Question

I’ve stared at the Boost.Interprocess documentation for hours but still haven’t been able to figure this out. In the doc, they have an example of creating a vector in shared memory like so:

//Define an STL compatible allocator of ints that allocates from the managed_shared_memory.
//This allocator will allow placing containers in the segment
typedef allocator<int, managed_shared_memory::segment_manager>  ShmemAllocator;

//Alias a vector that uses the previous STL-like allocator so that allocates
//its values from the segment
typedef vector<int, ShmemAllocator> MyVector;

int main(int argc, char *argv[])
{
    //Create a new segment with given name and size
    managed_shared_memory segment(create_only, "MySharedMemory", 65536);
    //Initialize shared memory STL-compatible allocator
    const ShmemAllocator alloc_inst (segment.get_segment_manager());
    //Construct a vector named "MyVector" in shared memory with argument alloc_inst
    MyVector *myvector = segment.construct<MyVector>("MyVector")(alloc_inst);

Now, I understand this. What I’m stuck is how to pass a second parameter to segment.construct() to specify the number of elements. The interprocess document gives the prototype for construct() as

MyType *ptr = managed_memory_segment.construct<MyType>("Name") (par1, par2...);

but when I try

MyVector *myvector = segment.construct<MyVector>("MyVector")(100, alloc_inst);

I get compilation errors.

My questions are:

1. Who actually gets passed the parameters par1, par2 from segment.construct, the constructor of the object, e.g. vector? My understanding is that the template allocator parameter is being passed. Is that correct?
2. How can I add another parameter, in addition to alloc_inst that is required by the constructor of the object being created in shared memory?

There’s very little information other than the terse Boost docs on this.

Answer 1

I asked the same question on the boost users mailing list and Steven Watanabe replied that the problem was simple: std::vector does not have a constructor of the type (size, allocator). Looking at its documentation I see that the constructor is

vector ( size_type n, const T& value= T(), const Allocator& = Allocator() );

so the correct call should be

MyVector *myvector = segment.construct<MyVector>("MyVector")(100, 0, alloc_inst);

Elementary, my dear, Watson, elementary!