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std::async

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this_thread spazio dei nomi
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this_thread namespace
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Defined in header <future>
template< class Function, class... Args>

std::future<typename std::result_of<Function(Args...)>::type>

    async( Function&& f, Args&&... args );
(1) (dal C++11)
template< class Function, class... Args >

std::future<typename std::result_of<Function(Args...)>::type>

    async( std::launch policy, Function&& f, Args&&... args );
(2) (dal C++11)
La funzione async modello esegue la funzione f in modo asincrono (potenzialmente in un thread separato) e restituisce un std::future che finirà per contenere il risultato di questa chiamata di funzione.
Original:
The template function async runs the function f asynchronously (potentially in a separate thread) and returns a std::future that will eventually hold the result of that function call.
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1)
Si comporta come async(std::launch::async | std::launch::deferred, f, args...). In altre parole, f può essere avviato in un nuovo thread o può essere eseguito in modo sincrono quando il std::future risultante viene eseguita una query per un valore.
Original:
Behaves the same as async(std::launch::async | std::launch::deferred, f, args...). In other words, f may be started in a new thread or it may be run synchronously when the resulting std::future is queried for a value.
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2)
Chiama una funzione con argomenti f args secondo una politica di lancio specifica policy:
Original:
Calls a function f with arguments args according to a specific launch policy policy:
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  • Se la bandiera async è impostato (cioè policy & std::launch::async != 0), poi async genera un nuovo thread di esecuzione come per std::thread(f, args...), salvo che, se la funzione restituisce un valore f o genera un'eccezione, viene memorizzato nello stato condiviso accessibile attraverso la std::future che async restituisce al chiamante.
    Original:
    If the async flag is set (i.e. policy & std::launch::async != 0), then async spawns a new thread of execution as if by std::thread(f, args...), except that if the function f returns a value or throws an exception, it is stored in the shared state accessible through the std::future that async returns to the caller.
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  • Se la differita bandiera' è impostato (cioè policy & std::launch::deferred != 0), poi convertiti async args... allo stesso modo dal costruttore std::thread, ma non genera un nuovo thread di esecuzione. Invece, valutazione pigro viene eseguita: la prima chiamata a un non-temporizzato attendere funzione sul std::future che async restituito al chiamante causerà f(args...) da eseguire nel thread corrente. Tutti gli ulteriori accessi al std::future stesso restituirà immediatamente il risultato.
    Original:
    If the deferred flag is set (i.e. policy & std::launch::deferred != 0), then async converts args... the same way as by std::thread constructor, but does not spawn a new thread of execution. Instead, lazy evaluation is performed: the first call to a non-timed wait function on the std::future that async returned to the caller will cause f(args...) to be executed in the current thread. All further accesses to the same std::future will return the result immediately.
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  • Se entrambe le bandierine std::launch::async e std::launch::deferred si trovano in policy, spetta alla realizzazione se eseguire l'esecuzione asincrona o valutazione pigra.
    Original:
    If both the std::launch::async and std::launch::deferred flags are set in policy, it is up to the implementation whether to perform asynchronous execution or lazy evaluation.
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Indice

[modifica] Parametri

f -
oggetto funzione o la funzione da chiamare
Original:
function or function object to call
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args... -
parametri da passare al f
Original:
parameters to pass to f
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policy -
maschera di bit valore, dove i singoli bit controllare i metodi consentiti di esecuzione
Bit
Spiegazione
Original:
Explanation
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std::launch::async
consentire una valutazione asincrono
Original:
enable asynchronous evaluation
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std::launch::deferred
consentire una valutazione pigra
Original:
enable lazy evaluation
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Original:
bitmask value, where individual bits control the allowed methods of execution
Bit
Spiegazione
Original:
Explanation
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std::launch::async
consentire una valutazione asincrono
Original:
enable asynchronous evaluation
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std::launch::deferred
consentire una valutazione pigra
Original:
enable lazy evaluation
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[modifica] Valore di ritorno

std::future riferimento al valore di ritorno della funzione.
Original:
std::future referring to the return value of the function.
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[modifica] Eccezioni

Lanci std::system_error con std::errc::resource_unavailable_try_again condizione di errore se la politica è std::launch::async lancio e l'implementazione è in grado di avviare un nuovo thread.
Original:
Throws std::system_error with error condition std::errc::resource_unavailable_try_again if the launch policy is std::launch::async and the implementation is unable to start a new thread.
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[modifica] Note

L'implementazione può estendere il comportamento del sovraccarico prima std::async consentendo supplementari (attuazione definito) bit nella politica di lancio di default.
Original:
The implementation may extend the behavior of the first overload of std::async by enabling additional (implementation-defined) bits in the default launch policy.
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[modifica] Esempio

#include <iostream>
#include <vector>
#include <algorithm>
#include <numeric>
#include <future>
 
template <typename RAIter>
int parallel_sum(RAIter beg, RAIter end)
{
    typename RAIter::difference_type len = end-beg;
    if(len < 1000)
        return std::accumulate(beg, end, 0);
 
    RAIter mid = beg + len/2;
    auto handle = std::async(std::launch::async,
                              parallel_sum<RAIter>, mid, end);
    int sum = parallel_sum(beg, mid);
    return sum + handle.get();
}
 
int main()
{
    std::vector<int> v(10000, 1);
    std::cout << "The sum is " << parallel_sum(v.begin(), v.end()) << '\n';
}

Output:

The sum is 10000