对《C++ GUI Programming with Qt 4 (2nd Edition)》书上例子的疑惑,该怎么解决

#ifndef FINDDIALOG_H#define FINDDIALOG_H#include <QDialog>class QCheckBox;class QLabel;class QLineEdit;class QPushButton;class FindDialog : public QDialog{    Q_OBJECTpublic:    FindDialog(QWidget *parent = 0);signals:    void findNext(const QString &str, Qt::CaseSensitivity cs);    void findPrevious(const QString &str, Qt::CaseSensitivity cs);private slots:    void findClicked();    void enableFindButton(const QString &text);private:    QLabel *label;    QLineEdit *lineEdit;    QCheckBox *caseCheckBox;    QCheckBox *backwardCheckBox;    QPushButton *findButton;    QPushButton *closeButton;};#endif

#include <QtGui>#include "finddialog.h"FindDialog::FindDialog(QWidget *parent)//疑问一：parent由始至终没有使用    : QDialog(parent){    label = new QLabel(tr("Find &what:"));    lineEdit = new QLineEdit;    label->setBuddy(lineEdit);    caseCheckBox = new QCheckBox(tr("Match &case"));    backwardCheckBox = new QCheckBox(tr("Search &backward"));    findButton = new QPushButton(tr("&Find"));    findButton->setDefault(true);    findButton->setEnabled(false);    closeButton = new QPushButton(tr("Close"));    connect(lineEdit, SIGNAL(textChanged(const QString &)),            this, SLOT(enableFindButton(const QString &)));    connect(findButton, SIGNAL(clicked()),            this, SLOT(findClicked()));    connect(closeButton, SIGNAL(clicked()),            this, SLOT(close()));    QHBoxLayout *topLeftLayout = new QHBoxLayout;    topLeftLayout->addWidget(label);    topLeftLayout->addWidget(lineEdit);    QVBoxLayout *leftLayout = new QVBoxLayout;    leftLayout->addLayout(topLeftLayout);    leftLayout->addWidget(caseCheckBox);    leftLayout->addWidget(backwardCheckBox);    QVBoxLayout *rightLayout = new QVBoxLayout;    rightLayout->addWidget(findButton);    rightLayout->addWidget(closeButton);    rightLayout->addStretch();    QHBoxLayout *mainLayout = new QHBoxLayout;    mainLayout->addLayout(leftLayout);    mainLayout->addLayout(rightLayout);    setLayout(mainLayout);//疑问2：为什么不是“对象.setLayout(mainLayout);”的形式    setWindowTitle(tr("Find"));//疑问同上    setFixedHeight(sizeHint().height());//疑问同上}