CODEWITHSUNDEEP
c++shared-ptr
Jimm
Jimm

Reputation: 8515

thread safety for objects stored in the shared pointer

I have a boost::shared_ptr<Map>, and this map is being modified by multiple threads, do i need to use explicit mutex or just the fact that threads are modifying the map through shared pointer, i will get some sort of implicit thread safety?

Upvotes: 0

Views: 191

Answers (2)

Alex Korban
Alex Korban

Reputation: 15136

shared_ptr has the same level of thread safety as built-in types.

You can perform read (const) operations from multiple threads simultaneously.

You can also perform write operations (i.e. use mutable operations such as operator= or reset) on different shared_ptr instances simultaneosly from multiple threads. This includes the case when these instances share the same reference count (because shared_ptr provides atomic increment/decrement for the ref counter).

If you need any other type of access, you will need to synchronize it or otherwise you'll get undefined behavior.

Upvotes: 1

Antimony
Antimony

Reputation: 39491

Did you even look at the docs?

shared_ptr objects offer the same level of thread safety as built-in types. A shared_ptr instance can be "read" (accessed using only const operations) simultaneously by multiple threads. Different shared_ptr instances can be "written to" (accessed using mutable operations such as operator= or reset) simultaneosly by multiple threads (even when these instances are copies, and share the same reference count underneath.)

Any other simultaneous accesses result in undefined behavior.

Edit: It looks like you're asking about the object pointed to, not the pointer itself. There are no thread safety gaurentees at all in that case. Why would there be?

Upvotes: 2

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