std::map thread-safety

Question

Is reference to object in std::map is thread safe?

std::map< std::string, Object >   _objects;

map can be changed from many threads and this access is synchronized, but reference to value (Object &) accessable just from 1 instance and thread. is write operations with Object & is safe if another thread will add items to map? will it reallocate?

Answer 1

The C++11 standard guarantees that const method access to containers is safe from different threads (ie, both use const methods).

In addition, [container.requirements.dataraces] states

implementations are required to avoid data races when the contents of the contained object in different elements in the same sequence, excepting vector<bool>

In other words, except for vector<bool> modifying distinct contents is not a data race.

Now, if one thread invalidates an iterator used by another thread, clearly this is a data race (and results in undefined behavior). If one thread does non-const access to a container, and another does const access, that is a data race (and undefined behavior). (Note: a number of functions are "considered const" for the purpose of multithreading, including begin, end and other functions (and methods) that are non-const simply because they return non-const iterators. [] is included in this set of pseudo-const for thread safety reasons, except for map and unordered_set etc -- 23.2.2.1).

However, it appears that if you have a reference to an element within the container, and engage in operations that do not invalidate that reference in another thread, and never write to that element in another thread, you can safely read from that reference. Similarly, if other threads never even read from the element, then writing to that element shouldn't result in undefined behavior.

For standards references, 17.6.5.9.5 seems to guarantee that functions from the standard library won't run away and read/write elements needlessly.

So the short answer: you are safe, so long as the other thread doesn't directly mess with that particular entry in the map.