QLineEdit with icon (or button)

lineedit.cpp

// depending on the stylesheet:
const int defaultPadding = 10;
const int border = 3;

const int iconSize = lineedit->fontMetrics().xHeight()*4; // or whatever size looks good
auto lineedit = QLineEdit();
lineedit->setStyleSheet(QString(R"CSS(
  QLineEdit {
    padding-left: %1px;
  }
)CSS").arg(defaultPadding*2 + iconSize)); // make space for the icon in the padding

auto label = new QLabel(lineedit);
label->setPixmap(icon.pixmap(iconSize));
label->setFixedSize(iconSize, iconSize);
label->setStyleSheet(R"CSS(
  QLabel {
    padding: 0px;
    border: none;
  }
)CSS");

// reposition the label when the lineedit changes size
lineedit->installEventFilter(new FunctionalEventFilter([=](QObject* obj, QEvent* event) // or create a QObject subclass that does that
{
  if (auto resizeEvent = dynamic_cast<QResizeEvent*>(event))
    label->move(defaultPadding + border, resizeEvent->size().height()/2 - label->height()/2); // position is relative to top-left of lineedit
}, label));