Static members

Some data belongs to the type rather than to any one object: a count of how many have been made, a shared table, a setting. A static member exists once, however many objects there are, including none.

type Ticket
    declare constructor( )
    declare static function Issued( ) as int32
    declare static function Reset( ) as int32
    declare function Number( ) as int32
  private:
    static issued_ as int32         '' one copy, shared by every Ticket
    number_ as int32                '' one per Ticket
end type

'' A definition outside the type gives the static field its starting
'' value. Without one, it starts at 0.
dim as int32 Ticket.issued_ = 100

constructor Ticket( )
    issued_ += 1
    number_ = issued_
end constructor

'' A static method has no object: it is called on the type itself.
static function Ticket.Issued( ) as int32
    return issued_
end function

static function Ticket.Reset( ) as int32
    dim as int32 before = issued_
    issued_ = 0
    return before
end function

function Ticket.Number( ) as int32
    return number_
end function

print "counter before any ticket:"; Ticket.Issued( )
dim as Ticket a, b, c
print "counter:"; Ticket.Issued( ); ", b is number"; b.Number( )

print "reset after"; Ticket.Reset( )
dim as Ticket d
print "d is number"; d.Number( ); ", c is still"; c.Number( )

Output

counter before any ticket: 100
counter: 103, b is number 102
reset after 103
d is number 1, c is still 103

A static field is declared in the type with static and shared by every Ticket. Each constructor call increments the same issued_, while each ticket keeps its own number_. The line outside the type, dim as int32 Ticket.issued_ = 100, gives the shared field its starting value; without it, the field starts at 0.

A static method has no object, so it has no this and cannot use number_ or any other per-object field; it is called on the type, Ticket.Issued( ). That is why it can answer before any ticket exists. Reaching for a per-object field from one is refused with TKO0937: 'number_' needs an object, and 'issued' has none: it is a static member.

Static members follow the same access rules as any other: issued_ is private, so only Ticket's own code can change it, and the outside world reads it through Issued( ).

A static field takes no room in an object: it is stored once, apart from them all.

Activities

1. Add a static Peak( ) that reports the highest number ever issued, even after a Reset.
2. Try to print Ticket.issued_ from outside the type, then move it to the public section and try again.