-----Original Message----- From: boost-bounces@lists.boost.org [mailto:boost-bounces@lists.boost.org] On Behalf Of Lars Viklund Sent: Sunday, May 29, 2011 12:10 AM To: boost@lists.boost.org Subject: Re: [boost] [shared_ptr] calling functions by reference
Concurrent access to different shared_ptr instances in the same family (sourced from the same source) is safe. Concurrent access to a single shared_ptr instance is generally unsafe.
You are confirming my suspicion about multi-threading issues.
Passing the shared_ptr by reference puts the onus on you to ensure that the shared_ptr instance you pass in is valid for the duration of the function call, which is non-trivial in general.
I understand. But doesn't it help to have guidelines and standards when it comes to working with shared_ptr<>?
Examples of the above would be foo(some_global_ptr); or foo(bar->baz); where some_global_ptr is destroyed somewhere deep in the call chain, or bar is destroyed during the call sequence.
If I understand you correctly, somebody performed a reset() on something along the call sequence. Would I be correct in saying that if reset() isn't used in a busy call sequence, then this problem wouldn't arise? Wouldn't using a const-reference, like you mention below, identify potential problems like that?
I've had real-life bugs of this kind. It's not pretty. Unless you have a genuine reason to access the same instance, try to avoid passing it by reference-to-non-const. If you feel you need to avoid the non-free copying cost, pass it by reference-to-const.
-- Lars Viklund | zao@acc.umu.se