问题
Short introduction: I am working on multithread code and I have to share dynamically allocated objects between two threads. To make my code cleaner (and less error-prone) I want to explicitly "delete" objects in each thread and that's why I want to use shared_ptr.
First question:
I want to know if implementation of -> operator in shared_ptr has some extra overhead (e.g. larger then unique_ptr) during run time. Objects I am talking about are usually longlife instances copied only once after creation (when i distribute them between threads), then I only access these objects' methods and fields.
I am aware, that shared_ptr only protect reference counting.
Second question:
How well are shared_ptr optimized in libstdc++? Does it always use mutex or take advantage of atomic operations (I focus on x86 and ARM platforms)?
回答1:
First question: using
operator->
All the implementations I have seen have a local cache of T* right in the shared_ptr<T> class so that the field is on the stack, operator-> has thus a comparable cost to using a stack local T*: no overhead at all.
Second question: mutex/atomics
I expect libstdc++ to use atomics on x86 platform, whether through standard facilities or specific g++ intrinsics (in the older versions). I believe the Boost implementation already did so.
I cannot, however, comment on ARM.
Note: C++11 introducing move semantics, many copies are naturally avoided in the usage of shared_ptr.
Note: read about correct usage of shared_ptr here, you can use references to shared_ptr (const or not) to avoid most of the copies/destruction in general, so the performance of those is not too important.
回答2:
GCC's shared_ptr will use no locking or atomics in single-threaded code. In multi-threaded code it will use atomic operations if an atomic compare-and-swap instruction is supported by the CPU, otherwise the reference counts are protected by a mutex. On i486 and later it uses atomics, i386 doesn't support cmpxchg so uses a mutex-based implementation. I believe ARM uses atomics for the ARMv7 architecture and later.
(The same applies to both std::shared_ptr and std::tr1::shared_ptr.)
来源：https://stackoverflow.com/questions/10902227/overhead-and-implementation-of-using-shared-ptr