Modern SharedPtr WeakPtr

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WeakPtr

Holds a link to a shared object that does not keep it alive, to break cycles.

Syntax

WeakPtr( of T )

Notes

• The only way to break a cycle: two objects holding SharedPtrs to each other never reach zero, and one of them holding a WeakPtr instead is what fixes it.
• It cannot be dereferenced. Lock( ) promotes it to a SharedPtr, or answers null if the object is gone -- and that is the only safe way to use one.
• Expired( ) is a question about the instant it was asked; between it and a later use the object may go. Lock and test the handle instead.
• The control block outlives the object, which is why a WeakPtr can safely read a count for something already destroyed: strong reaching 0 destroys the object, weak reaching 0 frees the block.

Examples

#import "std/memory"
using memory

type Node
    as int32 id
end type

dim w as WeakPtr( of Node )

scope
    dim p as SharedPtr( of Node ) = MakeShared( of Node )( )
    p.Ref( ).id = 7
    w = p
    print w.Expired( )       '' false

    '' Lock is the ONLY safe way to use one
    dim s as SharedPtr( of Node ) = w.Lock( )
    print s.Ref( ).id        '' 7
end scope

print w.Expired( )           '' true
print w.Lock( ).IsNull( )    '' true -- no dangling pointer handed back

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Library: SharedPtr and WeakPtr
Category: Smart Pointers
Documented under: Members