What's the performance penalty of weak_ptr?

Question

I'm currently designing a object structure for a game, and the most natural organization in my case became a tree. Being a great fan of smart pointers I use shared_ptr's exclusively. However, in this case, the children in the tree will need access to it's parent (example -- beings on map need to be able to access map data -- ergo the data of their parents.

The direction of owning is of course that a map owns it's beings, so holds shared pointers to them. To access the map data from within a being we however need a pointer to the parent -- the smart pointer way is to use a reference, ergo a weak_ptr.

However, I once read that locking a weak_ptr is a expensive operation -- maybe that's not true anymore -- but considering that the weak_ptr will be locked very often, I'm concerned that this design is doomed with poor performance.

Hence the question:

What is the performance penalty of locking a weak_ptr? How significant is it?

Answer 1

From the Boost 1.42 source code (<boost/shared_ptr/weak_ptr.hpp> line 155):

shared_ptr<T> lock() const // never throws
{
    return shared_ptr<element_type>( *this, boost::detail::sp_nothrow_tag() );
}

ergo, James McNellis's comment is correct; it's the cost of copy-constructing a shared_ptr.

Answer 2

For my own project, I was able to improve performance dramatically by adding #define BOOST_DISABLE_THREADS before any boost includes. This avoids the spinlock/mutex overhead of weak_ptr::lock which in my project was a major bottleneck. As the project is not multithreaded wrt boost, I could do this.

Answer 3

Using/dereferencing a shared_ptr is almost like accessing raw ptr, locking a weak_ptr is a perf "heavy" operation compared to regular pointer access, because this code has to be "thread-aware" to work correctly in case if another thread triggers release of the object referenced by the pointer. At minimum, it has to perform some sort of interlocked/atomic operation that by definition is much slower than regular memory access.

As usual, one way to see what's going on is to inspect generated code:

#include <memory>

class Test
{
public:
    void test();
};

void callFuncShared(std::shared_ptr<Test>& ptr)
{
    if (ptr)
        ptr->test();
}

void callFuncWeak(std::weak_ptr<Test>& ptr)
{
    if (auto p = ptr.lock())
        p->test();
}

void callFuncRaw(Test* ptr)
{
    if (ptr)
        ptr->test();
}

Accessing through shared_ptr and raw pointer is the same. Since shared_ptr was passed as a reference, we need to load referenced value, that's why the difference is only one extra load for shared_ptr version.

callFuncShared:

callFuncWeak:

Calling through weak_ptr produces 10x more code and at best it has to go through locked compare-exchange, which by itself will take more than 10x CPU time than dereferencing raw or shared_ptr:

Only if the shared counter isn't zero, only then it can load the pointer to actual object and use it (by calling the object, or creating a shared_ptr).