How can I corretcly declare a QVariant that will map to NULL in an SQLite tabe?

l3u_

Hi all,

Qt 6.8 introduced some new deprecation warnings, among others that one shouldn't QVariant(QVariant::Type type) anymore.

Until now, I used this static constant here to get a NULL value when it's inserted into an SQLite table:

static const QVariant s_sqliteNull(QVariant::Int);

I now replaced this using the advised QMetaType declaration with

static const QVariant s_sqliteNull(QMetaType(QMetaType::Int));

which seemed to work fine. However, I now tried to compile my stuff using Clang and not GCC, and it seems this one doesn't like this approach: I get:

.../Database.cpp:746:43: error: conversion function from 'const QVariant ()' to 'const QVariant' invokes a deleted function
746 |                                           s_sqliteNull,
    |                                           ^~~~~~~~~~~~
/usr/include/qt6/QtCore/qvariant.h:314:5: note: 'QVariant<const QVariant (*)(), false>' has been explicitly marked deleted here
314 |     QVariant(T) = delete;
    |     ^
/usr/include/qt6/QtCore/qlist.h:289:43: note: passing argument to parameter 'args' here
289 |     inline QList(std::initializer_list<T> args)
    |                                           ^
.../Database.cpp:1535:42: error: incompatible operand types ('int' and 'const QVariant (*)()')
1535 |         query.bindValue(1, markerId != 0 ? markerId : s_sqliteNull);
    |                                          ^ ~~~~~~~~   ~~~~~~~~~~~~

I don't fully get this. How can I correctly declare a variable that will map to QSLite's NULL that both GCC and CLang like?

Thanks for all help!

Christian Ehrlicher

What's wrong with QVariant()?

l3u_

Yeah, I meanwhile found out that using a plain, uninitialized QVariant() will do. That's why I deleted this ;-)

JonB

@l3u_
It's better to mark @Christian-Ehrlicher's reply as the correct one and close the topic than to delete your post/topic. Others reading this in future may benefit from it.

l3u_

Okay, I'll do so – I just thought I was simply dumb ;-)

GabrielBruce

Excited to know more about this qvariant.

JonB

@GabrielBruce
What's to be excited about and what more do you want to learn? :) QVariant() returns a default, invalid QVariant, and you can e.g. pass that as a bound value where you would use NULL in SQL.