Sharing generic bodies across instantiations.
in [ADA]
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From: Robert A Duff on 3 Aug 2010 10:37 "Randy Brukardt" <randy(a)rrsoftware.com> writes: > You need to read about the Ada contract model for generics. (Off-hand, I > can't think of a good reference, however.) The AARM talks about it. Not exactly an "easy read". ;-) I think maybe the Ada 83 Rationale talks about it. There were some holes in the contract model in Ada 83, which were patched in Ada 95. >... Part of the Ada contract model is > that there cannot be an error caused by the actual parameters of an instance > in an instance body. Tucker Taft liked to describe the model as > "assume-the-best" in the specification and "assume-the-worst" in the generic > body. Thus there are a lot of things that are illegal in a generic body in > Ada simply because they *could* be illegal in some instance of that generic. > C++ has no such counterpart (indeed, I recall reading that Stoustrup [sp] > explicitly eliminated it from C++ templates because it was too complex). > > In addition, binding of names in the generic body always takes place at the > point of the compilation of that body. That means that most operations at > determined at that point, facilitating sharing. (This also causes the > sometimes bug of "reemergence" in generics, where a hidden predefined > operator is used instead of the user-defined one for the actual type.) C++ > uses a more macro approach to template expansion. It makes C++ templates more powerful than Ada generics. The downside is that if you're writing a reusable and widely-reused library, it's difficult to know (in C++) whether some change might break clients. The other downside is that error messages are typically incomprehensible; in Ada you don't get normally get errors in instance bodies, so they can't be incomprehensible. ;-) - Bob |