How Can RAII with OpenGL Objects in C Lead to Unexpected Behavior When Copying or Moving Objects?

RAII with OpenGL Objects in C : Understanding the Hidden Issues

In C object-oriented programming, resource acquisition is initialization (RAII) is a technique used to manage resources efficiently. When using OpenGL objects within C classes, it's common to want to use RAII to ensure that the OpenGL object is correctly released when the class is destroyed.

Consider the following code snippet:

class BufferObject
{
private:
  GLuint buff_;

public:
  BufferObject()
  {
    glGenBuffers(1, &buff_);
  }

  ~BufferObject()
  {
    glDeleteBuffers(1, &buff_);
  }
};

This class seems to implement RAII correctly for OpenGL objects. However, problems arise when the class is copied or moved, as exemplified by the following code:

vector<BufferObject> bufVec;
{
  BufferObject some_buffer;
  //Initialize some_buffer;
  bufVec.push_back(some_buffer);
}
bufVec.back(); //buffer doesn't work.

BufferObject InitBuffer()
{
  BufferObject buff;
  //Do stuff with `buff`
  return buff;
}

auto buff = InitBuffer(); //Returned buffer doesn't work.

In these scenarios, the OpenGL objects become unusable, leading to errors.

The reason for this behavior lies in the default copy constructors and assignment operators generated by the compiler. These operations simply copy the members of the object, resulting in multiple C objects referencing the same underlying OpenGL object. When the original C object is destroyed, it releases the OpenGL object, causing the other objects to reference a destroyed resource.

To resolve this issue, the BufferObject class should be a move-only type. This means eliminating the copy constructor and assignment operator and providing move equivalents that transfer the ownership of the OpenGL object to the new object.

class BufferObject
{
private:
  GLuint buff_;

public:
  BufferObject()
  {
    glGenBuffers(1, &buff_);
  }

  BufferObject(const BufferObject &) = delete;
  BufferObject &operator=(const BufferObject &) = delete;

  BufferObject(BufferObject &&other) : buff_(other.buff_)
  {
    other.buff_ = 0;
  }

  BufferObject &operator=(BufferObject &&other)
  {
    //ALWAYS check for self-assignment
    if(this != &other)
    {
      Release();
      buff_ = other.buff_;
      other.buff_ = 0;
    }

    return *this;
  }

  ~BufferObject() {Release();}

  void Release();
  {
    if(buff_)
      glDeleteBuffers(1, &buff_);
  }

//Other members.
};

With these changes, the class ensures that the OpenGL object is correctly managed and released, even when copying or moving the class.

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