Question

Sorry not sure if this has been asked before, I really dont know what to look up either. I’m new to C++ from Java. When we want to call a function on an object in Java, we say picture.rotateRight();

Then, in rotateRight(), we’d have something like int height=this.getHeight();. However, how do we do this in C++? I have a method named invertcolors(); and then I have something like:

Image* myImage = new Image();
bool b = myImage->ReadFromFile("in_01.bmp");
myImage->invertcolors();

void invertcolors(){
    int width=TellWidth();
    int height=TellHeight();
    ...
}

How do I access myImage from the method definition without actually saying myImage (since that name can later be changed).

Also, the function parameters are non-negotiable.

Answer 1

First of all, your invertcolors() function definition is a non-member function. Although you’ve declared it inside the Image class, you haven’t linked the implementation to the class in any way so the compiler thinks its a non-member function. To make it a member of Image, you need to use Image::invertcolors like this:

void Image::invertcolors(){
    int width=TellWidth();
    int height=TellHeight();
    ...
}

You do get this in C++, but it’s a pointer so you have to use this->getHeight() in C++. However, note that it is redundant in this case. As a beginner you’ll probably find the only real use in a method having the same argument name as an attribute. In this case, you’ll need to use this->height = height for example. However, note that C++ has a nice syntax addition here. This code does the same as a simple setter:

void Image::setHeight(int height): height(height) {}

Note that neither in Java nor C++ is this an operator. ., -> and + are examples of operators.