tbb::concurrent_vector allows multiple threads to grow and access the vector at the same time. This is the level of thread safety that tbb_concurrent_vector has over std::vector. Simultaneously growing and accessing elements is a recurring idiom in parallel programming.

For example, if x is a std::vector and thread #1 executes x.resize(x.size()+1) while thread #2 executes x[0]=...; internal corruption may result, because in typical implementations of std::vector, a resize operation may cause the internal storage to move to a different address. Thus the location of x[0] in memory may move between the time that the reference to x[0] is computed and x[0] is assigned to.

tbb::concurrent_vector deals with the problem by not moving existing storage when resizing. Instead, in order to grow, it creates a new chunk of storage. The chunks are accessed by a fast constant-time indexing scheme. See tbb/concurrent_vector.h for how it is implemented.

However, there is no free lunch here. The concurrent_vector indexing introduces an extra level of complexity compared to std::vector. So if you do not need to concurrently grow and access a vector, I recommend using std::vector. But if you do have multiple threads appending to a dynamically growing array, then concurrent_vector can be a good fit.

concurrent_vector allows multiple threads to access (read, write, take the address) of the same element. It does not serialize the accesses, so whether those accesses are safe depends upon the data type.