How to pass a shared_ptr to a thread.
Hi, I'm new to boost. I'm using it for the shared_ptr implementation. I have a usage question. On Windows, a thread function takes a void* parameter. If I have a shared_ptr to an object, what's the correct way to pass that to a new thread? E.g.: DWORD WINAPI threadproc (LPVOID something) { // how to get a shared_ptr through "something"? ... } void create5threads () { ... shared_ptr p(new Object); // what should "something" be if i want to pass p to the new threads, incrementing its use count? CreateThread(..., &threadproc, (LPVOID)something, ...); CreateThread(..., &threadproc, (LPVOID)something, ...); CreateThread(..., &threadproc, (LPVOID)something, ...); CreateThread(..., &threadproc, (LPVOID)something, ...); CreateThread(..., &threadproc, (LPVOID)something, ...); } Sorry if this is a stupid question, but the only thing I could think of was "(LPVOID)new shared_ptr(p)", but that means each thread must explicitly delete the new shared_ptr and that kind of defeats the purpose! Thanks, Jason
AMDG Jason Cipriani wrote:
Hi, I'm new to boost. I'm using it for the shared_ptr implementation. I have a usage question. On Windows, a thread function takes a void* parameter. If I have a shared_ptr to an object, what's the correct way to pass that to a new thread? E.g.:
<snip>
Sorry if this is a stupid question, but the only thing I could think of was "(LPVOID)new shared_ptr(p)", but that means each thread must explicitly delete the new shared_ptr and that kind of defeats the purpose!
One solution would be to use Boost.Thread... Another would be to have the thread function immediately copy the shared_ptr to a local variable and delete the one on the heap. Finally, you could use enable_shared_from this, to allow a shared_ptr to be recovered from a raw pointer. In Christ, Steven Watanabe
On Wed, Dec 3, 2008 at 10:34 PM, Steven Watanabe
AMDG
Jason Cipriani wrote:
Hi, I'm new to boost. I'm using it for the shared_ptr implementation. I have a usage question. On Windows, a thread function takes a void* parameter. If I have a shared_ptr to an object, what's the correct way to pass that to a new thread? E.g.:
<snip>
Sorry if this is a stupid question, but the only thing I could think of was "(LPVOID)new shared_ptr(p)", but that means each thread must explicitly delete the new shared_ptr and that kind of defeats the purpose!
Thanks for the quick reply.
One solution would be to use Boost.Thread...
It's too late to refactor, unfortunately.
Another would be to have the thread function immediately copy the shared_ptr to a local variable and delete the one on the heap.
This is the way I'll do it, that's a good idea. It's not what I was hoping for but I guess it's not that bad, it's easy to understand, and it does what I want, thanks!
Finally, you could use enable_shared_from this, to allow a shared_ptr to be recovered from a raw pointer.
Hmm, is there a way to do something like this (the following code is broken): class Item : public enable_shared_from_this<Item> { ... }; DWORD WINAPI threadproc (LPVOID vitem) { shared_ptr<Item> s = ((Item *)vitem)->shared_from_this(); ... } void start5threads () { shared_ptr<Item> p(new Item); CreateThread(..., &threadproc, (LPVOID)p.get(), ...); CreateThread(..., &threadproc, (LPVOID)p.get(), ...); CreateThread(..., &threadproc, (LPVOID)p.get(), ...); CreateThread(..., &threadproc, (LPVOID)p.get(), ...); CreateThread(..., &threadproc, (LPVOID)p.get(), ...); } ... that doesn't require any special synchronization to avoid 'p' in start5threads going out of scope (and deleting the Item) before 's' is initialized in any of the threads? I've got the solution though now, thanks again! Jason
AMDG Jason Cipriani wrote:
Finally, you could use enable_shared_from this, to allow a shared_ptr to be recovered from a raw pointer.
Hmm, is there a way to do something like this (the following code is broken):
class Item : public enable_shared_from_this<Item> { ... };
DWORD WINAPI threadproc (LPVOID vitem) { shared_ptr<Item> s = ((Item *)vitem)->shared_from_this(); ... }
void start5threads () { shared_ptr<Item> p(new Item); CreateThread(..., &threadproc, (LPVOID)p.get(), ...); CreateThread(..., &threadproc, (LPVOID)p.get(), ...); CreateThread(..., &threadproc, (LPVOID)p.get(), ...); CreateThread(..., &threadproc, (LPVOID)p.get(), ...); CreateThread(..., &threadproc, (LPVOID)p.get(), ...); }
... that doesn't require any special synchronization to avoid 'p' in start5threads going out of scope (and deleting the Item) before 's' is initialized in any of the threads?
Oh, right. Never mind that idea. In Christ, Steven Watanabe
On Wed, Dec 3, 2008 at 22:29, Jason Cipriani
Sorry if this is a stupid question, but the only thing I could think of was "(LPVOID)new shared_ptr(p)", but that means each thread must explicitly delete the new shared_ptr and that kind of defeats the purpose!
That's the usual way to pass things through void* callbacks. If you don't want to do that, then use boost.thread, which does that trick for you with boost.function so you can use arbitrary functors, allowing state. boost::thread mythread(boost::bind(threadproc, something));
participants (3)
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Jason Cipriani -
Scott McMurray -
Steven Watanabe