Buddhika Gamage Buddhika Gamage - 4 years ago 209
C++ Question

Where to use std::variant over union?

Please explain what is the difference between

union
and
std::variant
and why
std::variant
was introduced into the standard? In what situations should we use
std::variant
over the old-school
union
?

Answer Source

Generally speaking, you should prefer variant unless one of the following comes up:

  1. You're cheating. You're doing type-punning or other things that are UB but you're hoping your compiler won't break your code.

  2. You're doing some of the pseudo-punnery that C++ unions are allowed to do: conversion between layout-compatible types or between common initial sequences.

  3. You explicitly need trivial copyability and/or layout compatibility. variant<Ts> are not required to have any particular layout or trivial copyability. unions of standard layout types are standard layout, and unions of trivially copyable types are trivially copyable.

  4. You need low-level support for in-place switching of objects. Using a memory buffer for such things doesn't provide the trivial copying guarantees that you could get out of a union.

The basic difference between the two is that variant knows which type it stores, while union expects you to keep track of that externally. So if you try to access the wrong item in a variant, you get an exception or nullptr. By contrast, doing so with a union is merely undefined behavior.

union is a lower-level tool, and thus should only be used when you absolutely need that lower-level.

variant also has machinery for doing visitation, which means you get to avoid having a bunch of if statements where you ask "if it is type X, do this. If it is type Y, do that, etc".

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