Espaços nominais
Variantes
Acções

std::atomic::compare_exchange_weak, std::atomic::compare_exchange_strong

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< cpp‎ | atomic‎ | atomic

 
 
Biblioteca operações atômica
Tipos
Original:
Types
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atomic(C++11)
atomic_is_lock_free(C++11)
Funções
Original:
Functions
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atomic_store
atomic_store_explicit
(C++11)
(C++11)
atomic_load
atomic_load_explicit
(C++11)
(C++11)
atomic_exchange
atomic_exchange_explicit
(C++11)
(C++11)
atomic_compare_exchange_weak
atomic_compare_exchange_weak_explicit
atomic_compare_exchange_strong
atomic_compare_exchange_strong_explicit
(C++11)
(C++11)
(C++11)
(C++11)
atomic_fetch_add
atomic_fetch_add_explicit
(C++11)
(C++11)
atomic_fetch_sub
atomic_fetch_sub_explicit
(C++11)
(C++11)
atomic_fetch_and
atomic_fetch_and_explicit
(C++11)
(C++11)
atomic_fetch_or
atomic_fetch_or_explicit
(C++11)
(C++11)
atomic_fetch_xor
atomic_fetch_xor_explicit
(C++11)
(C++11)
Bandeiras atômicas
Original:
Atomic flags
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atomic_flag(C++11)
atomic_flag_test_and_set
atomic_flag_test_and_set_explicit
(C++11)
(C++11)
atomic_flag_clear
atomic_flag_clear_explicit
(C++11)
(C++11)
Inicialização
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Initialization
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atomic_init(C++11)
ATOMIC_VAR_INIT(C++11)
ATOMIC_FLAG_INIT(C++11)
Memória de encomenda
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Memory ordering
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memory_order(C++11)
kill_dependency(C++11)
atomic_thread_fence(C++11)
atomic_signal_fence(C++11)
 
std::atomic
Funções de membro
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Member functions
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atomic::atomic
atomic::operator=
atomic::is_lock_free
atomic::store
atomic::load
atomic::operator T
atomic::exchange
atomic::compare_exchange
Membro funções especializadas
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Specialized member functions
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atomic::fetch_add
atomic::fetch_sub
atomic::fetch_and
atomic::fetch_or
atomic::fetch_xor
atomic::operator++
atomic::operator++(int)
atomic::operator--
atomic::operator--(int)
atomic::operator+=
atomic::operator-=
atomic::operator&=
atomic::operator|=
atomic::operator^=
 
Defined in header <atomic>
bool compare_exchange_weak( T& expected, T desired,

                            std::memory_order success,
                            std::memory_order failure );

bool compare_exchange_weak( T& expected, T desired,
                            std::memory_order success,

                            std::memory_order failure ) volatile;
(1) (a partir do C++ 11)
bool compare_exchange_weak( T& expected, T desired,

                            std::memory_order order =
                                std::memory_order_seq_cst );

bool compare_exchange_weak( T& expected, T desired,
                            std::memory_order order =

                                std::memory_order_seq_cst ) volatile;
(2) (a partir do C++ 11)
bool compare_exchange_strong( T& expected, T desired,

                              std::memory_order success,
                              std::memory_order failure );

bool compare_exchange_strong( T& expected, T desired,
                              std::memory_order success,

                              std::memory_order failure ) volatile;
(3) (a partir do C++ 11)
bool compare_exchange_strong( T& expected, T desired,

                              std::memory_order order =
                                  std::memory_order_seq_cst );

bool compare_exchange_strong( T& expected, T desired,
                              std::memory_order order =

