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Difference between revisions of "cpp/thread/packaged task"

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< cpp‎ | thread
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{{cpp/title|packaged_task}}
 
{{cpp/title|packaged_task}}
−
{{cpp/thread/packaged_task/sidebar}}
+
{{cpp/thread/packaged_task/navbar}}
−
{{ddcl list begin}}
+
{{dcl begin}}
−
{{ddcl list header | future}}
+
{{dcl header|future}}
−
{{ddcl list item | num=1 | notes={{mark c++11 feature}} |
+
{{dcl|num=1|since=c++11|
 
template< class > class packaged_task; //not defined
 
template< class > class packaged_task; //not defined
 
}}
 
}}
−
{{ddcl list item | num=2 | notes={{mark c++11 feature}} |
+
{{dcl|num=2|since=c++11|
−
template< class Function, class Args... >  
+
template< class R, class ...ArgTypes >  
−
class packaged_task<Function(Args...)>;
+
class packaged_task<R(ArgTypes...)>;
 
}}
 
}}
−
{{ddcl list end}}
+
{{dcl end}}
  
−
The class template {{tt|std::packaged_task}} packages a function to store its return value that is later acquired asynchronously via a {{cpp|std::future}} object, that the {{tt|std::packaged_task}} can supply.
+
The class template {{tt|std::packaged_task}} wraps any {{named req|Callable}} target (function, lambda expression, bind expression, or another function object) so that it can be invoked asynchronously. Its return value or exception thrown is stored in a shared state which can be accessed through {{lc|std::future}} objects.
−
 
+
−
{{todo}}
+
  
 +
{{rrev|until=c++17|
 +
Just like {{lc|std::function}}, {{tt|std::packaged_task}} is a polymorphic, allocator-aware container: the stored callable target may be allocated on heap or with a provided allocator.
 +
}}
  
 
===Member functions===
 
===Member functions===
−
{{dcl list begin}}
+
{{dsc begin}}
−
{{dcl list mem ctor | cpp/thread/packaged_task/packaged_task | constructs the task object}}
+
{{dsc inc|cpp/thread/packaged_task/dsc constructor}}
−
{{dcl list mem dtor | cpp/thread/packaged_task/~packaged_task | destructs the task object}}
+
{{dsc inc|cpp/thread/packaged_task/dsc destructor}}
−
{{dcl list mem fun | cpp/thread/packaged_task/operator{{=}} | moves the task object}}
+
{{dsc inc|cpp/thread/packaged_task/dsc operator{{=}}}}
−
{{dcl list mem fun | cpp/thread/packaged_task/valid | checks if the task object has a valid function}}
+
{{dsc inc|cpp/thread/packaged_task/dsc valid}}
−
{{dcl list mem fun | cpp/thread/packaged_task/swap | swaps two task objects}}
+
{{dsc inc|cpp/thread/packaged_task/dsc swap}}
  
−
{{dcl list h2 | Getting the result}}
+
{{dsc h2|Getting the result}}
−
{{dcl list mem fun | cpp/thread/packaged_task/get_future | returns a {{rlt|future}} associated with the promised result}}
+
{{dsc inc|cpp/thread/packaged_task/dsc get_future}}
  
−
{{dcl list h2 | Execution}}
+
{{dsc h2|Execution}}
−
{{dcl list mem fun | cpp/thread/packaged_task/operator() | executes the function}}
+
{{dsc inc|cpp/thread/packaged_task/dsc operator()}}
−
{{dcl list mem fun | cpp/thread/packaged_task/make_ready_at_thread_exit | executes the function ensuring that the result is ready<br> only once the current thread exits}}
+
{{dsc inc|cpp/thread/packaged_task/dsc make_ready_at_thread_exit}}
−
{{dcl list mem fun | cpp/thread/packaged_task/reset | resets the state abandoning any stored results of previous executions}}  
+
{{dsc inc|cpp/thread/packaged_task/dsc reset}}
−
{{dcl list end}}
+
{{dsc end}}
  
