qt的ui-＞setupUi的调用

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>

namespace Ui {
class Widget;
}

class Widget : public QWidget
{
    Q_OBJECT

public:
    explicit Widget(QWidget *parent = 0);
    ~Widget();

private:
    Ui::Widget *ui;
};

#endif // WIDGET_H

以上代码用qt的人经常见到，6-8行是一个类的声明（第一个类），之后的是对另外一个类的定义（第二个类），在这个类的定义里面又创建了一个第一个类的指针。第一个类的定义在类似ui_widget.h里面，也就是qtdisigner创建出来的那个文件，如下：

/********************************************************************************
** Form generated from reading UI file 'widget.ui'
**
** Created by: Qt User Interface Compiler version 5.6.1
**
** WARNING! All changes made in this file will be lost when recompiling UI file!
********************************************************************************/

#ifndef UI_WIDGET_H
#define UI_WIDGET_H

#include <QtCore/QVariant>
#include <QtWidgets/QAction>
#include <QtWidgets/QApplication>
#include <QtWidgets/QButtonGroup>
#include <QtWidgets/QHeaderView>
#include <QtWidgets/QPushButton>
#include <QtWidgets/QWidget>

QT_BEGIN_NAMESPACE

class Ui_Widget
{
public:
    QPushButton *pushButton;
    QPushButton *pushButton_2;
    QPushButton *pushButton_3;
    QPushButton *pushButton_4;
    QPushButton *pushButton_5;
    QPushButton *pushButton_6;
    QPushButton *pushButton_7;
    QPushButton *pushButton_8;

    void setupUi(QWidget *Widget)
    {
        if (Widget->objectName().isEmpty())
            Widget->setObjectName(QStringLiteral("Widget"));
        Widget->resize(400, 300);
        pushButton = new QPushButton(Widget);
        pushButton->setObjectName(QStringLiteral("pushButton"));
        pushButton->setGeometry(QRect(50, 70, 75, 23));
        pushButton_2 = new QPushButton(Widget);
        pushButton_2->setObjectName(QStringLiteral("pushButton_2"));
        pushButton_2->setGeometry(QRect(220, 70, 75, 23));
        pushButton_3 = new QPushButton(Widget);
        pushButton_3->setObjectName(QStringLiteral("pushButton_3"));
        pushButton_3->setGeometry(QRect(60, 120, 75, 23));
        pushButton_4 = new QPushButton(Widget);
        pushButton_4->setObjectName(QStringLiteral("pushButton_4"));
        pushButton_4->setGeometry(QRect(230, 120, 75, 23));
        pushButton_5 = new QPushButton(Widget);
        pushButton_5->setObjectName(QStringLiteral("pushButton_5"));
        pushButton_5->setGeometry(QRect(70, 160, 75, 23));
        pushButton_6 = new QPushButton(Widget);
        pushButton_6->setObjectName(QStringLiteral("pushButton_6"));
        pushButton_6->setGeometry(QRect(70, 230, 75, 23));
        pushButton_7 = new QPushButton(Widget);
        pushButton_7->setObjectName(QStringLiteral("pushButton_7"));
        pushButton_7->setGeometry(QRect(230, 170, 75, 23));
        pushButton_8 = new QPushButton(Widget);
        pushButton_8->setObjectName(QStringLiteral("pushButton_8"));
        pushButton_8->setGeometry(QRect(240, 230, 75, 23));

        retranslateUi(Widget);

        QMetaObject::connectSlotsByName(Widget);
    } // setupUi

    void retranslateUi(QWidget *Widget)
    {
        Widget->setWindowTitle(QApplication::translate("Widget", "Widget", 0));
        pushButton->setText(QApplication::translate("Widget", "\351\242\234\350\211\262", 0));
        pushButton_2->setText(QApplication::translate("Widget", "\346\226\207\344\273\266", 0));
        pushButton_3->setText(QApplication::translate("Widget", "\345\255\227\344\275\223", 0));
        pushButton_4->setText(QApplication::translate("Widget", "\350\276\223\345\205\245", 0));
        pushButton_5->setText(QApplication::translate("Widget", "\346\266\210\346\201\257", 0));
        pushButton_6->setText(QApplication::translate("Widget", "\351\224\231\350\257\257\344\277\241\346\201\257", 0));
        pushButton_7->setText(QApplication::translate("Widget", "\350\277\233\345\272\246", 0));
        pushButton_8->setText(QApplication::translate("Widget", "\345\220\221\345\257\274", 0));
    } // retranslateUi

};

namespace Ui {
    class Widget: public Ui_Widget {};
} // namespace Ui

QT_END_NAMESPACE

#endif // UI_WIDGET_H

在下面几行就是对Ui::Widget的定义，他只是一个Ui_Widget的子类。至于QT_BEGIN_NAMESPACE和QT_END_NAMESPACE，由于

#if !defined(QT_NAMESPACE) || defined(Q_MOC_RUN) /* user namespace */

# define QT_PREPEND_NAMESPACE(name) ::name
# define QT_USE_NAMESPACE
# define QT_BEGIN_NAMESPACE
# define QT_END_NAMESPACE
# define QT_BEGIN_INCLUDE_NAMESPACE
# define QT_END_INCLUDE_NAMESPACE
#ifndef QT_BEGIN_MOC_NAMESPACE
# define QT_BEGIN_MOC_NAMESPACE
#endif
#ifndef QT_END_MOC_NAMESPACE
# define QT_END_MOC_NAMESPACE
#endif
# define QT_FORWARD_DECLARE_CLASS(name) class name;
# define QT_FORWARD_DECLARE_STRUCT(name) struct name;
# define QT_MANGLE_NAMESPACE(name) name

#else /* user namespace */

# define QT_PREPEND_NAMESPACE(name) ::QT_NAMESPACE::name
# define QT_USE_NAMESPACE using namespace ::QT_NAMESPACE;
# define QT_BEGIN_NAMESPACE namespace QT_NAMESPACE {
# define QT_END_NAMESPACE }
# define QT_BEGIN_INCLUDE_NAMESPACE }
# define QT_END_INCLUDE_NAMESPACE namespace QT_NAMESPACE {
#ifndef QT_BEGIN_MOC_NAMESPACE
# define QT_BEGIN_MOC_NAMESPACE QT_USE_NAMESPACE
#endif

QT_NAMESPACE没有被定义，因此这个命名空间是没有的。