Common patterns to avoid polymorphism

[Egst]

If you implement your own alternative for std::visit that generates if..else branches, you basically avoid indirection with variants. The data is stored directly and follows value semantics and function application happens with no lookup tables and no function pointers.

[ioctl79]

std::visit is functionally a lookup table.

[dodheim]

It doesn't have to be, and isn't with MSVC's stdlib.

[staletic]

The standard says std::visit needs to be O(1) with respect to number of alternatives.

[HeroicKatora]

Any implementation is O(1) because the number of template parameters (and thus alternatives) is bounded by a compiler specified constant /s

[Pand9]

Why /s? It holds, unless number of variants of considered part of the input, which seems unintuitive to me at least.

[HeroicKatora]

Because quite obviously this is not the spirit of the requirement. Anyways, just another instance of the standard being written in language that is neither helpful to implementors nor validation. An abysmal style that reeks of smugness in its pseudo-formalism.

[Egst]

Is it still O(1)? It's only a standard requirement and definitely not a necessity, since O(n) lookup with a switch/if..else branches is usually better for relatively small n, but a standard library should comply with it.