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10+ < title > alloc::arc - Rust</ title >
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30+ < p class ='location '> < a href ='../index.html '> alloc</ a > </ p > < script > window . sidebarCurrent = { name : 'arc' , ty : 'mod' , relpath : '../' } ; </ script > < script defer src ="../sidebar-items.js "> </ script >
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44+ < section id ='main ' class ="content mod ">
45+ < h1 class ='fqn '> < span class ='in-band '> Module < a href ='../index.html '> alloc</ a > ::< wbr > < a class ='mod ' href =''> arc</ a > </ span > < span class ='out-of-band '> < span id ='render-detail '>
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50+ < div class ='docblock '> < p > Threadsafe reference-counted boxes (the < code > Arc<T></ code > type).</ p >
51+
52+ < p > The < code > Arc<T></ code > type provides shared ownership of an immutable value.
53+ Destruction is deterministic, and will occur as soon as the last owner is
54+ gone. It is marked as < code > Send</ code > because it uses atomic reference counting.</ p >
55+
56+ < p > If you do not need thread-safety, and just need shared ownership, consider
57+ the < a href ="../rc/struct.Rc.html "> < code > Rc<T></ code > type</ a > . It is the same as < code > Arc<T></ code > , but
58+ does not use atomics, making it both thread-unsafe as well as significantly
59+ faster when updating the reference count.</ p >
60+
61+ < p > The < code > downgrade</ code > method can be used to create a non-owning < code > Weak<T></ code > pointer
62+ to the box. A < code > Weak<T></ code > pointer can be upgraded to an < code > Arc<T></ code > pointer, but
63+ will return < code > None</ code > if the value has already been dropped.</ p >
64+
65+ < p > For example, a tree with parent pointers can be represented by putting the
66+ nodes behind strong < code > Arc<T></ code > pointers, and then storing the parent pointers
67+ as < code > Weak<T></ code > pointers.</ p >
68+
69+ < h1 id ='examples ' class ='section-header '> < a href ='#examples '> Examples</ a > </ h1 >
70+ < p > Sharing some immutable data between threads:</ p >
71+ < pre class ='rust rust-example-rendered '>
72+ < span class ='kw '> use</ span > < span class ='ident '> std</ span > ::< span class ='ident '> sync</ span > ::< span class ='ident '> Arc</ span > ;
73+ < span class ='kw '> use</ span > < span class ='ident '> std</ span > ::< span class ='ident '> thread</ span > ;
74+
75+ < span class ='kw '> let</ span > < span class ='ident '> five</ span > < span class ='op '> =</ span > < span class ='ident '> Arc</ span > ::< span class ='ident '> new</ span > (< span class ='number '> 5</ span > );
76+
77+ < span class ='kw '> for</ span > _ < span class ='kw '> in</ span > < span class ='number '> 0</ span > ..< span class ='number '> 10</ span > {
78+ < span class ='kw '> let</ span > < span class ='ident '> five</ span > < span class ='op '> =</ span > < span class ='ident '> five</ span > .< span class ='ident '> clone</ span > ();
79+
80+ < span class ='ident '> thread</ span > ::< span class ='ident '> spawn</ span > (< span class ='kw '> move</ span > < span class ='op '> ||</ span > {
81+ < span class ='macro '> println</ span > < span class ='macro '> !</ span > (< span class ='string '> "{:?}"</ span > , < span class ='ident '> five</ span > );
82+ });
83+ }</ pre >
84+
85+ < p > Sharing mutable data safely between threads with a < code > Mutex</ code > :</ p >
86+ < pre class ='rust rust-example-rendered '>
87+ < span class ='kw '> use</ span > < span class ='ident '> std</ span > ::< span class ='ident '> sync</ span > ::{< span class ='ident '> Arc</ span > , < span class ='ident '> Mutex</ span > };
88+ < span class ='kw '> use</ span > < span class ='ident '> std</ span > ::< span class ='ident '> thread</ span > ;
89+
90+ < span class ='kw '> let</ span > < span class ='ident '> five</ span > < span class ='op '> =</ span > < span class ='ident '> Arc</ span > ::< span class ='ident '> new</ span > (< span class ='ident '> Mutex</ span > ::< span class ='ident '> new</ span > (< span class ='number '> 5</ span > ));
91+
92+ < span class ='kw '> for</ span > _ < span class ='kw '> in</ span > < span class ='number '> 0</ span > ..< span class ='number '> 10</ span > {
93+ < span class ='kw '> let</ span > < span class ='ident '> five</ span > < span class ='op '> =</ span > < span class ='ident '> five</ span > .< span class ='ident '> clone</ span > ();
94+
95+ < span class ='ident '> thread</ span > ::< span class ='ident '> spawn</ span > (< span class ='kw '> move</ span > < span class ='op '> ||</ span > {
96+ < span class ='kw '> let</ span > < span class ='kw-2 '> mut</ span > < span class ='ident '> number</ span > < span class ='op '> =</ span > < span class ='ident '> five</ span > .< span class ='ident '> lock</ span > ().< span class ='ident '> unwrap</ span > ();
97+
98+ < span class ='op '> *</ span > < span class ='ident '> number</ span > < span class ='op '> +=</ span > < span class ='number '> 1</ span > ;
99+
100+ < span class ='macro '> println</ span > < span class ='macro '> !</ span > (< span class ='string '> "{}"</ span > , < span class ='op '> *</ span > < span class ='ident '> number</ span > ); < span class ='comment '> // prints 6</ span >
101+ });
102+ }</ pre >
103+ </ div > < h2 id ='structs ' class ='section-header '> < a href ="#structs "> Structs</ a > </ h2 >
104+ < table >
105+ < tr class =' module-item '>
106+ < td > < a class ='struct ' href ='struct.Arc.html '
107+ title ='alloc::arc::Arc '> Arc</ a > </ td >
108+ < td class ='docblock short '>
109+ < p > An atomically reference counted wrapper for shared state.</ p >
110+
111+ </ td >
112+ </ tr >
113+
114+ < tr class =' module-item '>
115+ < td > < a class ='struct ' href ='struct.Weak.html '
116+ title ='alloc::arc::Weak '> Weak</ a > </ td >
117+ < td class ='docblock short '>
118+ < p > A weak pointer to an < code > Arc</ code > .</ p >
119+
120+ </ td >
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