ¤ Item E1, P9

const int GamePlayer::NUM_TURNS; // mandatory definition;
// goes in class impl. file
There's no need to lose sleep worrying about this detail. If you forget the definition, your linker should remind you. ¤ Item E1, P10

const int GamePlayer::NUM_TURNS;
请问这一句怎么理解？里面的::怎么理解？