Question

您好我正在尝试了解如何使用'this'指针。现在我编写了一个示例程序，它使用类Image，它是类BMP的子类。现在，函数TellWidth和TellHeight在BMP类中声明。现在编译器给出了一个错误，表示在Image中不存在TellWidth函数。但由于Image是BMP的子类，因此它不能继承BMP中的函数。我该如何解决这个

void Image :: invertcolors()
{
    int x;
    int y;

    int width  =(*this).TellWidth();
    int height = (*this)->TellHeight();

    for(x=0,x<=height-1;x++){
        for(y=0,y<=width-1;y++){
            (*this)(x,y)->Red = (255 - (*this)(x,y)->Red);
            (*this)(x,y)->Blue = (255 - (*this)(x,y)->Blue);
            (*this)(x,y)->Green = (255 - (*this)(x,y)->Green);

        }
    }
    delete width;
    delete height;
}

图像

class Image : public BMP  
{
public:

    void invertcolors();

    void flipleft();
    void adjustbrightness(int r, int g, int b) ;

};

这个类太大了，不能在这里发布，这是一个相关的摘录

class BMP {
private:
   int Width;
   int Height;
public:
   int TellBitDepth(void) const;
   int TellWidth(void) const;
   int TellHeight(void) const;
};

Answer 1

TellWidth()很可能在private类中声明为BMP（或没有访问者修饰符）。 protected类需要public或Image才能访问它，如果您希望能够覆盖它，则需要virtual它在Image班。

适当的this用法是这样的：

int width = this->TellWidth();
int height = this->TellHeight();

阅读this，了解this的快速教程。

Answer 2

关于this的一点：您很少需要明确提及它。通常的例外是当你需要将它传递给非成员函数时（这里似乎不是这种情况。）

当您在班级成员函数内时，this->field只能被field访问，而this->function(x)可以被调用为function(x)。

以下是对您的代码的一些评论。我希望他们有所帮助。

void Image :: invertcolors()
{
    // Don't define these here; that's old C-style code. Declare them where
    // they're needed (in the loop: for (int x=0...)
    int x;
    int y;

    // Change the lines below to
    // int width  = TellWidth();
    // int height = TellHeight();
    //    (*this).TellWidth() should work, but is redundant;
    //    (*this)->TellHeight() should probably *not* work, as once you've
    //    dereferenced *this, you're dealing with an object instance, not a
    //    pointer. (There are ways to make (*this)->that() do something useful,
    //    but you're probably not trying something like that.)
    int width  =(*this).TellWidth();
    int height = (*this)->TellHeight();

    for(x=0,x<=height-1;x++){
        for(y=0,y<=width-1;y++){
            // After locating the BMP class through google (see Edit 2),
            // I've confirmed that (*this)(x,y) is invoking a (int,int) operator
            // on the BMP class. It wasn't obvious that this operator 
            // was defined; it would have been helpful if you'd posted
            // that part of the header file.
            (*this)(x,y)->Red = (255 - (*this)(x,y)->Red);
            (*this)(x,y)->Blue = (255 - (*this)(x,y)->Blue);
            (*this)(x,y)->Green = (255 - (*this)(x,y)->Green);

        }
    }
    // These are int values. They can't be deleted, nor do they need to be.
    // I'm sure the compiler has told you the same thing, though perhaps not
    // in the same way.
    delete width;
    delete height;
}

编辑：看起来有人以same course作为OP。这里给出的例子更清楚地表明Image应该有某种数组访问器，这可以解释(*this)(x,y)->Red = (255 - (*this)(x,y)->Red)想要实现的目标。

编辑2 ：Here's the source原始BMP类。

Answer 3

类图像定义为

class Image : public BMP  
{
public:

    void invertcolors();

    void flipleft();
    void adjustbrightness(int r, int g, int b) ;

};

'this'指针，使用'this'指针继承子类中超类的函数

3 个答案: