Qt|C++工作笔记-对虚函数的进一步认识（关键是Qt中如何正确使用，达到想要的效果）

关于虚函数的简单例子

首先是一个很简单例子，刚学C++的朋友都能理解

源码如下：

#include <cstdio>
#include <windows.h>

static int criticalRes = 1;		//critical resource

class Base {
public:
	virtual void print() {
		printf("print called in Base Class!\n");

		//critical resource will be modified by Base Class
		criticalRes *= 1000;
	}
};

class Child:public Base {
public:
	virtual void print() {	// "virtual" can be removed or reserve
		printf("print called in Child Class!\n");
		criticalRes -= 1;
		Base::print();

		printf("The criticalRes's value is %d\n", criticalRes);
	}
};

void main() {

	Child *child = new Child;
	child->print();

	delete child;
	getchar();
}

运行截图如下：

程序逻辑：先把临界资源的值减少了1，再调用父类的函数，乘1000。

 

把代码改下：

#include <cstdio>
#include <windows.h>

static int criticalRes = 1;		//critical resource

class Base {
public:
	virtual void print() {
		printf("print called in Base Class!\n");

		//critical resource will be modified by Base Class
		criticalRes *= 1000;
	}
};

class Child:public Base {
public:
	virtual void print() {	// "virtual" can be removed or reserve
		Base::print();
		printf("print called in Child Class!\n");
		criticalRes -= 1;

		printf("The criticalRes's value is %d\n", criticalRes);
	}
};

void main() {

	Child *child = new Child;
	child->print();

	delete child;
	getchar();
}

运行截图如下：

程序运行逻辑：先调用父类把临界资源的指乘了1000，再减少了1。

 

在Qt中应该注意的问题

上面那个只是小小的知识点，在Qt中，要利用这个特性，完成我们想要达到的功能。

比如上次说的三只小猪的例子【链接如下：https://blog.csdn.net/qq78442761/article/details/82182054】

 

给出鼠标单击时候的代码：

void MyDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option, const QModelIndex &index) const
{
    if(index.data().canConvert<MyPic>()){
        //qDebug()<<"can be convert!";
        MyPic mypic=qvariant_cast<MyPic>(index.data());
        mypic.paint(painter,option.rect,option.palette);
    }
    else{
        qDebug()<<"can't be convert!";
    }

    QStyledItemDelegate::paint(painter, option, index);
}

运行截图如下：

这种运行太令人糟糕了，我们不要他被覆盖掉！

 

改成如下的代码：

void MyDelegate::paint(QPainter *painter, const QStyleOptionViewItem &option, const QModelIndex &index) const
{
    QStyledItemDelegate::paint(painter, option, index);

    if(index.data().canConvert<MyPic>()){
        //qDebug()<<"can be convert!";
        MyPic mypic=qvariant_cast<MyPic>(index.data());
        mypic.paint(painter,option.rect,option.palette);
    }
    else{
        qDebug()<<"can't be convert!";
    }
}

运行截图如下：