Does Qt have its own boost::optional alternative?

Question

Does Qt have its own boost::optional alternative, or should I just use boost::optional?

Toby Speight · Answer 1 · 2021-04-21T06:33:37.550

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Qt has a 'variant' type, but not an 'optional' type. I think that boost::optional (or, in newer C++ versions, std::optional) is a reasonable choice.

edited Apr 21 '21 at 06:33

answered Nov 16 '15 at 13:11

Toby Speight

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As an addendum, programmers prefer Qt types over the stdlib alternatives (for example, QVariant vs std::variant, QVector vs std::vector, etc) due to the Qt functions doing processing with them natively, obviating conversions. However due to the nonexistence of a Qt optional type, it's perfectly reasonable to use std::optional. – Aug 05 '21 at 03:40

score 6 · Answer 2 · answered Nov 16 '15 at 13:35

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Seems like QVariant already can do what boost::optional was intended to do. You have:

bool QVariant::isNull() const
bool QVariant::isValid() const
Type QVariant::type() 
static QVariant QVariant::fromValue(const T & value)
T QVariant::value()

So you can wrap any type, check if the variant is null or valid or even get the type to use with an if or switch statement.

answered Nov 16 '15 at 13:35

dtech

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Sure but QVariant is an overkill. It has more overhead and hides type of its contents. – Sergey Galin Apr 11 '18 at 14:07
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If one already uses Qt it is a viable option, I doubt it will end up used in places where its memory or performance overheads will be exacerbated. – dtech Apr 11 '18 at 14:11
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QVariant isn't a good fit for the use case of optionals. From the docos for QVariant: Null variants is not a single state and two null variants may easily return false on the == operator if they do not contain similar null values. – Andrew Lipscomb Mar 03 '19 at 02:51

score 3 · Answer 3 · answered Dec 25 '20 at 13:14

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In C++17 there was introduced std::optional: https://en.cppreference.com/w/cpp/utility/optional

I think it's more usefull in Qt project, than boost one. There is still no any QOptional class in Qt.

answered Dec 25 '20 at 13:14

Nikxp

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score 2 · Answer 4 · answered Nov 16 '15 at 13:11

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There is qt-maybe template implemented on top of Qt's QVariant container, but it's not in "standard" qt lib.

answered Nov 16 '15 at 13:11

Konrad 'Zegis'

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score 1 · Answer 5 · edited May 23 '17 at 11:48

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No there is no alternative for boost::optional at present. You can also refer this thread: boost::optional alternative in C++ Standard Library

edited May 23 '17 at 11:48

Community

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answered Nov 16 '15 at 13:15

Rahul Tripathi

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Does Qt have its own boost::optional alternative?

5 Answers5