                                  std::memory_order_seq_cst ) volatile;
(4) (a partir do C++ 11)
Atomicamente compara o valor armazenado em *this com o valor apontado por expected, e se estas forem iguais, substitui o ex-desired (executa ler-modificar-escrever a operação). Caso contrário, carrega o valor real armazenado no *this em *expected (realiza a operação de carga).
Original:
Atomically compares the value stored in *this with the value pointed to by expected, and if those are equal, replaces the former with desired (performs read-modify-write operation). Otherwise, loads the actual value stored in *this into *expected (performs load operation).
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Os modelos de memória para a leitura-modificação-gravação e operações de carga são success e failure respectivamente. No (2) e (4) as versões order é usado tanto para ler-modificar-escrever e carregar operações, excepto que std::memory_order_release std::memory_order_relaxed e são utilizadas para a operação de carregamento se order == std::memory_order_acq_rel ou order == std::memory_order_release respectivamente.
Original:
The memory models for the read-modify-write and load operations are success and failure respectively. In the (2) and (4) versions order is used for both read-modify-write and load operations, except that std::memory_order_release and std::memory_order_relaxed are used for the load operation if order == std::memory_order_acq_rel, or order == std::memory_order_release respectively.
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As formas fracas (1-2) das funções podem falhar spuriously, isto é, actuar como se *this != *expected mesmo se eles forem iguais. Quando uma comparação e troca-se em um loop, a versão fraca trará melhor desempenho em algumas plataformas. Quando um fraco comparar-e-troca exigiria um laço e um forte não, o forte é preferível.
Original:
The weak forms (1-2) of the functions are allowed to fail spuriously, that is, act as if *this != *expected even if they are equal. When a compare-and-exchange is in a loop, the weak version will yield better performance on some platforms. When a weak compare-and-exchange would require a loop and a strong one would not, the strong one is preferable.
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Índice

[editar] Parâmetros

expected -
ponteiro para o valor esperado para ser encontrado no objeto atómica
Original:
pointer to the value expected to be found in the atomic object
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desired -
o valor para armazenar no objeto atômico se for como o esperado
Original:
the value to store in the atomic object if it is as expected
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success -
a sincronização de memória de ordenação para a leitura-modificação-gravação operação, se a comparação for bem-sucedido. Todos os valores são permitidos .
Original:
the memory synchronization ordering for the read-modify-write operation if the comparison succeeds. All values are permitted.
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failure -
a sincronização da memória de ordenação para a operação de carregamento se a comparação falhar. Não pode ser std::memory_order_release ou std::memory_order_ack_rel e não pode especificar mais forte do que success ordenação
Original:
the memory synchronization ordering for the load operation if the comparison fails. Cannot be std::memory_order_release or std::memory_order_ack_rel and cannot specify stronger ordering than success
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order -
a sincronização da memória de ordenação para as duas operações
Original:
the memory synchronization ordering for both operations
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[editar] Valor de retorno

true se o valor subjacente atômica foi alterado, caso contrário false.
Original:
true if the underlying atomic value was changed, false otherwise.
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[editar] Exceções

noexcept specification:  
noexcept
  (a partir do C++ 11)

[editar] Exemplo

Demonstra como compare_exchange_strong quer altera o valor da variável atómica ou a variável usada para comparação .
Original:
Demonstrates how compare_exchange_strong either changes the value of the atomic variable or the variable used for comparison.
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#include <atomic>
#include <iostream>
 
std::atomic<int>  ai;
 
int  tst_val= 4;
int  new_val= 5;
bool exchanged= false;
 
void valsout()
{
    std::cout << "ai= " << ai
	      << "  tst_val= " << tst_val
	      << "  new_val= " << new_val
	      << "  exchanged= " << std::boolalpha << exchanged
	      << "\n";
}
 
int main()
{
    ai= 3;
    valsout();
 
    // tst_val != ai   ==>  tst_val is modified
    exchanged= ai.compare_exchange_strong( tst_val, new_val );
    valsout();
 
    // tst_val == ai   ==>  ai is modified
    exchanged= ai.compare_exchange_strong( tst_val, new_val );
    valsout();
 
    return 0;
}

Output:

ai= 3  tst_val= 4  new_val= 5  exchanged= false
ai= 3  tst_val= 3  new_val= 5  exchanged= false
ai= 5  tst_val= 3  new_val= 5  exchanged= true

[editar] Veja também

atomicamente compara o valor do objeto atômico com não atômica argumento e realiza troca atômica se carga igual ou atômica se não
Original:
atomically compares the value of the atomic object with non-atomic argument and performs atomic exchange if equal or atomic load if not
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(modelo de função) [edit]