'this'指针，使用'this'指针在子类中继承超类的函数

Question

您好，我想了解如何使用“this”指针。现在我编写了一个示例程序，它使用了一个类 Image，它是 BMP 类的子类。现在函数 TellWidth 和 TellHeight 在 BMP 类中声明。现在编译器给了我一个错误，说 Image 中不存在 TellWidth 函数。但是由于 Image 是 BMP 的子类，它不应该继承 BMP 中的功能。 我该如何解决这个问题

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;
}

图片

class Image : public BMP  
{
public:

    void invertcolors();

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

};

这个类太大了，这里不能发，这里是相关的摘录

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 BMP 类中声明为 private（或没有访问器修饰符）。它必须是protected 或public 才能使Image 类能够访问它，并且它还需要是virtual，如果您希望能够在Image 类中覆盖它。

正确的this用法是这样的：

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

阅读this 以获取有关this 的快速教程。

Answer 2

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

当您在类成员函数内部时，this->field 可以简单地作为field 访问，this->function(x) 可以作为function(x) 调用。

以下是您的代码中的一些 cmets。我希望它们对您有所帮助。

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：看起来有人将same course 作为 OP。此处提供的示例更清楚地表明 Image 应该具有某种数组访问器，这可以解释 (*this)(x,y)->Red = (255 - (*this)(x,y)->Red) 的意图。

编辑 2：Here's the source 用于原始 BMP 类。

Answer 3

类Image被定义为

class Image : public BMP  
{
public:

    void invertcolors();

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

};