Exactly matching 1 tab to K spaces in QPlainTextEdit

[cadol001]

#include <QPlainTextEdit>
#include <QVBoxLayout>
MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    QVBoxLayout* layout = new QVBoxLayout{};
    QPlainTextEdit* tedit = new QPlainTextEdit{this};
    layout->addWidget(tedit);
    ui->centralwidget->setLayout(layout);

    QTextOption opt;
    opt.setWrapMode(QTextOption::NoWrap);
    opt.setFlags(QTextOption::ShowTabsAndSpaces);
    tedit->document()->setDefaultTextOption(opt);

    int N = 100;
    int pointsize = 18;
    int spaces_per_tab = 2;

    QTextCursor c = tedit->textCursor();
    c.insertText(QString("\t").repeated(N) + "\n");
    c.insertText(QString(" ").repeated(N*spaces_per_tab));

    QFont f = QFont{};
    f.setPointSize(pointsize);
    f.setWordSpacing(pointsize * 0.75);
    tedit->setFont(f);

    QFontMetrics fmt{f};
    int adv = fmt.horizontalAdvance(" ")*spaces_per_tab;
    tedit->setTabStopDistance(adv);
}

I am trying to make a tab equal to spaces_per_tab spaces in q QPlainTextEdit. However, if you run this code you will see that the estimated horizontalAdvance is slightly off such that 2 spaces is not at all equal to 1 tab.

How do I get the actual width of a space?

[jsulm]

@cadol001 See https://doc.qt.io/qt-6/qplaintextedit.html#tabStopDistance-prop

[JonB]

@jsulm
? The OP is trying to use that, tedit->setTabStopDistance(adv). Issue is (apparently) that his calculation of adv is not right, either in general or at least for 2 spaces, and he wants to know why.

@cadol001
Does your code do same as claimed solution at https://stackoverflow.com/a/50192868/489865 ? It does seems to be. I assume the setWordSpacing() is irrelevant and can be removed? Does yours work for other spaces_per_tab values, e.g. 4 or 8, or is it always wrong? What about of you pick a different pointsize?

QFontMetrics fmt{f};

Ah ha! You use QFontMetrics. Example uses QFontMetricsF. That should be why your figures are wrong? The former returns stuff (including horizontalAdvance()) as ints, the latter as qreals (cf. QRect vs QRectF). Presumably if you change over to QFontMetricsF you will be good?