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SPVM-0.989083
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/ SPVM::Document::Language::GarbageCollection

Name

SPVM::Document::Language::GarbageCollection - Garbage Collection in SPVM Language

Description

This document describes garbage collection in SPVM language.

Garbage Collection

Reference Count GC

Garbage collection in SPVM is a reference counted GC.

The object is destroyed when its reference count reaches 0.

The reference count of an object assigned by the assignment operator is incremented by 1.

# The reference count is incremented by 1
my $object = Point->new;

The reference count of an object whose assignment is removed by an assignment operator is decreased by 1.

# The reference count is decremented by 1
$object = undef;

Reference counts incremented by assignments to local variables are decremented at the end of its scope.

{
  # The reference count is incremented by 1
  my $object = Point->new;
  
  # The reference count of $object is decremented by 1 at the end of this scope
}

Scope

A scope is the part surrounded by a scope block.

# Scope block
{
  # Beginning of scope
  
  my $point = Point->new;
  
  # End of scope
}

Entering Scope

The entering scope operation is the operation at the start of a scope block.

Memorizes the current top position of the mortal stack.

Mortal Stack

The mortal stack is the stack that stores objects that are decremented by 1 at the end of the scope.

Local variables of the object type are pushed to the mortal stack at the same time as its declaration.

Leaving Scope

The leaving scope operation is the operation at the end of a scope block.

Reference count of the objects from the top position memorized by the entering scope operation to the current top position of the mortal stack are decremented by 1.

Weak Reference

SPVM supports weak references. Weak references are used to avoid circular references.

This is an example that objects have circular references.

{
  my $foo = new Foo;
  my $bar = new Bar;
  
  $foo->{bar} = $bar;
  $bar->{foo} = $foo;
}

The weaken operator converts a reference to a weak reference.

{
  my $foo = new Foo;
  my $bar = new Bar;
  
  $foo->{bar} = $bar;
  $bar->{foo} = $foo;
  
  weaken $foo->{bar};
}

If a reference is convertd to a weak reference, the reference count of the referenced object is decremented by 1.

And the weaken flag of the field trun on, and the back reference from the field is added to the referenced object.

The isweak operator checks if the weaken flag of the field turn on.

my $isweak = isweaken $foo->{bar};

The unweaken operator converts a weak reference to a reference.

unweaken $foo->{bar};

If a weak reference is convertd to a reference, the reference count of the referenced object is incremented by 1.

And the weaken flag of the field trun off, and the back reference from the field is removed from the referenced object.

Copyright & License

Copyright (c) 2023 Yuki Kimoto

MIT License