Execution library

 

C++

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Execution Library

Execution concepts scheduler receiver receiver_of operation_state    sender sender_in sender_to Execution components Execution contexts run_loop Execution domains default_domain Forward progress guarantee forward_progress_guarantee Environments empty_env    get_env    env_of_t Queries std::stop_token_of_t std::get_stop_token std::get_allocator std::forwarding_query get_scheduler get_domain get_forward_progress_guarantee get_delegation_scheduler get_completion_scheduler Completion signatures completion_signatures completion_signatures_of_t transform_completion_signatures transform_completion_signatures_of    value_types_of_t error_types_of_t    tag_of_t sends_stopped    Sender adaptor closures sender_adaptor_closure Coroutine utility as_awaitable with_awaitable_senders

Core operations Operation state connect connect_result_t start Completion set_value set_error set_stopped Transformation transform_env transform_sender apply_sender Sender algorithms Sender factories just just_error just_stopped read_env schedule schedule_result_t Sender adaptors starts_on continues_on    on then upon_error upon_stopped let_value let_error let_stopped schedule_from when_all when_all_with_variant into_variant stopped_as_optional stopped_as_error bulk split Sender consumers this_thread::sync_wait this_thread::sync_wait_with_variant

 

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.

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.

Library utilities

Schedulers

Defined in header <execution>
execution::scheduler (C++26)	specifies that a type is a scheduler (concept)

Execution Contexts

Defined in header <execution>
execution::run_loop (C++26)	execution resource holding a thread-safe MPSC task queue and a manually-driven event loop (class)

Senders

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:

Template:cpp/execution/dsc read

Defined in header <execution>
Defined in namespace std::execution
execution::just (C++26)	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 (customization point object)
execution::just_error (C++26)	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 (customization point object)
execution::just_stopped (C++26)	creates a sender that completes immediately by calling its receiver's set_stopped (customization point object)
execution::schedule (C++26)	prepares a task graph for execution on a given scheduler (customization point object)

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
execution::bulk (C++26)	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 (customization point object)
ensure_started	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)
execution::into_variant (C++26)	returns a sender which sends a variant of tuples of all the possible sets of types sent by the input sender (customization point object)
execution::let_error (C++26)	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 (customization point object)
execution::let_value (C++26)	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 (customization point object)
execution::let_stopped (C++26)	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 (customization point object)
execution::on (C++26)	start the provided sender on an execution agent belonging to the execution resource associated with the provided scheduler (customization point object)
execution::split (C++26)	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 (customization point object)
execution::stopped_as_error (C++26)	returns a sender that maps the stopped channel to an error (customization point object)
execution::stopped_as_optional (C++26)	returns a sender that maps the value channel to std::optional<std::decay_t<T>> and the stopped channel to std::nullopt (customization point object)
execution::then (C++26)	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 (customization point object)
transfer	Returns a sender describes transition from the execution context of the input sender to the execution context of the target scheduler (function template)
execution::upon_error (C++26)	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 (customization point object)
execution::upon_stopped (C++26)	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 (customization point object)
execution::when_all (C++26)	completes once all of the input senders have completed (customization point object)

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
start_detached	Completes when the provided sender completes, or calls std::terminate if the sender sends an error. (function template)
Functions managing the current thread
Defined in header <execution>
Defined in namespace std::this_thread
this_thread::sync_wait (C++26)	blocks current thread until the specified sender completes and returns its async result (customization point object)

Eager execution

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

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

Example

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

Run this code

#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

See also

async (C++11)

runs a function asynchronously (potentially in a new thread) and returns a std::future that will hold the result (function template)