Всплывающие подсказки в QT

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Эта штука реализуется через QCompleter и наследование от QTextEdit.

textedit.h

#ifndef TEXTEDIT_H
#define TEXTEDIT_H

#include <QTextEdit>

QT_BEGIN_NAMESPACE
class QCompleter;
QT_END_NAMESPACE

class TextEdit : public QTextEdit
{
    Q_OBJECT

public:
    TextEdit(QWidget *parent = 0);
    ~TextEdit();

    void setCompleter(QCompleter *c);
    QCompleter *completer() const;

protected:
    void keyPressEvent(QKeyEvent *e) override;
    void focusInEvent(QFocusEvent *e) override;

private slots:
    void insertCompletion(const QString &completion);

private:
    QString textUnderCursor() const;

private:
    QCompleter *c;
};

#endif // TEXTEDIT_H

textedit.cpp

#include "textedit.h"
#include <QCompleter>
#include <QKeyEvent>
#include <QAbstractItemView>
#include <QtDebug>
#include <QApplication>
#include <QModelIndex>
#include <QAbstractItemModel>
#include <QScrollBar>

TextEdit::TextEdit(QWidget *parent)
: QTextEdit(parent), c(0)
{
    setPlainText(tr("This TextEdit provides autocompletions for words that have more than"
                    " 3 characters. You can trigger autocompletion using ") +
                    QKeySequence("Ctrl+E").toString(QKeySequence::NativeText));
}

TextEdit::~TextEdit()
{
}

void TextEdit::setCompleter(QCompleter *completer)
{
    if (c)
        QObject::disconnect(c, 0, this, 0);

    c = completer;

    if (!c)
        return;

    c->setWidget(this);
    c->setCompletionMode(QCompleter::PopupCompletion);
    c->setCaseSensitivity(Qt::CaseInsensitive);
    QObject::connect(c, SIGNAL(activated(QString)),
                     this, SLOT(insertCompletion(QString)));
}

QCompleter *TextEdit::completer() const
{
    return c;
}

void TextEdit::insertCompletion(const QString& completion)
{
    if (c->widget() != this)
        return;
    QTextCursor tc = textCursor();
    int extra = completion.length() - c->completionPrefix().length();
    tc.movePosition(QTextCursor::Left);
    tc.movePosition(QTextCursor::EndOfWord);
    tc.insertText(completion.right(extra));
    setTextCursor(tc);
}

QString TextEdit::textUnderCursor() const
{
    QTextCursor tc = textCursor();
    tc.select(QTextCursor::WordUnderCursor);
    return tc.selectedText();
}

void TextEdit::focusInEvent(QFocusEvent *e)
{
    if (c)
        c->setWidget(this);
    QTextEdit::focusInEvent(e);
}

void TextEdit::keyPressEvent(QKeyEvent *e)
{
    if (c && c->popup()->isVisible()) {
        // The following keys are forwarded by the completer to the widget
       switch (e->key()) {
       case Qt::Key_Enter:
       case Qt::Key_Return:
       case Qt::Key_Escape:
       case Qt::Key_Tab:
       case Qt::Key_Backtab:
            e->ignore();
            return; // let the completer do default behavior
       default:
           break;
       }
    }

    bool isShortcut = ((e->modifiers() & Qt::ControlModifier) && e->key() == Qt::Key_E); // CTRL+E
    if (!c || !isShortcut) // do not process the shortcut when we have a completer
        QTextEdit::keyPressEvent(e);

    const bool ctrlOrShift = e->modifiers() & (Qt::ControlModifier | Qt::ShiftModifier);
    if (!c || (ctrlOrShift && e->text().isEmpty()))
        return;

    static QString eow("~!@#$%^&*()_+{}|:\"<>?,./;'[]\\-="); // end of word
    bool hasModifier = (e->modifiers() != Qt::NoModifier) && !ctrlOrShift;
    QString completionPrefix = textUnderCursor();

    if (!isShortcut && (hasModifier || e->text().isEmpty()|| completionPrefix.length() < 3
                      || eow.contains(e->text().right(1)))) {
        c->popup()->hide();
        return;
    }

    if (completionPrefix != c->completionPrefix()) {
        c->setCompletionPrefix(completionPrefix);
        c->popup()->setCurrentIndex(c->completionModel()->index(0, 0));
    }
    QRect cr = cursorRect();
    cr.setWidth(c->popup()->sizeHintForColumn(0)
                + c->popup()->verticalScrollBar()->sizeHint().width());
    c->complete(cr); // popup it up!
}

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

QT_BEGIN_NAMESPACE
class QAbstractItemModel;
class QComboBox;
class QCompleter;
class QLabel;
class QLineEdit;
class QProgressBar;
QT_END_NAMESPACE
class TextEdit;

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = 0);

private slots:
    void about();

private:
    void createMenu();
    QAbstractItemModel *modelFromFile(const QString& fileName);

    QCompleter *completer;
    TextEdit *completingTextEdit;
};

#endif // MAINWINDOW_H

mainwindow.cpp

#include <QtWidgets>
#include "mainwindow.h"
#include "textedit.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent), completer(0)
{
    createMenu();

    completingTextEdit = new TextEdit;
    completer = new QCompleter(this);
    completer->setModel(modelFromFile(":/resources/wordlist.txt"));
    completer->setModelSorting(QCompleter::CaseInsensitivelySortedModel);
    completer->setCaseSensitivity(Qt::CaseInsensitive);
    completer->setWrapAround(false);
    completingTextEdit->setCompleter(completer);

    setCentralWidget(completingTextEdit);
    resize(500, 300);
    setWindowTitle(tr("Completer"));
}

void MainWindow::createMenu()
{
    QAction *exitAction = new QAction(tr("Exit"), this);
    QAction *aboutAct = new QAction(tr("About"), this);
    QAction *aboutQtAct = new QAction(tr("About Qt"), this);

    connect(exitAction, SIGNAL(triggered()), qApp, SLOT(quit()));
    connect(aboutAct, SIGNAL(triggered()), this, SLOT(about()));
    connect(aboutQtAct, SIGNAL(triggered()), qApp, SLOT(aboutQt()));

    QMenu* fileMenu = menuBar()->addMenu(tr("File"));
    fileMenu->addAction(exitAction);

    QMenu* helpMenu = menuBar()->addMenu(tr("About"));
    helpMenu->addAction(aboutAct);
    helpMenu->addAction(aboutQtAct);
}

QAbstractItemModel *MainWindow::modelFromFile(const QString& fileName)
{
    QFile file(fileName);
    if (!file.open(QFile::ReadOnly))
        return new QStringListModel(completer);

#ifndef QT_NO_CURSOR
    QApplication::setOverrideCursor(QCursor(Qt::WaitCursor));
#endif
    QStringList words;

    while (!file.atEnd()) {
        QByteArray line = file.readLine();
        if (!line.isEmpty())
            words << line.trimmed();
    }

#ifndef QT_NO_CURSOR
    QApplication::restoreOverrideCursor();
#endif
    return new QStringListModel(words, completer);
}

void MainWindow::about()
{
    QMessageBox::about(this, tr("About"), tr("This example demonstrates the "
        "different features of the QCompleter class."));
}

Фактически получается, что нужно написать весь необходимый функционал самостоятельно. В данном примере Completer формируется из ресурсного файла, который добавлен в приложение.

Более полный код этого примера есть в самом Qt Creator. Называется Custom Completer