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Subject: Re: [boost] An extension to boost::variant
From: Chris Hamilton (chamilton_at_[hidden])
Date: 2009-03-04 07:19:23

> Chris Hamilton wrote:
>> Caveat: There would be a small violation to the "Never Empty Guarantee"
>> if a reference was the first templated type and the variant was default
>> constructed; in this case, it would be default constructible (as
>> internally it would be stored using a pointer), but the pointer would
>> have a null value. Trying to use this value would cause an error,
>> obviously.
> Just forbid default construction in this case. It isn't very hard.

Okay. So, I dug into the variant source code to see how hard this would
be to implement. In my mind, it came down to the following:

(1) variant currently wraps reference types with reference_content<>.
Instead, make it wrap them with reference_wrapper.
(2) Replace all reference_content<> handling throughout variant with
reference_wrapper<> handlers.
(3) Make reference_wrapper support operator= (by removing the explicit
from the constructors). reference_wrappers already forbid default
construction, so the above (quoted discussion) is a non-issue.
(4) Add second versions of the variant assign, constructor, etc,
functions that can pass non-const references (so non-const reference
types can be handled as variant input types).

This now lets me do something like:

// The reference type is the second one, so that this object
// can be default constructible.
typedef boost::variant<const char *, const std::string &> String;

const char * s1 = "string1";
std::string s2("string2");

String s;

// This doesn't throw
s = s1;
boost::get<const char *>(s);

// Nor does this.
s = s2;
boost::get<const std::string&>(s);

Anybody see the utility of this? (In my mind, this is a feature I want
variants to have.) Anybody see any problems with the approach I've
taken? I haven't run the entire variant test suite yet, nor have I
explored all possible uses (and misuses) of reference types, but I would
appreciate some early feedback. I've attached a unified patch (taken
against boost-trunk).



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