为什么自定义类的对象无显示(已解决) [复制链接]

#include<QLabel>
#include<QWidget>
#include"stdio.h"
#include<QString>
#include<Qt>
class counter:public QWidget
{
    Q_OBJECT
    public:
        counter();
    public slots:
        void IncCounter();
        void DecCounter();
    private:
        int Counter;
        QLabel *label;
};

#include"counter.h"
counter::counter()
{
    Counter=0;
    label=new QLabel("0",this);
}
void counter::IncCounter()
{
    char str[30];
    sprintf(str,"%d",++Counter);
    label->setText(str);
}

void counter::DecCounter()
{
    char str[30];
    sprintf(str,"%d",--Counter);
    label->setText(str);
}

#include<QPushButton>
#include"counter.h"

class MainWindow: public QWidget
{
    Q_OBJECT
    public :
        MainWindow();
    private:
        QPushButton *AddButton;
        QPushButton *DecButton;
        counter *ct;
};

#include"mainwindow.h"
#include<QFont>

MainWindow::MainWindow()
{
    QFont f("Helvetica",14,QFont::Bold);
    setFont(f);
   
    AddButton=new QPushButton("&Add",this);
    AddButton->setGeometry(50,15,90,40);
    DecButton=new QPushButton("&Dec",this);
    DecButton->setGeometry(50,70,90,40);

    ct=new counter();
    ct->setGeometry(50,125,90,40);

    connect(AddButton,SIGNAL(clicked()),ct,SLOT(counter::IncCounter()));
    connect(DecButton,SIGNAL(clicked()),ct,SLOT(counter::DecCounter()));   
}

#include"mainwindow.h"
#include<QApplication>

int main(int argc,char **argv)
{
    QApplication app(argc,argv);
    MainWindow mwindow;
    mwindow.setGeometry(50,20,200,200);
    mwindow.show();
    return app.exec();
}