David Abrahams wrote:
"Peter Dimov" <pdimov@mmltd.net> writes:
Conceptual issues with (2) aside (the external format of X is determined by X itself and you have no idea whether the structure of X[0] also describes X[1]),
I don't know what you mean.
When your mpi_archive is given an arbitrary X, there is no way to know whether serializing an array of X can be transformed into serializing a X and then building an array datatype out of the result. That's because X's serialize functions can do _anything_. Your archive is simply not allowed to alter them. (*) That is why the optimization must be explicitly enabled by the author of X. My approach deals with that by allowing him to write a save_sequence overload for X: void save_sequence( mpi_archive & a, X const * x, size_t n ) { a.save_pod_array( x, n ); } You are trying to generalize this by allowing X's author to define a trait that isn't archive specific. I claim that this doesn't work, because whether an array of X supports optimized writes into an archive A is (in my experience) determined by the combination of X and A, and nothing is gained by defining separate traits with the hope that another archive might reuse them (YAGNI principle). In short, you'll end up defining an is_mpi_pod trait, give it another, non-MPI name, and pretend that it's generally useful. It's not. It's MPI specific. (*) Consider a X that is essentially a discriminated union as an example. struct X { int valid_field_; // 0..1 int field1_; double field2_; }; that serializes valid_field_ and then either field1_ or field2_ depending on valid_field_.