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Re: std::variant - going from Alternative& to the enclosing variant&

From: Arthur O'Dwyer <arthur.j.odwyer_at_[hidden]>
Date: Mon, 7 Sep 2020 19:46:19 -0400
On Mon, Sep 7, 2020 at 12:50 PM Victor Khomenko via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> Problem: How to change the value of a variant from within a visitor?
> Unfortunately, the relevant operator()(Alternative&) of the visitor has no
> access to the enclosing std::variant. Not only this problem is annoying,
> but it's also messy to work around, as explained below. [...]
> Motivating Example 1: Visitor turning a Caterpillar to a Butterfly:
> using MyVariant = variant<Caterpillar,Butterfly>;
> struct Visitor{
> void operator()(Caterpillar& c){
> variant_from_alternative<MyVariant>(c) = Butterfly(); }
> ...
> };

Could you show both sides of this example — both callee *and *caller? I
would expect the caller to look like this:

    MyVariant foo;
    Visitor visitor;
    std::visit(visitor, foo);

which means it'd be trivial to change it to look like this instead:

    struct Visitor {
        void operator()(Caterpillar&, MyVariant& v) const { v =
Butterfly(); }

    MyVariant foo;
    Visitor visitor;
    std::visit([&](auto& c) {
        visitor(c, foo);
    }, foo);

Can you show a motivating example (both sides — caller *and *callee) where
the solution isn't this simple?


Received on 2020-09-07 18:50:00