On 2015-02-20 14:42, Thiago Macieira wrote:
> On Friday 20 February 2015 12:53:24 Matthew Woehlke wrote:
>> for (auto const i : qtEnumerate(map))
>>
>> Maybe it would be nice for Qt to provide one or both of these?
>
> Sounds easy enough. Want to give it a try?
I'm happy to give you my headers; not sure when/if I'd have time to
clean them up as actual patches against Qt, however. For enumerate,
though, I technically need the customer's permission to share it.
> Note that this should also work for foreach:
>
> foreach (const auto i, qtEnumerate(map))
>
> Something like:
>
> template <typename Map>
> struct QEnumerateMap : private Map
> {
> struct const_iterator {
> typename Map::iterator i;
> iterator(typename Map::iterator i) : i(i) {}
>
> // need to return by value
> std::pair<typename Map::key_type, typename Map::value_type>
> value() const
> { return std::make_pair(i.key(), i.value()); }
> };
>
> const_iterator begin() const
> { return iterator(Map::begin()); }
> const_iterator end() const
> { return iterator(Map::end()); }
> };
No, that doesn't seem right at all (unless you were going for a
non-broken cref?).
The way I did it is I construct a minimal class that has a reference to
the map (though, as discussed, this might need to be a copy), containing
a private iterator type, and begin()/end() methods. The private iterator
type is constructed from the container iterator, and needs to implement
its own operator++, operator== and operator*. The last one is key; it
returns the underlying iterator, rather than value() of the same.
Index range works on basically the same idea, only the 'underlying
iterator' is a number, you construct it from a number, and begin()
returns an iterator(0) while end returns an iterator(m_end). The
iterator needs the same operators, with the obvious implementations.
This all works with range-based for. Not sure about foreach...
One thing I notice you *did* get right is separating the utility class
from the function to create it; you do need to do this for template
deduction.
--
Matthew
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