33 Execution control library [exec]

33.9 Senders [exec.snd]

33.9.13 Sender consumers [exec.consumers]

33.9.13.3 execution​::​spawn [exec.spawn]

1
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spawn attempts to associate the given input sender with the given token's async scope and, on success, eagerly starts the input sender.
2
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The name spawn denotes a customization point object.
For subexpressions sndr, token, and env,
  • (2.1)
    let Sndr be decltype((sndr)),
  • (2.2)
    let Token be remove_cvref_t<decltype((token))>, and
  • (2.3)
    let Env be remove_cvref_t<decltype((env))>.
If any of sender<Sndr>, scope_token<Token>, or queryable<Env> are not satisfied, the expression spawn(​sndr, token, env) is ill-formed.
3
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Let spawn-state-base be the exposition-only class:
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namespace std::execution { struct spawn-state-base { // exposition only virtual void complete() noexcept = 0; // exposition only }; }
4
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Let spawn-receiver be the exposition-only class:
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namespace std::execution { struct spawn-receiver { // exposition only using receiver_concept = receiver_t; spawn-state-base* state; // exposition only void set_value() && noexcept { state->complete(); } void set_stopped() && noexcept { state->complete(); } }; }
5
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Let spawn-state be the exposition-only class template:
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namespace std::execution { template<class Alloc, scope_token Token, sender Sender> struct spawn-state : spawn-state-base { // exposition only using op-t = connect_result_t<Sender, spawn-receiver>; // exposition only spawn-state(Alloc alloc, Sender&& sndr, Token token); // exposition only void complete() noexcept override; // exposition only void run() noexcept; // exposition only private: using assoc-t = // exposition only remove_cvref_t<decltype(declval<Token&>().try_associate())>; Alloc alloc; // exposition only op-t op; // exposition only assoc-t assoc; // exposition only }; }
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spawn-state(Alloc alloc, Sender&& sndr, Token token);
6
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Effects: Initializes alloc with std​::​move(alloc), op with connect(std​::​move(sndr), spawn-receiver(this)), and assoc with token.try_associate().
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void run() noexcept;
7
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Effects: Equivalent to: if (assoc) start(op); else complete();
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void complete() noexcept override;
8
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Effects: Equivalent to: auto assoc = std::move(this->assoc); { using traits = allocator_traits<Alloc>::template rebind_traits<spawn-state>; typename traits::allocator_type alloc(this->alloc); traits::destroy(alloc, this); traits::deallocate(alloc, this, 1); }
9
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For the expression spawn(sndr, token, env) let new_sender be the expression token.wrap(sndr) and let alloc and senv be defined as follows:
  • (9.1)
    if the expression get_allocator(env) is well-formed, then alloc is the result of get_allocator(env) and senv is the expression env,
  • (9.2)
    otherwise if the expression get_allocator(get_env(new_sender)) is well-formed, then alloc is the result of get_allocator(get_env(new_sender)) and senv is the expression JOIN-ENV(prop(get_allocator, alloc), env),
  • (9.3)
    otherwise alloc is allocator<void>() and senv is the expression env.
10
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The expression spawn(sndr, token, env) is of type void and has the following effects:
  • (10.1)
    Uses alloc to allocate and construct an object o of type decltype(spawn-state(alloc, write_env(token.wrap(sndr), senv), token)) from alloc, write_env(token.wrap(sndr), senv), and token and then invokes o.run().
    If an exception is thrown then any constructed objects are destroyed and any allocated memory is deallocated.
11
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The expression spawn(sndr, token) is expression-equivalent to spawn(sndr, token, execution​::​env<>(​)).