[Development] QMetaType and non-const references

[Volker Hilsheimer]

> On 19 Jul 2022, at 18:08, Thiago Macieira <thiago.macieira at intel.com> wrote:
> 
>> Lastly, an aside: Q_DECLARE_METATYPE being effectively unnecessary was the
>> goal for Qt 6, but is not the case yet. I consider that a bug that should
>> finally be fixed in 6.5.
> 
> Right. There should be exactly one use for Q_DECLARE_METATYPE: when the name 
> you want to register is not the name that the C++ language recognises for your 
> type, and this should only be done for legacy reasons.
> 
> 	typedef QMap<int, int> IntMap;
> 	Q_DECLARE_METATYPE(IntMap);
> 
> And you may have the string "IntMap" somewhere in your meta objects.

Interestingly,

    qDe0bug() << QMetaType::fromType<std::string>().name();
    qDebug() << QMetaType::fromName("std::string");

prints, with Apple clang:

std::__1::basic_string<char>
QMetaType()

but with msvc 22:

std::basic_string<char,std::char_traits<char>,std::allocator<char>>
QMetaType()

and then "std::string" in your meta object won’t work either, I suppose?

But interestingly, neither does

Q_DECLARE_METATYPE(std::string).

With

qRegisterMetaType<std::string>(“std::string”);

I get

std::__1::basic_string<char>
QMetaType(std::__1::basic_string<char>)

with clang and

td::basic_string<char,std::char_traits<char>,std::allocator<char>>
QMetaType(std::basic_string<char,std::char_traits<char>,std::allocator<char>>)

with msvc 22. What does that mean if e.g. "std::string" is present in a meta object?


> Any opinion on what must happen together or not at all?


I agree with Fabian, it seems uncontroversial to treat T& and T as different types in the QMetaType system. Code that relies on them being the same needs to be fixed anyway. Doing that doesn’t seem to prevent anything else discussed.


Volker