Is std::vector::size() allowed to require non-trivial computations? When would it make sense?

I would guess that your definition of "trivial" doesn't match that of the author of the code.

C++03 says in Table 65, found in §23.1, that size() should have a constant complexity. (In C++0x, this is required for all containers.) You'd be hard-pressed to find a std::vector where it's not.

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