问题
Suppose I have the need to do the following (This is just some imaginative code for discussion of the C++ standard, thus I won't discuss why I design it this way, so don't bother me with something like: your design is wrong.)
T* ptr = new T;
shared_ptr<T> p(ptr);
shared_ptr<T> q(ptr, SomeDeleterThatDoesnotDeleteButDoSomeOtherStuff());
Suppose the logic guarantees that p or some of its copies lives longer than all copies of q, so practically there won't be any problem. My question is, is it forbidden by C++ standard, e.g. explicitly stated as UB by C++ standard, for different shared_ptr counters to share the same address?
Thanks.
回答1:
I can't find anything in the standard (well, the final draft) that specifically rules it out. The closest I can find is a note in 20.9.11.2.10 shared_ptr casts
5 [ Note: The seemingly equivalent expression shared_ptr(static_cast(r.get())) will eventually result in undefined behavior, attempting to delete the same object twice. —end note ]
which actually seems to forget about your case with a custom deleter.
回答2:
If the first shared_ptr object is destroyed, then you get UB because objects using the second might access released object.
Since you made sure that your first shared_ptr object lives longer then the second, you do not get UB.
来源：https://stackoverflow.com/questions/11079770/same-address-multiple-shared-ptr-counters-is-it-forbidden-by-c-standard