I have been working on adding inline spell check to Sigil recently and ran into a quirk on Qt that isn’t immediately obvious. I ended up having to look though the Qt source code to understand exactly what was happening.

When dealing with a QPlainTextEdit you can get the QTextCursor and use the charFormat() function to retrieve the QTextCharFormat for the character before the cursor. This does not work when the formatting is set by a QSyntaxHighlighter!.

charFormat retrieves the character format that has explicitly been set on the QPlainTextEdit. QSyntaxHighlighter does not directly set the formatting on the QPlainTextEdit. Instead QSyntaxHighlighter sets the format in additionalFormats as part of the block layout. All formatting for the block the cursor is currently in can be accessed by using QPlainTextEdit::textCursor().block().layout()->additionalFormats().

QTextLayout::additionalFormats() returns a list of FormatRange objects. A FormatRange gives the start of the formatting (relative to the block not the full text in the QPlainTextEdit), the length and the formatting (as set by the QSyntaxHighlighter). Simply loop over all of the FormatRange objects and check if the cursor is within a range to determine what formatting is applied to a particular part of the block’s text. Use QTextCursor::positionInBlock() to determine the relative position of the cursor within the block.

Here is an example from Sigil that I use for spell checking. It determines if a particular segment of text has the misspelled word style applied to it. It then selects the text.

QTextCursor c = textCursor();
int pos = c.positionInBlock();
foreach (QTextLayout::FormatRange r, textCursor().block().layout()->additionalFormats()) {
    if (pos >= r.start && pos <= r.start + r.length && r.format.underlineStyle() == QTextCharFormat::SpellCheckUnderline) {
        c.setPosition(c.block().position() + r.start);
        c.movePosition(QTextCursor::Right, QTextCursor::KeepAnchor, r.length);

Note: QTextEdit can be substituted any place QPlainTextEdit is used. This applies to both not just QPlainTextEdit.