19 General utilities library [utilities]

Let n be sizeof...(Variants).

Let m be a pack of n values of type size_­t.

Such a pack is called valid if $0\le {\text{m}}_i<{\text{variant\_size\_v<remove\_reference\_t<Variants}}_i\text{>>}$ for all $0\le i<n$.

For each valid pack m, let e(m) denote the expression:

INVOKE(std::forward<Visitor>(vis), get<m>(std::forward<Variants>(vars))...) // see [func.require]

Requires: For each valid pack m, e(m) shall be a valid expression.

All such expressions shall be of the same type and value category; otherwise, the program is ill-formed.

Returns: e(m), where m is the pack for which m${}_i$ is vars${}_i$.index() for all $0\le i<n$.

The return type is decltype(e(m)).

Throws: bad_­variant_­access if any variant in vars is valueless_­by_­exception().

Complexity: For $n\le 1$, the invocation of the callable object is implemented in constant time, i.e., for $n=1$, it does not depend on the number of alternative types of Variants${}_0$.

For $n>1$, the invocation of the callable object has no complexity requirements.