Comments on: Update: A Smart Way to Send a std::vector with MPI - and why it Fails

By: Mahantesh

Mahantesh — Wed, 12 Mar 2008 17:32:44 +0000

Thanks! this works for me. I usually do vec1.reserve(N) within try and catch and reserve its place. The next operation will be .resize(N), that way you can hope for O(N) ((will not have to move data around if N is large.

By: Stuart Dootson

Stuart Dootson — Thu, 15 Feb 2007 18:26:07 +0000

Ummmm - I think your students code is reasonably OK. They access the vector contents using the interface only - operator[]. That returns a reference to the first element of the vector contents. Taking it’s address gives a pointer to the first location of the contiguous storage you mentioned. Nowhere are you accessing the *internals* of the vector object. It’s like this (view with a monospace font!):

+————————-+
| vector object internals |
+————————-+
|
|
V
+—————–+
| vector contents |
+—————–+
^
|
= &vec[0]

memcpy is fine with a vector (though not idiomatic C++/STL - I tend to use std::copy instead).

You might like to look at Boost.MPI (http://www.generic-programming.org/~dgregor/boost.mpi/doc/index.html) - a C++ interface to MPI that’s built on top of standard C MPI implementations.

By: teki

teki — Thu, 08 Feb 2007 11:10:34 +0000

Do not use resize, use reserve, that’s what you want.

The statement of the article is false.
Recommended readings:
http://www.gotw.ca/publications/mill10.htm

An I recommend to read a few book about STL.

By: wreel

wreel — Thu, 08 Feb 2007 11:05:17 +0000

That's actually safe. std::vector types are, by design, STL's interface into legacy C array API calls. The students looked like they did the proper research and it falls inline with Scott Meyers item 16 in "Effective STL". If Stepanov had his druthers, you would/should have been able to pass [

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] but quirkiness in certain STL implementations caused for the explicit call for the address of a reference.

By: Stefan Eilemann

Stefan Eilemann — Thu, 08 Feb 2007 10:58:11 +0000

vector.resize can also be terrible slow, since you are inserting n elements into the vector. I discovered that when storing image data in a vector, and sending it over the network as described above. I'm now back using C arrays.

By: teki

teki — Thu, 08 Feb 2007 10:56:36 +0000

"Counting on object-internals like the fact that it probably has a big block of memory inside to store the actual data will not do, although it may work with your present implementation!"

If it's a standard compliant STL implementation, then it have to work.

Recommended reading: http://www.gotw.ca/publications/mill10.htm