NAME¶

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

Synopsis¶

bool compare_exchange_weak( T&
expected, T desired,

std::memory_order success,

std::memory_order failure )
noexcept;
bool compare_exchange_weak( T&
expected, T desired,

std::memory_order success,

std::memory_order failure )
volatile noexcept;
bool compare_exchange_weak( T&
expected, T desired,

std::memory_order order =

std::memory_order_seq_cst )
noexcept;
bool compare_exchange_weak( T&
expected, T desired,

std::memory_order order =

std::memory_order_seq_cst )
volatile noexcept; (1) (since
bool compare_exchange_strong( T& C++11)
expected, T desired,

std::memory_order success,

std::memory_order failure )
noexcept; (2) (since
bool compare_exchange_strong( T& C++11)
expected, T desired,

std::memory_order success,

std::memory_order failure )
volatile noexcept; (3) (since
bool compare_exchange_strong( T& C++11)
expected, T desired,

std::memory_order order =

std::memory_order_seq_cst )
noexcept; (4) (since
bool compare_exchange_strong( T& C++11)
expected, T desired,

std::memory_order order =

std::memory_order_seq_cst )
volatile noexcept;

Atomically compares the
object representation
(until C++20)
value representation
(since C++20) of *this with that of expected, and if those are bitwise-equal,
replaces the former with desired (performs read-modify-write operation). Otherwise,
loads the actual value stored in *this into expected (performs load operation).

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_acquire 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.

The volatile-qualified versions are deprecated if (since C++20)
std::atomic<T>::is_always_lock_free is false.

Parameters¶

expected - reference to the value expected to be found in the atomic object. Gets
stored with the actual value of *this if the comparison fails.
desired - the value to store in the atomic object if it is as expected
success - the memory synchronization ordering for the read-modify-write operation
if the comparison succeeds. All values are permitted.
the memory synchronization ordering for the load operation if the
comparison fails. Cannot be std::memory_order_release or
failure - std::memory_order_acq_rel
and cannot specify stronger ordering than success
(until C++17)
order - the memory synchronization ordering for both operations

Return value¶

true if the underlying atomic value was successfully changed, false otherwise.

Notes¶

The comparison and copying are bitwise (similar to std::memcmp and std::memcpy); no
constructor, assignment operator, or comparison operator are used.

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 unless the object representation of T may include
padding bits,
(until C++20) trap bits, or offers multiple object representations for the same
value (e.g. floating-point NaN). In those cases, weak compare-and-exchange typically
works because it quickly converges on some stable object representation.

For a union with bits that participate in the value representations of some members
but not the others, compare-and-exchange might always fail because such padding bits
have indeterminate values when they do not participate in the value representation
of the active member.

Padding bits that never participate in an object's value (since C++20)
representation are ignored.

Example¶

Compare-and-exchange operations are often used as basic building blocks of lockfree
data structures

// Run this code

#include <atomic>
template<typename T>
struct node
{
T data;
node* next;
node(const T& data) : data(data), next(nullptr) {}
};

template<typename T>
class stack
{
std::atomic<node<T>*> head;
public:
void push(const T& data)
{
node<T>* new_node = new node<T>(data);

// put the current value of head into new_node->next
new_node->next = head.load(std::memory_order_relaxed);

// now make new_node the new head, but if the head
// is no longer what's stored in new_node->next
// (some other thread must have inserted a node just now)
// then put that new head into new_node->next and try again
while(!head.compare_exchange_weak(new_node->next, new_node,
std::memory_order_release,
std::memory_order_relaxed))
; // the body of the loop is empty

// Note: the above use is not thread-safe in at least
// GCC prior to 4.8.3 (bug 60272), clang prior to 2014-05-05 (bug 18899)
// MSVC prior to 2014-03-17 (bug 819819). The following is a workaround:
// node<T>* old_head = head.load(std::memory_order_relaxed);
// do {
// new_node->next = old_head;
// } while(!head.compare_exchange_weak(old_head, new_node,
// std::memory_order_release,
// std::memory_order_relaxed));
}
};
int main()
{
stack<int> s;
s.push(1);
s.push(2);
s.push(3);
}

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

This section is incomplete
Reason: more practical use of the strong CAS would be nice, such as where
Concurrency in Action uses it

// Run this code

#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();
}

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 atomically compares the value of the atomic
atomic_compare_exchange_strong object with non-atomic argument and performs
atomic_compare_exchange_strong_explicit atomic exchange if equal or atomic load if
(C++11) not
(C++11) (function template)
(C++11)
(C++11)