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From: John Maddock (john_at_[hidden])
Date: 2006-08-10 05:23:48

> Just to bring this issue forward to the list, let me state
> the problem in simple terms. Currently, Boost.Bimap is
> relying on the following three classes being layout-compatible
> for the same choice of arguments X and Y (actual names do
> not match those used inside B.B):
> template<typename X,typename Y>
> struct relation
> {
> X left;
> y right;
> };
> template<typename X,typename Y>
> struct pair_by_left
> {
> X first;
> Y second;
> };
> template<typename X,typename Y>
> struct pair_by_right
> {
> X second;
> Y first;
> };
> Assuming layout compatibility allows us to "see" the
> elements of a bimap as pair_by_left's in the left view
> and pair_by_right's in the right view, thus matching the
> (first=key,second=data) syntax of a regular std::map.
> Needless to say all C++ compilers we know of do in fact
> provide such a layout compatibility.
> If we adhered strictly to the C++ standard, this kind
> of mirroring could only be achieved with non-zero
> overhead --for instance, holding references to
> left and right properly named "first" and "second"
> accordingly to the view.
> If I'm not wrong B.B assumes layout compatibility
> by default but can decay to a standard compliant solution
> for those (as of yet unknown) compilers where it could
> be needed.
> What do others think about this issue?

Hmmm, if those classes are POD's then layout compatibilty is guarenteed,
otherwise it's an assumption. Can it be verified with asserts or
static_asserts ?


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