In the following code, while ( !Ref.expired() ); is joyfully optimized into an infinite loop. If the line of code is changed to while ( !Ref.lock() );. everything works as expected. So two questions really:
1) How can the compiler optimize away expired when std::weak_ptr::expired() accesses a memory-fenced counter?
2) Is Ref.lock() actually safe, or could this too be optimized away?
Sample code below.
#include <iostream>
#include <memory>
#include <thread>
#include <chrono>
class A
{
public:
A()
{
m_SomePtr = std::make_shared<bool>( false );
}
virtual ~A()
{
std::weak_ptr<bool> Ref = m_SomePtr;
m_SomePtr.reset();
// Spin (will be optimised into an infinite loop in release builds)
while ( !Ref.expired() );
}
std::shared_ptr<bool> GetPtr() const { return m_SomePtr; }
private:
std::shared_ptr<bool> m_SomePtr;
};
class B
{
public:
B( std::shared_ptr<bool> SomePtr ) : m_Ref( SomePtr ) {}
void LockPtr() { m_SomePtr = m_Ref.lock(); }
void UnLockPtr() { m_SomePtr.reset(); }
private:
std::shared_ptr<bool> m_SomePtr;
std::weak_ptr<bool> m_Ref;
};
int main()
{
std::unique_ptr<A> a( new A() );
std::unique_ptr<B> b( new B( a->GetPtr() ) );
b->LockPtr();
std::cout << "Starting " << std::endl;
std::thread first( [&]()
{
std::this_thread::sleep_for( std::chrono::seconds( 5 ) );
b->UnLockPtr();
} );
std::thread second( [&]()
{
a.reset( nullptr );
} );
first.join();
second.join();
std::cout << "Complete" << std::endl;
return 0;
}
