DetTab_Form::DetTab_Form(QWidget *parent)
    : QTabWidget(parent)
{
    Det_Form detForm;
    Limitcard_Form limitForm;

    detTabWidget = new QTabWidget;
    detTabWidget->addTab(&detForm, "details");
    detTabWidget->addTab(&limitForm, "limit");
    detTabWidget->show();
}

class Det_Form : public QDialog
{
    Q_OBJECT

public:
    Det_Form(QWidget *parent = 0);
    ~Det_Form();

...+разные виджеты и функции

class Det_Form : public QWidget
{
    Q_OBJECT

public:
    Det_Form();
    ~Det_Form();

...+разные виджеты и функции

Det_Form::Det_Form(QWidget *parent):QWidget(parent)