'this' pointer, inheriting functions of super class in subclass using 'this' pointer - c++

Question

Hi i am trying to understand how to use the 'this' pointer. Now i wrote a sample program which uses a class Image which is a subclass of a class BMP. Now the functions TellWidth and TellHeight are declared in the BMP class. Now the compiler gives me an error which says that the TellWidth function does not exist in Image. But as Image is a subclass of BMP shouldnt it inherit the functions in BMP.
How do i resolve this
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;
}
Image
class Image : public BMP
{
public:
void invertcolors();
void flipleft();
void adjustbrightness(int r, int g, int b) ;
};
This class is too big to post here, here is a relavent excerpt
class BMP {
private:
int Width;
int Height;
public:
int TellBitDepth(void) const;
int TellWidth(void) const;
int TellHeight(void) const;
};

Answer 1

TellWidth() is most likely declared as private (or has no accessor modifier) in the BMP class. It needs to be protected or public for the Image class to be able to access it, and it needs to be also virtual, if you want to be able to override it in the Image class.
And the proper this usage is like this:
int width = this->TellWidth();
int height = this->TellHeight();
Read this for a quick tutorial on this.

Answer 2

One point about this: you rarely need to mention it explicitly. The usual exception is when you need to pass it into a non-member function (which doesn't seem to be the case here.)
When you're inside of a class member function, this->field can be accessed simply as field, and this->function(x) can be invoked as function(x).
Here are some comments on your code. I hope they're helpful.
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;
}
EDIT: Looks like there's someone else taking the same course as the OP. The example presented there makes it clearer that Image is supposed to have some sort of array accessor, which may explain what (*this)(x,y)->Red = (255 - (*this)(x,y)->Red) was intended to achieve.
EDIT 2: Here's the source for the original BMP class.

Answer 3

class Image is defined as
class Image : public BMP
{
public:
void invertcolors();
void flipleft();
void adjustbrightness(int r, int g, int b) ;
};