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Execution library

From cppreference.com

The Execution library provides a framework for managing asynchronous execution on generic execution resources, targeting the standard C++ library.

The library aims to provide vocabulary types for async operations and to allow the construction of task execution graphs in a simple, composable way.

Contents

[edit] Library-wide definitions

  • Sender: A description of asynchronous work to be sent for execution. Produces an operation state (below).
  • Senders asynchronously “send” their results to listeners called “receivers” (below).
  • Senders can be composed into task graphs using generic algorithms.
  • Sender factories and adaptors are generic algorithms that capture common async patterns in objects satisfying the sender concept.
  • Receiver: A generalized callback that consumes or “receives” the asynchronous results produced by a sender.
  • Receivers have three different “channels” through which a sender may propagate results: success, failure, and canceled, so-named “value”, “error”, and “stopped”.
  • Receivers provide an extensible execution environment: a set of key/value pairs that the consumer can use to parameterize the asynchronous operation.
  • Operation State: An object that contains the state needed by the asynchronous operation.
  • A sender and receiver are connected when passed to the std::execution::connect function.
  • The result of connecting a sender and a receiver is an operation state.
  • Work is not enqueued for execution until “start” is called on an operation state.
  • Once started, the operation state’s lifetime cannot end before the async operation is complete, and its address must be stable.
  • Scheduler: A lightweight handle to an execution context.
  • An execution context is a source of asynchronous execution such as a thread pool or a GPU stream.
  • A scheduler is a factory for a sender that completes its receiver from a thread of execution owned by the execution context.

[edit] Library utilities

[edit] Senders

[edit] Sender factories

A sender factory is a function that returns a sender and whose parameters have types for which the sender concept is false.

The following are sender factories:

Defined in header <execution>
Defined in namespace std::execution
Accepts a variadic number of arguments and returns a sender that, when connected and started, completes synchronously by passing the arguments to the receiver's value completion function.
(function template) [edit]
Accepts a single argument and returns a sender that, when connected and started, completes synchronously by passing the argument to the receiver's error completion function.
(function template) [edit]
A function callable with no arguments that returns a sender that, when connected and started, completes synchronously by invoking the receiver's stopped completion function.
(function) [edit]
A function that is callable with a query object (e.g., std::get_stop_token) and returns a sender that, when connected to a receiver and started, synchronously invokes the query object with the receiver’s environment, passing the result to the receiver's value completion function.
(function template) [edit]
A function that accepts a scheduler and returns a sender that, when connected and started, will attempt to enqueue a value completion operation with no result datums to an execution agent belonging to the scheduler's associated execution resource. The operation may also complete with an error (e.g., if the attempt to enqueue work fails) or with stopped (e.g., if stop is requested on the execution resource before the work is dequeued).
(function template) [edit]

[edit] Sender adaptors

A sender adaptor is a function returning a sender whose parameters include at least one whose type satisfies the sender concept, and for which the returned sender is a parent sender of the adaptor function's sender arguments.

The following are sender adaptors:

Defined in header <execution>
Defined in namespace std::execution
Creates a multi-shot sender that invokes the function with every index in the provided shape along with the values sent by the input sender. The sender completes once all invocations have completed, or an error has occurred.
(function template) [edit]
Eagerly starts a sender, returning a sender that will deliver the results to a receiver to which it is connected and started, if any. When the result sender is not connected to a receiver, or if the resulting operation state is not started, the results are ignored. If such a sender is destroyed before the underlying operation completes, the operation continues running detached.
(function template) [edit]
Returns a sender which sends a variant of tuples of all the possible sets of types sent by the input sender.
(function template) [edit]
Returns a sender which represents a node chained to the input sender, which invokes the provided function with the error from the input sender, if occurred.
(function template) [edit]
Returns a sender which represents a node chained to the input sender, which when started, invokes the provided function with the values sent by the input sender as arguments.
(function template) [edit]
Returns a sender which represents a node chained to the input sender, which invokes the provided function with the stop token from the input sender, if the "stopped" signal is sent.
(function template) [edit]
Start the provided sender on an execution agent belonging to the execution resource associated with the provided scheduler.
(function template) [edit]
If the provided sender is a multi-shot sender, returns that sender. Otherwise, returns a multi-shot sender which sends values equivalent to the values sent by the provided sender.
(function template) [edit]
Returns a sender that maps the stopped channel to an error.
(function template) [edit]
Returns a sender that maps the value channel to std::optional<std::decay_t<T>> and the stopped channel to std::nullopt.
(function template) [edit]
Chains the task graph by the input sender with a node represents invoking the provided function with the values sent by the input sender as arguments.
(function template) [edit]
Returns a sender describes transition from the execution context of the input sender to the execution context of the target scheduler
(function template) [edit]
Chains the task graph by the input sender with a node representing invoking the provided function with the error sent by the input sender if an error occurred.
(function template) [edit]
Chains the task graph by the input sender with a node representing invoking the provided function with the stopped behavior by the input sender if a "stopped" signal is sent.
(function template) [edit]
Completes once all of the input senders have completed. The values are the values sent by each of the input senders, in order of the arguments passed to when_all. Completes inline on the execution resource on which the last input sender completes.
(function template) [edit]

[edit] Sender consumers

A sender consumer is an algorithm that takes one or more senders as parameters and that does not return a sender.

The following are sender consumers:

Defined in header <execution>
Defined in namespace std::execution
Completes when the provided sender completes, or calls std::terminate if the sender sends an error.
(function template) [edit]
Functions managing the current thread
Defined in header <thread>
Defined in namespace std::this_thread
(C++26)
blocks current thread until passed sender has completed, and returns values (if any) the sender completed with
(function template) [edit]

[edit] Eager execution

Allows a fire-and-forget eager submission of an invocable to a scheduler.

Defined in header <execution>
Defined in namespace std::execution
Submits the provided function for execution on the provided scheduler.
(function template) [edit]

[edit] Example

A version of this example is available on godbolt.org, where it uses stdexec, an experimental reference implementation of std::execution.

#include <cstdio>
#include <execution>
#include <string>
#include <thread>
#include <utility>
using namespace std::literals;
 
int main()
{
    std::execution::run_loop loop;
 
    std::jthread worker([&](std::stop_token st)
    {
        std::stop_callback cb{ st, [&]{ loop.finish(); }};
        loop.run();
    });
 
    std::execution::sender auto hello = std::execution::just("hello world"s);
    std::execution::sender auto print
        = std::move(hello)
        | std::execution::then([](std::string msg)
        {
            std::puts(msg.c_str());
            return 0;
        });
 
    std::execution::scheduler auto io_thread = loop.get_scheduler();
    std::execution::sender auto work = std::execution::on(io_thread, std::move(print));
 
    auto [result] = std::this_thread::sync_wait(std::move(work)).value();
 
    return result;
}

Output:

hello world

[edit] See also

(C++11)
runs a function asynchronously (potentially in a new thread) and returns a std::future that will hold the result
(function template) [edit]