复制构造函数和重载赋值运算符的困难

Question

I am working on a problem to open a PGM file, make two copies of it, modify the original, and save all three separately. I am having a lot of trouble understanding how to write a copy constructor that accepts an array and also includes an overloaded assignment operator. I also need to include a destructor so it follows the law of three, but each iteration I've done so far gives me an error for heap corruption.

As of right now, my code modifies the original and exports it fine. But my output files for the two unmodified copies only contain three zeroes for one and the first three lines for another.

PGMImage::PGMImage(const PGMImage & anotherImage) {
  int i;
  pixels = new int[numberPixels + 1];
  for (i = 0; i < numberPixels; ++i) {
    pixels[i] = anotherImage.pixels[i];
  }
  cout << "Copy constructor called." << endl;
}

//Overloaded Assignment Operator
PGMImage & PGMImage::operator = (const PGMImage & anotherImage) {
  if (this != & anotherImage) {
    //delete pixels;                  
    pixels = new int[numberPixels];

    * pixels = * (anotherImage.pixels);
  }
  return *this;
}

//Destructor
PGMImage::~PGMImage() {}

Answer 1

I'm assuming that int* pixels and int numberPixels are private members of your class that represent the image data. I'm going to also take a wild guess that you got other members of PGMImage such as width , height , and bitdepth . Both the assignment operator and the copy constructor need to copy all member variables over to the target object.

Also, I don't know why you are doing a +1 on the memory allocation.

I'm only using this-> for clarity to indicate a member on the target object and to distinguish it from anotherImage .

Here's what I think you methods should look like:

void PGMImage::InitializeFromImage(const PGMImage& anotherImage) {
  this->pixels = new int[anotherImage.numberPixels];
  memcpy(this->pixels, anotherImage.pixels, sizeof(int)*anotherImage.numberPixels);
  this->numberPixels = anotherImage.numberPixels;

  this->width = anotherImage.width;
  this->height = anotherImage.height;
  this->bitdepth = anotherImage.bitdepth;
}

PGMImage::PGMImage(const PGMImage& anotherImage) {
  InitializeFromImage(anotherImage);
  cout << "Copy constructor called." << endl;
}

PGMImage& PGMImage::operator = (const PGMImage &anotherImage) {
  if (this != &anotherImage) {
      delete [] this->pixels;
      this->pixels = nullptr;
      InitializeFromImage(anotherImage);
  }
  return *this;
}