Re: [boost] [Concepts] Definition. Was [GSoC] [Boost.Hana] Formal review request

2014-08-05 17:42 GMT+02:00 Robert Ramey :

Please don't forget, that not all requirements can be checked at compile time. For example: TotallyOrdered. Just by checking, that an expression (a

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This is not true.

requirements are on types - not on instantiations of types. If it can't be checked at compile time - it's not a type requirement - it's something else (perhaps a pre-condition).

TotallyOrdered doesn't refer to some container that may or may not be sorted. It refers to the existent of a comparison function for the type which fulfills a number of requirements. To see this look at https://www.sgi.com/tech/stl/StrictWeakOrdering.html

Here we can specify type requirements on functions which provide for a strict weak ordering. These can all be verified at compile time based on the properties of the types. Note that not all can be verified by the compiler as some depend upon the semantic definition of the function. But one doesn't have to know that runtime data to verify that the function meets the conditions for a strict weak ordering. To summarize - all type requirements can be verified when the code is written - but not all type requirements can be verified automatically by the compiler.

So verification of by the compiler isn't perfectly exhaustive - but it's a heck of a lot better than what we usually do - which is nothing.

Robert Ramey

What about this example: struct Circ { unsigned x; explicit Circ(unsigned i=0) : x(i%256) {} }; bool operator<(Circ a, Circ b) { return (b.x-a.x)%256 < (a.x-b.x)%256; } Is std::set<Circ> valid? I think generally not, but... suppose that we know, that we're only going to use values 0-127. In this case std::set<Circ> is valid. Regards, Kris