每个QWidget都有contentsMargins函数，善用QMargins

m_pSearchLineEdit = new QLineEdit();
QPushButton *pSearchButton = new QPushButton(this);

pSearchButton->setCursor(Qt::PointingHandCursor);
pSearchButton->setFixedSize(22, 22);
pSearchButton->setToolTip(QStringLiteral("搜索"));
pSearchButton->setStyleSheet("QPushButton{border-image:url(:/images/icon_search_normal); background:transparent;} 
                                     QPushButton:hover{border-image:url(:/images/icon_search_hover)} 
                                     QPushButton:pressed{border-image:url(:/images/icon_search_press)}");

//防止文本框输入内容位于按钮之下
QMargins margins = m_pSearchLineEdit->textMargins();
m_pSearchLineEdit->setTextMargins(margins.left(), margins.top(), pSearchButton->width(), margins.bottom());
m_pSearchLineEdit->setPlaceholderText(QStringLiteral("请输入搜索内容"));

QHBoxLayout *pSearchLayout = new QHBoxLayout();
pSearchLayout->addStretch();
pSearchLayout->addWidget(pSearchButton);
pSearchLayout->setSpacing(0);
pSearchLayout->setContentsMargins(0, 0, 0, 0);
m_pSearchLineEdit->setLayout(pSearchLayout);

connect(pSearchButton, SIGNAL(clicked(bool)), this, SLOT(search()));

http://blog.csdn.net/u011012932/article/details/50357523

http://doc.qt.io/qt-5/qwidget.html#contentsMargins