stl map find thread safe

Question

Is find call on stl map thread safe?

Answer 1

No, the C++ spec makes no guarantees on thread safety in the spec for operations on any STL containers. If thread safety is important, the you should provide your own locking.

That being said, different implementations seem to offer different guarantees. Most seem to allow multiple concurrent readers, for example, as long as no writing is being performed concurrently. If you don't care about portability, you can research the documentation for your implementation. For example from here for SGI STL:

The SGI implementation of STL is thread-safe only in the sense that simultaneous accesses to distinct containers are safe, and simultaneous read accesses to to shared containers are safe. If multiple threads access a single container, and at least one thread may potentially write, then the user is responsible for ensuring mutual exclusion between the threads during the container accesses.

From this answer, a similar guarantee seems to be made by Dinkumware (they make Microsoft's STL implementation).

Multiple threads can safely read the same container object. (There are nunprotected mutable subobjects within a container object.)

Two threads can safely manipulate different container objects of the same type. (There are no unprotected shared static objects within a container type.)

You must protect against simultaneous access to a container object if at least one thread is modifying the object. (The obvious synchronization primitives, such as those in the Dinkum Threads Library, will not be subverted by the container object.)