[C++][Qt]Replace usage of QVariant with a more intuitive optional wrapper

[etherealjoy]

Fixes #8934

Issue was due to Optional support with QVariant here #8733

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Changes

• QVariant cannot construct user types hence it is not suitable for all cases
• A more intuitive approach is provided, which also shows the type of optional in the api instead of plain QVariant
• Added handling of optional body

Function declaration now looks like this

void deletePet(const qint64 &pet_id, const OptionalParam<QString> &api_key = OptionalParam<QString>());
void updatePetWithForm(const qint64 &pet_id, const OptionalParam<QString> &name = OptionalParam<QString>(), 
           const OptionalParam<QString> &status = OptionalParam<QString>());

instead of this

void deletePet(const qint64 &pet_id, const QVariant &api_key = QVariant());
void updatePetWithForm(const qint64 &pet_id, const QVariant &name = QVariant(), const QVariant &status = QVariant());

User types are supported like this

    QList<OAIObject> list;
    QMap<QString, OAIObject> map;
    QList<QString> strl;
    list.append(OAIObject());
    api->notifyTopicV1(list);
    api->notifyTopicV1();
    api->notifyTopicV2();
    api->notifyTopicV2(map);
    api->notifyTopicV3();
    api->notifyTopicV3(strl);

cc @ravinikam @stkrwork @MartinDelille @muttleyxd

@xconverge
I had to rework this due to side effects, made a more simpler and intuitive approach.

[xconverge]

LGTM, we probably should just use std::optional later but this works for now. Looks like it wasn't too bad as an incremental step on what I had done, sorry about that. The ability to sub in a hacky qvariant and not break peoples code obviously didn't fully work, better to find out sooner rather than later

You can remove the type information I put in the doxygen comments to supplement the qvariant downsides if you want to

[etherealjoy]

LGTM, we probably should just use std::optional later but this works for now. Looks like it wasn't too bad as an incremental step on what I had done, sorry about that. The ability to sub in a hacky qvariant and not break peoples code obviously didn't fully work, better to find out sooner rather than later

std::optional would have same issue like QVariant

You can remove the type information I put in the doxygen comments to supplement the qvariant downsides if you want to

This is fine, the doxygen is still meaningful. From user point of view, the pass the parameters normally without bothering about the OptionalParam, construction will happen in the template automatically.

[xconverge]

LGTM, we probably should just use std::optional later but this works for now. Looks like it wasn't too bad as an incremental step on what I had done, sorry about that. The ability to sub in a hacky qvariant and not break peoples code obviously didn't fully work, better to find out sooner rather than later

std::optional would have same issue like QVariant

You can remove the type information I put in the doxygen comments to supplement the qvariant downsides if you want to

This is fine, the doxygen is still meaningful. From user point of view, the pass the parameters normally without bothering about the OptionalParam, construction will happen in the template automatically.

Yep makes sense, it looks so straightforward and obvious now vs before when we were first considering QVariant vs std::optional. This looks great

[xconverge]

:/ thanks for the cleanup @etherealjoy I had been using master successfully for the past few days without seeing these issues

[wing328]

@etherealjoy thanks for the PR, which has been included in the v5.1.0 release: https://twitter.com/oas_generator/status/1373636654024380423