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

 

C++

Language

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

 

Atomic operations library

Types

atomic (C++11)

atomic_is_lock_free (C++11)

Functions

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)

Atomic flags

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)

Initialization

atomic_init (C++11)

ATOMIC_VAR_INIT (C++11)

ATOMIC_FLAG_INIT (C++11)

Memory ordering

memory_order (C++11)

kill_dependency (C++11)

atomic_thread_fence (C++11)

atomic_signal_fence (C++11)

 

std::atomic

Template:cpp/atomic/atomic/navbar content

 

Defined in header <atomic>
bool compare_exchange_weak(T & expected, T desired,                            memory_order success, memory_order failure) volatile;	(1)	(since C++11)
bool compare_exchange_weak(T & expected, T desired,                            memory_order success, memory_order failure);	(2)	(since C++11)
bool compare_exchange_weak(T & expected, T desired,                            memory_order order = memory_order_seq_cst) volatile;	(3)	(since C++11)
bool compare_exchange_weak(T & expected, T desired,                            memory_order order = memory_order_seq_cst);	(4)	(since C++11)
bool compare_exchange_strong(T & expected, T desired,                              memory_order success, memory_order failure) volatile;	(5)	(since C++11)
bool compare_exchange_strong(T & expected, T desired,                              memory_order success, memory_order failure);	(6)	(since C++11)
bool compare_exchange_strong(T & expected, T desired,                              memory_order order = memory_order_seq_cst) volatile;	(7)	(since C++11)
bool compare_exchange_strong(T & expected, T desired,                              memory_order order = memory_order_seq_cst);	(8)	(since C++11)

Return value

Boolean indicating, if the underlying atomic variable was changed.

Example

Demonstrates how compare_exchange_strong either changes the value of the atomic variable or the variable used for comparison.

#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

See also

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)	atomically compares the value of the atomic object with non-atomic argument and performs atomic exchange if equal or atomic load if not (function template)