Re: struct puzzle

On 12-04-17 18:15, Patrick Welche wrote:
> Following Seth's excellent example, the attached simple program parses
> an int and a map of strings. The puzzle is why it no longer compiles
> when I remove the int with the following patch:
I think it's the single-element fusion sequence edge-case again.

This has been a pain-point in Qi for a long time, and apparently still
crops up in X3 in specific circumstances (looks like, detecting whether
the bound argument in context is a container fails since it's a sequence
containing just the container).

This is very hard to get right - given the C++ type system and methods
to do meta-programming, I think. For now I'd accept the workaround of
adding padding on single-element (adapted) fusion sequences with a
container element.

    struct result {
        Map both;
        //
        boost::spirit::x3::unused_type padding;
    };

    BOOST_FUSION_ADAPT_STRUCT(result, padding, both)

            auto placebo_ = attr(nullptr);
            auto all_     = placebo_ >> +map_;

It's not pretty, but at least you know why.

Note that you can trivially use something non-x3 instead of unused_type:

    struct Blackhole {
        template <typename T> Blackhole& operator=(T&&) { return *this; }
    };

    struct result {
        Map both;
        //
        Blackhole padding;
    };


See http://coliru.stacked-crooked.com/a/7a55bf8e1529c3f4

------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot

_______________________________________________
Spirit-general mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/spirit-general