Re: [patch] : Improve std::search on v7

[Gabriel Dos Reis <gdr at integrable-solutions dot net>]

Chris Jefferson <caj@cs.york.ac.uk> writes:

| Gabriel Dos Reis wrote:
| > chris jefferson <caj@cs.york.ac.uk> writes:
| > 
| > | This is quite a simple patch that does a few things.
| > | 
| > | 1) Adds a new version of a more generic bind2nd to predefined_ops.h
| > | (if anyone has the old copy, it should be dumped and the file replaced
| > | with a clean CVS version, then this patch applied.
| > | 
| > | This uses the "Empty Base Optimization" to avoid PR20408, but even if
| > 
| > 
| > I think we have already proven in the past that non-careful uses of
| > EBO change program semantics.  Think of a function object with virtual
| > operator(). 
| > 
| Hmm.. out of interest, could you give an example of exactly how this can
| break? I've heard of such problems with EBO, but couldn't find a
| concrete example of it breaking, and failed to do so myself (possibly a
| lack of imagination).

I remember that someone raised that issue ont this list (having to do
with Allocators) for containers, and eventually we reworked the
container implementation.  Basically, when a class is polymorphic,
deriving, internally in our implementation, from it can change
semantics of user program. 

How does your patch perform with this comparator?

    #include <typeinfo>

    struct CompareInt {
       virtual bool operator()(int i, int j) const
       {
          // Derived classes must override this function.
          assert(typeid(*this) == typeid(Compare));
          return i == j;
       }
    };

-- Gaby