Hello
what are the current plans for supporting atomic_* free functions for
shared_ptr, as defined in C++11 (20.6.2 "Header <memory> synopsis")?
These would be very useful for passing shared_ptr objects between
threads, esp. if there are multiple readers and single writer.
Currently the only safe and viable way to prevent race condition (when
smart pointer being copied in one thread is concurrently reset in
another) is to always use mutexes when accessing these objects, but that
is more expensive than it needs to be, esp. if multiple readers are
trying to read the data at roughly the same moment (which is typical in
publisher/subscriber model).
Example:
shared_ptr<Data> data; // Written by one thread, read by many
mutex mutex; // Required due to lack of atomic_ overloads
thread 1:
auto newData = make_shared<Data>(a, b, c);
{
lock_guard l(mutex);
swap(data, newData);
}
thread 2:
shared_ptr<const Data> readData;
{
lock_guard l(mutex);
readData = data;
}
// act on readData
thread 3: (same as thread 2)
thread 4: (same as thread 2)
etc.
... could be replaced with, hopefully more efficient:
thread 1:
auto newData = make_shared<Data>(a, b, c);
atomic_store(&data, newData);
thread 2:
shared_ptr<const Data> readData = atomic_load(&data);
thread 3: (same as thread 2)
thread 4: (same as thread 2)
etc.
Current behaviour, g++ 4.8.1 (mingw):
#include <memory>
#include <atomic>
int main()
{
std::shared_ptr<int> a;
std::atomic_store(&a, std::shared_ptr<int>());
}
In file included from main.cpp:2:0:
d:\programdata\mingw\include\c++\4.8.1\atomic:890:5: note:
template<class _ITp> void std::atomic_store(std::atomic<_ITp>*, _ITp)
atomic_store(atomic<_ITp>* __a, _ITp __i) noexcept
^
d:\programdata\mingw\include\c++\4.8.1\atomic:890:5: note: template
argument deduction/substitution failed:
main.cpp:7:47: note: 'std::shared_ptr<int>' is not derived from
'std::atomic<_ITp>'
std::atomic_store(&a, std::shared_ptr<int>());
^
In file included from main.cpp:2:0:
d:\programdata\mingw\include\c++\4.8.1\atomic:895:5: note:
template<class _ITp> void std::atomic_store(volatile std::atomic<_ITp>*,
_ITp)
atomic_store(volatile atomic<_ITp>* __a, _ITp __i) noexcept
^
d:\programdata\mingw\include\c++\4.8.1\atomic:895:5: note: template
argument deduction/substitution failed:
main.cpp:7:47: note: 'std::shared_ptr<int>' is not derived from
'volatile std::atomic<_ITp>'
std::atomic_store(&a, std::shared_ptr<int>());
^
B.
