Check boost::interprocess::vector has enough capacity before writing?

I am using a shared vector to share objects across memory:

using ShmemAllocator = bip::allocator<T, bip::managed_shared_memory::segment_manager>;
using MyVector = bip::vector<T, ShmemAllocator>;

bip::permissions perm;
perm.set_unrestricted();
segment.reset(new bip::managed_shared_memory(bip::open_or_create, shared_memory_name, numBytes, 0, perm));

const ShmemAllocator alloc_inst(segment->get_segment_manager());
vec = segment->find_or_construct<MyVector>(shared_vector_name)(alloc_inst);

Note the vector is created within a managed_shared_memory object and this is created by specifying a number of bytes, not number of vector elements.

I then write elements to the vector:

int write(const std::vector<T>& vec)
{
    bip::scoped_lock<bip::named_mutex> lock(*sdc.mutex);

    for(const auto& item : vec)
    {
        sdc.vec->push_back(item);
    }

    sdc.cond_empty->notify_all();
}

What is the safest way to check whether I have enough space to write all my elements, prior to writing? I would really like to avoid simply assigning a large number of bytes and hoping I never hit it!