Managing C++ Associated Asynchronous State

ISO/IEC JTC1 SC22 WG21 N3192 = 10-0182 - 2010-11-12

Lawrence Crowl, crowl@google.com, Lawrence@Crowl.org

This paper revises ISO/IEC JTC1 SC22 WG21 N3129 = 10-0119 - 2010-08-20.

This paper addresses national body comment US 194. The changes in this paper are editorial in intent. However, some of the FCD wording is broken with respect to the intent. In that sense, the changes fix real bugs in the standard.

Problem
Solution
Wording
    30.6.4 Associated asynchronous state [futures.state]
    30.6.5 Class template promise [futures.promise]
    30.6.6 Class template future [futures.unique_future]
    30.6.7 Class template shared_future [futures.shared_future]
    30.6.8 Class template atomic_future [futures.atomic_future]
    30.6.10.1 packaged_task member functions [futures.task.members]

Problem

Within the Final Committee Draft, the specification for managing associated asynchronous state [futures.state] is confusing, sometimes omitted, and redundantly specified.

Solution

Define terms-of-art for releasing, making ready, and abandoning an associated asynchronous state. Define a term-of-art for waiting functions on an associated asynchronous state. Use those terms where appropriate.

This solution implements the intent of the existing wording. That is, there is no new functionality.

Wording

The wording is relative to the FCD.

30.6.4 Associated asynchronous state [futures.state]

Edit paragraph 3 as follows.

An asynchronous return object is an object that reads results from an associated asynchronous state. A waiting function of an asynchronous return object is one that potentially blocks to wait for the associated asynchronous state to be made ready.

Edit paragraph 5 as follows.

When the last reference to an associated asynchronous state is given up, any resources held by that associated asynchronous state are released. When an asynchronous return object or an asynchronous provider is said to release its associated asynchronous state, it means:

• if the return object or provider holds the last reference to its associated asynchronous state, the associated state is destroyed; and
• the return object or provider gives up its reference to its associated asynchronous state.

When an asynchronous provider is said to make its associated asynchronous state ready, it means:

• first, the provider marks its associated asynchronous state as ready; and
• second, the provider unblocks any execution agents waiting for its associated asynchronous state to become ready.

When an asynchronous provider is said to abandon its associated asynchronous state, it means:

• first, if that state is not ready, the provider

  • stores an exception object of type future_error with an error condition of broken_promise within its associated asynchronous state; and then
  • makes its associated asynchronous state ready;

• second, the provider releases its associated asynchronous state.

30.6.5 Class template promise [futures.promise]

Edit paragraph 7, regarding the destructor, as follows.

Effects: if the associated asynchronous state of *this is not ready, stores an exception object of type future_error with an error condition of broken_promise Any threads blocked in a function waiting for the asynchronous state associated with *this to become ready are unblocked. Destroys *this and releases its reference to its associated asynchronous state if any. If this is the last reference to that associated asynchronous state, destroys that state. abandons any associated asynchronous state ([futures.state]).

Edit paragraph 8, regarding the move assignment operator, as follows.

Effects: abandons any associated asynchronous state ([futures.state]) and then as if promise<R>(std::move(rhs)).swap(*this).

Remove paragraph 9, as it is now redundant with the effects.

Postcondition: rhs has no associated asynchronous state. *this has the associated asynchronous state of rhs prior to the assignment.

Edit paragraph 18, regarding set_value, as follows.

Effects: atomically stores the value r in the associated asynchronous state and sets that state to ready. Any threads blocked in a call of a blocking function of any future that refers to the same associated asynchronous state as *this are unblocked. makes that state ([futures.state]) ready.

Edit paragraph 22, regarding set_exception, as follows.

Effects: atomically stores the exception p in the associated asynchronous state and sets that state to ready. Any threads blocked in a call of a blocking function of any future that refers to the same associated asynchronous state as *this are unblocked. makes that state ([futures.state]) ready.

Edit paragraph 26, regarding set_value_at_thread_exit, as follows.

Effects: Stores the value r in the associated asynchronous state without making the associated asynchronous that state ready immediately. Schedules the associated asynchronous that state to be made ready when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed.

Edit paragraph 29, regarding set_exception_at_thread_exit, as follows.

Effects: Stores the exception p in the associated asynchronous state without making the associated asynchronous that state ready immediately. Schedules the associated asynchronous that state to be made ready when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed.

30.6.6 Class template future [futures.unique_future]

Edit paragraph 10, regarding the destructor, as follows.

Effects:

• gives up the reference to its releases any associated asynchronous state ([futures.state]).
• destroys *this.

Edit paragraph 11, regarding the move assignment operator, as follows.

Effects:

• if *this referred to an associated asynchronous state prior to the assignment it gives up this reference. releases any associated asynchronous state ([futures.state]).

• move assigns the contents of rhs to *this.

30.6.7 Class template shared_future [futures.shared_future]

Edit paragraph 13, regarding the destructor, as follows.

Effects:

• gives up the reference to its releases any associated asynchronous state ([futures.state]).
• destroys *this.

Edit paragraph 14, regarding the move assignment operator, as follows.

Effects:

• if *this refers to an associated asynchronous state it gives up this reference. releases any associated asynchronous state ([futures.state]).
• assigns the contents of rhs to *this.

Edit paragraph 16, regarding the copy assignment operator, as follows.

Effects:

• if *this refers to an associated asynchronous state it gives up this reference. releases any associated asynchronous state ([futures.state]).
• assigns the contents of rhs to *this. [Note: as a result, *this refers to the same associated asynchronous state as rhs (if any). —end note]

30.6.8 Class template atomic_future [futures.atomic_future]

Edit paragraph 9, regarding the destructor, as follows.

Effects:

• gives up the reference to its releases any associated asynchronous state ([futures.state]).
• destroys *this.

Edit paragraph 10, regarding the copy assignment operator, as follows.

Effects:

• releases any associated asynchronous state.
• assigns the contents of rhs to *this. [Note: as a result, *this refers to the same associated asynchronous state as rhs (if any). —end note]

30.6.10.1 packaged_task member functions [futures.task.members]

Edit paragraph 9, regarding the move assignment operator, as follows.

Effects:

• releases any associated asynchronous state ([futures.state]).
• packaged_task<R, ArgTypes...>(other).swap(*this).

Edit paragraph 10, regarding the destructor, as follows.

Effects: if the associated asynchronous state of *this is not ready, stores an exception object of type future_error with an error code of broken_promise. Any threads blocked in a function waiting for the associated asynchronous state of *this to become ready are unblocked. Destroys *this and releases its reference to its associated asynchronous state (if any). If this is the last reference to that associated asynchronous state, destroys that state. abandons any associated asynchronous state ([futures.state]).

Edit within paragraph 24, regarding make_ready_at_thread_exit, as follows.

.... this shall be done without making the state ready making that state ([futures.state]) ready immediately. ....

Edit paragraph 27, regarding reset, as follows.

Effects: returns the object to a state as if a newly-constructed instance had just been assigned to *this by *this = packaged_task(std::move(f)), where f is the task stored in *this. [Note: this constructs a new associated asynchronous state for *this. The old state is discarded, abandoned ([futures.state]). as described in the destructor for packaged_task. get_future may now be called again for *this. —end note]