效果图

二、代码之路

整体思路即根据focusInEvent和focusOutEvent事件判断焦点是否在输入框中，同时重写setPlaceholderText方法，把最开始设置的占位字符保存下来，然后焦点进入的时候如果输入框中没有字符 就把占位字符清空，如果本身就有内容字符，占位字符是不显示的。

然后在焦点离开输入框时，判断当前输入框是否有内容字符，如果没有则恢复之前保存的占位字符。

// 鼠标是否点击进行编辑;
void signalIsMouseIn(bool isFocusIn);

MyLineEdit

class MyLineEdit : public QLineEdit
{
    Q_OBJECT

public:
    MyLineEdit(QWidget* parent = NULL)
        : QLineEdit(parent)
    {


    }

    void setPlaceholderText(const QString &text)
    {
        m_placeHolderText = text;
        __super::setPlaceholderText(text);
    }

private:

    void focusInEvent(QFocusEvent *event)
    {
        if (this->text().isEmpty())
        {
            __super::setPlaceholderText("");
        }

        emit signalIsMouseIn(true);
        return __super::focusInEvent(event);
    }

    void focusOutEvent(QFocusEvent *event)
    {
        if (this->text().isEmpty())
        {
            this->setPlaceholderText(m_placeHolderText);
        }

        emit signalIsMouseIn(false);
        return __super::focusOutEvent(event);
    }

signals:
    // 鼠标是否点击进行编辑;
    void signalIsMouseIn(bool isFocusIn);

private:
    QString m_placeHolderText;

};

注：
因为QLineEdit不提供焦点进出的信号，所有这里添加了一个信号，在focusInEvent和focusOutEvent事件中进行触发，

这里为什么要添加这个信号呢？因为QQ的输入框前面的图标在有焦点的时候会变成蓝色，没有焦点的时候会变成灰色，所以这里需要这个信号来通知进行变换图标。可以见下方代码中此信号的用处。
QQLineEditWidget.h

class QQLineEditWidget : public QWidget
{
    Q_OBJECT

public:
    QQLineEditWidget(QWidget *parent = Q_NULLPTR);

private:
    void initWidget();

private:
    void paintEvent(QPaintEvent *event);
};

QQLineEditWidget.cpp

#include "QQLineEdit.h"
#include <QHBoxLayout>
#include <QPainter>
#include <QGraphicsDropShadowEffect>

#pragma execution_character_set("utf-8")

QQLineEditWidget::QQLineEditWidget(QWidget *parent)
    : QWidget(parent)
{

    initWidget();

    this->setFixedSize(QSize(430, 330));
    this->setAttribute(Qt::WA_TranslucentBackground);
    this->setWindowFlag(Qt::FramelessWindowHint);

    // 设置阴影边框;
    auto shadowEffect = new QGraphicsDropShadowEffect(this);
    shadowEffect->setOffset(0, 0);
    shadowEffect->setColor(Qt::black);
    shadowEffect->setBlurRadius(10);
    this->setGraphicsEffect(shadowEffect);
}

void QQLineEditWidget::initWidget()
{
    // 设置两个输入框前后的图标及按钮;
    QLabel* m_labelQQ = new QLabel;
    m_labelQQ->setFixedSize(QSize(14, 20));
    m_labelQQ->setPixmap(QIcon(":/Resources/LoginWindow/QQ_Focus.png").pixmap(m_labelQQ->size()));

    QLabel* m_labelLocker = new QLabel;
    m_labelLocker->setFixedSize(QSize(12, 14));
    m_labelLocker->setPixmap(QIcon(":/Resources/LoginWindow/Locker_Normal.png").pixmap(m_labelLocker->size()));

    QPushButton* m_pButtonPullDown = new QPushButton;
    m_pButtonPullDown->setCursor(Qt::ArrowCursor);
    m_pButtonPullDown->setFixedSize(QSize(16, 16));
    m_pButtonPullDown->setStyleSheet("QPushButton{border-image:url(:/Resources/LoginWindow/PullDownButton_Normal.png);}\
										QPushButton:hover{border-image:url(:/Resources/LoginWindow/PullDownButton_Hover.png);}");

