Niklas Wiberg wrote:
Hi, I have the following situation: In the constructor body of class A, I need to call functions that take a shared_ptr<A> as a parameter. These functions do not store the shared_ptr<A>, but in some situations a weak_ptr<A>. There is not yet any shared_ptr owning the A instance, since this is during A construction...
At first, I thought using a shared_ptr<A> tmpThis(this, NullDeleter()) was a good idea. But when I later on (when another shared_ptr actually owns the A instance) want to use shared_from_this on A, I get an exception since the use_count of the internal weak_ptr is based on the first temporary shared_ptr that I used during construction (with the NullDeleter).
Am I correct in my assumption that the internal weak_ptr in enable_shared_from_this is initialized by the first shared_ptr and that will prevent any later use of shared_from_this when I have another shared_ptr (with a default deleter) owning the element?
In the current implementation, every shared_ptr constructed with a pointer to the object will overwrite the internal weak_ptr in enable_shared_from_this, so the last one wins. But this is an implementation detail; if you decide to port your code to a TR1 implementation of shared_ptr, it may or may not work.
I guess that a static create function would solve the problem, but changing that would be quite a big undertaking in this particular case so I thought I'd check if someone knows a simple solution to this...
The static create function is the simple solution. ;-)
I added a small example code snipped below...
Your example works for me. No exception is thrown. But:
void foo(shared_ptr<A> a); void otherFunction(shared_ptr<A> a);
class A : public enable_shared_from_this<A> { A() { // Can't use shared_from_this since no shared_ptr owns this yet shared_ptr<A> thisTmp(this, NullDeleter()); foo(thisTmp); }
If foo only stores a weak_ptr, it will expire immediately at the end of the constructor, so I'm not sure what are you trying to accomplish.