 
===Non-member functions===
 
===Non-member functions===
 +
{{dsc begin}}
 +
{{dsc inc|cpp/thread/packaged_task/dsc swap2}}
 +
{{dsc end}}
 +
 +
===Helper classes===
 +
{{dsc begin}}
 +
{{dsc inc|cpp/thread/packaged_task/dsc uses_allocator}}
 +
{{dsc end}}
 +
 +
==={{rl|deduction_guides|Deduction guides}}{{mark since c++17}}===
 +
 +
===Example===
 +
{{example
 +
|code=
 +
#include <cmath>
 +
#include <functional>
 +
#include <future>
 +
#include <iostream>
 +
#include <thread>
 +
 +
// unique function to avoid disambiguating the std::pow overload set
 +
int f(int x, int y) { return std::pow(x, y); }
 +
 +
void task_lambda()
 +
{
 +
    std::packaged_task<int(int, int)> task([](int a, int b)
 +
    {
 +
        return std::pow(a, b);
 +
    });
 +
    std::future<int> result = task.get_future();
 +
 +
    task(2, 9);
 +
 +
    std::cout << "task_lambda:\t" << result.get() << '\n';
 +
}
 +
 +
void task_bind()
 +
{
 +
    std::packaged_task<int()> task(std::bind(f, 2, 11));
 +
    std::future<int> result = task.get_future();
 +
 +
    task();
 +
 +
    std::cout << "task_bind:\t" << result.get() << '\n';
 +
}
 +
 +
void task_thread()
 +
{
 +
    std::packaged_task<int(int, int)> task(f);
 +
    std::future<int> result = task.get_future();
 +
 +
    std::thread task_td(std::move(task), 2, 10);
 +
    task_td.join();
 +
 +
    std::cout << "task_thread:\t" << result.get() << '\n';
 +
}
 +
 +
int main()
 +
{
 +
    task_lambda();
 +
    task_bind();
 +
    task_thread();
 +
}
 +
|output=
 +
task_lambda: 512
 +
task_bind:  2048
 +
task_thread: 1024
 +
}}
 +
 +
===Defect reports===
 +
{{dr list begin}}
 +
{{dr list item|wg=lwg|dr=3117|std=C++17|before=deduction guides for {{tt|packaged_task}} were missing|after=added}}
 +
{{dr list end}}
 +
 +
===See also===
 +
{{dsc begin}}
 +
{{dsc inc|cpp/thread/dsc future}}
 +
{{dsc end}}
  
−
{{todo}}
+
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

Latest revision as of 02:02, 23 October 2023

 
C++
 
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(C++20)
 
std::packaged_task
 
Defined in header <future>
template< class > class packaged_task; //not defined
 (1) (since C++11)
template< class R, class ...ArgTypes >
class packaged_task<R(ArgTypes...)>;
 (2) (since C++11)

The class template std::packaged_task wraps any Callable target (function, lambda expression, bind expression, or another function object) so that it can be invoked asynchronously. Its return value or exception thrown is stored in a shared state which can be accessed through std::future objects.

Just like std::function, std::packaged_task is a polymorphic, allocator-aware container: the stored callable target may be allocated on heap or with a provided allocator.

(until C++17)

Contents

[edit] Member functions

 constructs the task object
(public member function) [edit]
destructs the task object
(public member function) [edit]
moves the task object
(public member function) [edit]
checks if the task object has a valid function
(public member function) [edit]
swaps two task objects
(public member function) [edit]
Getting the result
returns a std::future associated with the promised result
(public member function) [edit]
Execution
executes the function
(public member function) [edit]
executes the function ensuring that the result is ready only once the current thread exits
(public member function) [edit]
resets the state abandoning any stored results of previous executions
(public member function) [edit]

[edit] Non-member functions

(C++11)
 specializes the std::swap algorithm
(function template) [edit]

[edit] Helper classes

(C++11) (until C++17)
 specializes the std::uses_allocator type trait
(class template specialization) [edit]

[edit] Deduction guides(since C++17)

[edit] Example

Run this code
#include <cmath>
#include <functional>
#include <future>
#include <iostream>
#include <thread>
 
// unique function to avoid disambiguating the std::pow overload set
int f(int x, int y) { return std::pow(x, y); }
 
void task_lambda()
{
    std::packaged_task<int(int, int)> task([](int a, int b)
    {
        return std::pow(a, b); 
    });
    std::future<int> result = task.get_future();
 
    task(2, 9);
 
    std::cout << "task_lambda:\t" << result.get() << '\n';
}
 
void task_bind()
{
    std::packaged_task<int()> task(std::bind(f, 2, 11));
    std::future<int> result = task.get_future();
 
    task();
 
    std::cout << "task_bind:\t" << result.get() << '\n';
}
 
void task_thread()
{
    std::packaged_task<int(int, int)> task(f);
    std::future<int> result = task.get_future();
 
    std::thread task_td(std::move(task), 2, 10);
    task_td.join();
 
    std::cout << "task_thread:\t" << result.get() << '\n';
}
 
int main()
{
    task_lambda();
    task_bind();
    task_thread();
}

Output: 

task_lambda: 512
task_bind:   2048
task_thread: 1024

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3117 C++17 deduction guides for packaged_task were missing added

[edit] See also

(C++11)
 waits for a value that is set asynchronously
(class template) [edit]
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