    QPushButton* m_pButtonKeyborad = new QPushButton;
    m_pButtonKeyborad->setCursor(Qt::PointingHandCursor);
    m_pButtonKeyborad->setFixedSize(QSize(16, 16));
    m_pButtonKeyborad->setStyleSheet("QPushButton{border-image:url(:/Resources/LoginWindow/keyboard.png) 0 80 0 0;}\
										QPushButton:hover{border-image:url(:/Resources/LoginWindow/keyboard.png) 0 40 0 40;}\
										QPushButton:pressed{border-image:url(:/Resources/LoginWindow/keyboard.png) 0 0 0 80;}");

    MyLineEdit* lineEditAccount = new MyLineEdit;
    lineEditAccount->setPlaceholderText("QQ号码/手机/邮箱");
    lineEditAccount->setTextMargins(20, 0, 20, 0);
    lineEditAccount->setFixedSize(QSize(235, 28));
    
    // 这里通过signalIsMouseIn信号进行图标切换；
    connect(lineEditAccount, &MyLineEdit::signalIsMouseIn, this, [=](bool isMouseIn) {
        if (isMouseIn)
        {
            m_labelQQ->setPixmap(QIcon(":/Resources/LoginWindow/QQ_Focus.png").pixmap(m_labelQQ->size()));
        }
        else
        {
            m_labelQQ->setPixmap(QIcon(":/Resources/LoginWindow/QQ_Normal.png").pixmap(m_labelQQ->size()));
        }
    });

    QHBoxLayout* hAccountLayout = new QHBoxLayout(lineEditAccount);
    hAccountLayout->addWidget(m_labelQQ);
    hAccountLayout->addStretch();
    hAccountLayout->addWidget(m_pButtonPullDown);
    hAccountLayout->setMargin(0);

    MyLineEdit* lineEditPassed = new MyLineEdit;
    // 测试setPlaceholderText在是否有焦点的时候显示占位字符;
//    lineEditPassed->setAlignment(Qt::AlignCenter);
    lineEditPassed->setPlaceholderText("密码");
    lineEditPassed->setTextMargins(20, 0, 20, 0);
    lineEditPassed->setFixedSize(QSize(235, 28));
    connect(lineEditPassed, &MyLineEdit::signalIsMouseIn, this, [=](bool isMouseIn) {
        if (isMouseIn)
        {
            m_labelLocker->setPixmap(QIcon(":/Resources/LoginWindow/Locker_Focus.png").pixmap(m_labelLocker->size()));
        }
        else
        {
            m_labelLocker->setPixmap(QIcon(":/Resources/LoginWindow/Locker_Normal.png").pixmap(m_labelLocker->size()));
        }
    });

    this->setStyleSheet("*{font-family:Microsoft YaHei;font-size:12px;}\
                            QLineEdit{font-size:13px;background:transparent;border:none;border-bottom:1px solid rgb(229, 229, 229);}\
                            QLineEdit:hover{border-bottom:1px solid rgb(193,193, 193);}\
                            QLineEdit:focus{border-bottom:1px solid rgb(18, 183, 245);}");

    QHBoxLayout* hPswLayout = new QHBoxLayout(lineEditPassed);
    hPswLayout->addWidget(m_labelLocker);
    hPswLayout->addStretch();
    hPswLayout->addWidget(m_pButtonKeyborad);
    hPswLayout->setMargin(0);

    QVBoxLayout* vBoxLayout = new QVBoxLayout();
    vBoxLayout->addStretch();
    vBoxLayout->addWidget(lineEditAccount);
    vBoxLayout->addWidget(lineEditPassed);
    vBoxLayout->addStretch();
    vBoxLayout->setSpacing(10);
    vBoxLayout->setMargin(0);

    QHBoxLayout* hMainLayout = new QHBoxLayout(this);
    hMainLayout->addStretch();
    hMainLayout->addLayout(vBoxLayout);
    hMainLayout->addStretch();
    hMainLayout->setMargin(10);
}

void QQLineEditWidget::paintEvent(QPaintEvent *event)
{
    QPainter painter(this);
    painter.setRenderHint(QPainter::Antialiasing);
    painter.setPen(Qt::NoPen);
    painter.setBrush(Qt::white);
    painter.drawRoundedRect(this->rect().adjusted(10, 10, -10, -10), 3, 3);
}