Re: [boost] [variant2] Need rationale for never-empty guarantee

On Sat, Mar 2, 2019 at 3:49 PM Steven Watanabe via Boost wrote:

AMDG

On 3/2/19 12:47 PM, Mathias Gaunard via Boost wrote:

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On Fri, 1 Mar 2019 at 09:59, Peter Dimov via Boost wrote:

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Kind of, but as written this implies that std::variant has no costs, which is not true. The checks for valueless do carry a cost. Each visit(), for example, starts with `if(valueless) throw`, which is not necessary in variant2.

That is not true. A typical implementation would just add an extra value to the switch, there is no extra branch.

That may be true in theory, but both libc++ and libstdc++ have if(__v.valueless_by_exception()) scattered everywhere. Not to mention that neither uses switch.

In Christ, Steven Watanabe

The code might say that, but the compiler probably still turns it into a switch. The variant only has one internal variable, such as "_which". It holds both the "is it A or B or C" and "or is it valueless". So the resulting code should only need to look at it once (ie something switch-like). If compilers and std::variant haven't yet eliminated the cost of valueless_by_exception on visitation, I think it is only a matter of time before they do. Tony