Re: [Boost-users] [Variant] Bug assigning similar, but different concrete types during assignment / ctor relay

Subject: Re: [Boost-users] [Variant] Bug assigning similar, but different concrete types during assignment / ctor relay
From: Michael Powell (mwpowellhtx_at_[hidden])
Date: 2018-11-18 23:33:45

On Sun, Nov 18, 2018 at 6:22 PM Steven Watanabe via Boost-users
<boost-users_at_[hidden]> wrote:
>
> AMDG
>
> On 11/18/2018 04:00 PM, Michael Powell via Boost-users wrote:
> > I've got a Boost.Variant confusion going on during ctor relay.
> >
> > I have two types participating in the Variant, an Integer type and a
> > Floating Point type. Both are Numeric as a base class. i.e. something
> > like:
> >
> > template<typename T>
> > struct numeric {};
> >
> > struct integer : numeric<long> {};
> >
> > struct floating_point : numeric<double> {};
> >
>
> integer and floating_point are unrelated.
> I don't see any for for variant to get confused
> about them.
>
> Something like this could happen during
> assignment, if an exception is thrown.
> Variant assignment only provides the basic
> exception guarantee and may fall back on a
> no-throw default constructor of one of the
> bounded types.
>
> > Then:
> >
> > using constant_value = boost::variant<..., floating_point, integer, ...>;
> >
> > Somewhere during the handling, I have a test case in which I have a
> > constant_value{} starting out in life as an integer{}.
> >
> > However, somewhere during handling, variant is misinterpreting integer
> > (which=3) as a floating_point (which=2).
> >
>
> So, the first `...` contains two types?
>
> > This is incorrect, of course.
> >
> > I could force the issue, I suppose and re-set the value to the
> > integer, but I'd like to understand the issue. I think it's a bug in
> > variant, or perhaps a "feature" confusing integer with floating_point
> > along base class numeric lines?
> >
>
> This code seems to work correctly:
>
> #include <boost/variant.hpp>
> #include <iostream>
>
> template<typename T>
> struct numeric {};
>
> struct integer : numeric<long> {};
>
> struct floating_point : numeric<double> {};
>
> using constant_value = boost::variant<char, floating_point, integer, void*>;
>
> int main() {
>     constant_value x = integer{};
>     std::cout << x.which() << std::endl;
>     std::cout << x.type().name() << std::endl;
> }
>
> $ ./scratch
> 2
> struct integer
> $

I'm trying to identify the issue. I am also passing them, or their
hosts/containers around as reference parameters. Shouldn't make a
difference, I think, since it is a reference?

No relation? Other than the fact they are both numeric. I do not know
why that would make a difference, but it seems to be.

> In Christ,
> Steven Watanabe
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