-----Original Message----- From: boost-users-bounces@lists.boost.org [mailto:boost-users- bounces@lists.boost.org] On Behalf Of Riccardo Murri Sent: 07 December 2010 08:20 To: boost-users@lists.boost.org Subject: Re: [Boost-users] mpi: sending/receiving different types

On Mon, Dec 6, 2010 at 6:01 PM, Hicham Mouline wrote:

...

About the broadcast() function, I obviously have many types of messages to send, would you rather:

1. send a broadcast with just an int (like a tag) to tell which broadcast is following next

or

2. construct a virtual base class my_mpi_broadcast , derive all the sendable broadcasts from it, from sender, send actual instance of derived from my_mpi_broadcast, and at receiver, can the mechanism reconstruct for me the derived class? (with boost::serialization)

I'm not sure I understand this question: mpi::broadcast() works like a send+recv in one call: one MPI rank (the *root*) sends, the others receive.

Any data that is allowed in send() can be used in a broadcast(); in particular, if you have classes with Serialization support, you send() or broadcast() them with no additional effort.

Best regards, Riccardo _______________________________________________ Hello

During a sequence of communication, the slave processes don't know what type to get broadcast of, next. What should be the T in the call to broadcast. 1. the root sends a tag for the T to tell the receivers I will broadcast you this and this. This means 2 broadcast each time 2. the root sends directly a reference to a base class of anything sendable, then the receivers construct the derived classes on reception with broadcast. The T is then the base class type. 3. I just thought of using boost::variant<> over all the possible types. rds,

Hi Hicham, On Wed, Dec 8, 2010 at 2:50 PM, Riccardo Murri wrote:

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2. the root sends directly a reference to a base class of anything sendable, then the receivers construct the derived classes on reception with broadcast. The T is then the base class type.

There's a risk of slicing here, i.e., if you pass a reference to a base class, only the base class data will be serialized and received.

As far as I understand, Boost.Serialization (hence Boost.MPI) will do the correct thing if you use *pointers* instead: i.e., you send a pointer to a (derived) class and you receive it through a pointer to the base class.

(Beware: I have not checked!)

Now I actually tried and could not get this to work; the main problem is that the mpi::broadcast expects an already-built object to call the (de)serialization procedure on, irrespective of whether the object was passed as a pointer or as a reference.

...

3. I just thought of using boost::variant<> over all the possible types.

I think this is the way to go; I'm attaching a sample program. Not sure this is exactly what you need, but could be enough to get you started. Best regards, Riccardo