[Development] RFC: more liberal 'auto' rules?

Marc Mutz marc.mutz at kdab.com
Tue Dec 22 13:28:44 CET 2015

On Tuesday 22 December 2015 11:26:40 Ziller Eike wrote:
> We have something similar in Qt:
>     QString a = "ABC";
>     QString b = "abc";
>     auto result = a + b;
>     a.replace("A", "C");
>     qDebug() << result;
> prints “CBCabc” instead of “ABCabc” when QT_USE_FAST_OPERATOR_PLUS is
> defined, because result is not a QString in that case.
> So funny/unwanted behavior can occur both because one used the wrong
> explicit type, and because one used auto instead of an explicit type.

This is not specific to auto, though:

    QStringView v = QLatin1Char('a');
    const char *data = str.toLatin1(); // implicit conversion by default
    QLatin1String l1 = str.toLatin1();
    QVector<int> v = {0, 1, 2};
    QVector<int>::iterator it = v.begin();
    const QVector<int> v2 = v;
    *it = 1;
    assert(v2.front() == 0); // boom

It's a general peril when working with C++.

Reminds me of the 1970s when we got a law requiring the use of the seat-belt 
in cars in Germany. My uncle used to tell the story how one of his friends 
survived a crash because he was ejected from the car since he was _not_ 
buckled up, just to follow up with the story how another friend was not harmed 
in a crash because he _was_ strapped in...


Marc Mutz <marc.mutz at kdab.com> | Senior Software Engineer
KDAB (Deutschland) GmbH & Co.KG, a KDAB Group Company
Tel: +49-30-521325470
KDAB - The Qt Experts

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