Re: [boost] [Variant] Documentation Questions/Notes

Subject: Re: [boost] [Variant] Documentation Questions/Notes
From: Brian Simpson (wheber_at_[hidden])
Date: 2012-11-02 02:38:51

Dave Abrahams <dave <at> boostpro.com> writes:

> 
>  <snip>
> 
> - Do the semantics for are_strict_equals at
>   http://www.boost.org/doc/libs/1_51_0/doc/html/variant/tutorial.html
>   differ from those of the built-in equality operator?
> 

The built-in equality operator compares two instances of identically-typed
variant.  As a binary visitor, however, are_strict_equals can compare
non-identically-typed variants.

boost::variant<int, std::string> v1("hello");
boost::variant<int, std::string> v2("g'bye");
boost::variant<double, std::string> v3("hello");
assert(!(v1 == v2));
assert(v1 == v3);//compile error
assert(boost::apply_visitor(are_strict_equals(), v1, v3));

Interestingly, it looks as if variant goes the extra mile to "prevent comparison
with foreign types" (that's a comment from variant.hpp).  I wonder why.

> <snip>