X3: Use traits to test if container is empty

[timangus]

::empty() is called on the Attribute in parse_into_container_impl. If Attribute has no ::empty(), if fails to compile. Probably instead it should use traits::is_empty(...).

[Kojoley]

Do you specialize traits::is_empty for your custom container because it does not expose empty method (but use some other way to determine if it is empty)?
I mostly see traits::is_empty is used to deal with unused_type, but parse_into_container_impl::call have override for it (added in ee4943d). Can you please provide a test case?

[timangus]

Ah right, I'll admit I was testing using 1.66 rather than the develop branch; it looks like ee4943d would resolve my problem albeit in a different way. FWIW, my custom container is QString, and my traits are:

namespace boost { namespace spirit { namespace x3 { namespace traits {

template<>
struct push_back_container<QString>
{
    template<typename T>
    static bool call(QString& c, T&& val)
    {
        c.push_back(std::move(val));
        return true;
    }
};

template<>
struct append_container<QString>
{
    template<typename Iterator>
    static bool call(QString& c, Iterator first, Iterator last)
    {
        c.append(first, std::distance(first, last));
        return true;
    }
};

template<>
struct is_empty_container<QString>
{
    static bool call(QString const& c)
    {
        return c.isEmpty();
    }
};

}}}}

[timangus]

Hmm well, quickly patching ee4943d into my code doesn't fix the problem. It does seem like traits is the answer to be honest, it seems to have been in issue #287, whose fix is in the same function.

[Kojoley]

It does seem like traits is the answer to be honest

Yeah. I have asked the first question just to be sure you are using a custom container and we are not fixing some hidden problem with the patch.

[timangus]

Ah right, cool. Thanks.

[djowel]

👍

[Kojoley]

Thanks for patch, and for Tremulous!

[timangus]

Haha, you're welcome! Believe it or not a small number of people still play it.

[Kojoley]

Nice to hear this! I had not played it since 2011 because of server population drop (it was hard to gather together even 2x2).