Why Do My OpenGL Objects in C RAII Classes Stop Working After Copying?

OpenGL Objects in C RAII Classes: Why They Stop Working after Copying

In C , it's common to utilize RAII (Resource Acquisition Is Initialization) to ensure objects are automatically cleaned up upon destruction. However, when dealing with OpenGL objects, it's crucial to understand how the RAII pattern affects object usage.

Consider the following class that encapsulates an OpenGL buffer object:

class BufferObject {
private:
  GLuint buff_;

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

Initially, this class seems to function correctly. However, issues arise when performing certain operations:

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.

Copying the class via push_back or return statements causes unexpected OpenGL errors. To understand why, it's necessary to delve into the mechanics of RAII.

When an object is copied, the default copy constructor is invoked (assuming one exists). In this case, a default copy constructor simply copies the member variables. However, this copy includes the OpenGL object handle (buff_), which is unique to each object.

The problem arises when the original object is destroyed (at the end of the first scope in our example). The destructor attempts to delete the OpenGL object, which has already been copied by the new object(s). This orphaned OpenGL object can no longer be used and leads to errors.

To resolve this issue, it's crucial to define custom copy and move semantics for OpenGL object wrappers. This approach ensures that the OpenGL object reference is properly transferred between objects without causing conflicts.

Moving an object involves transferring ownership of its resources to another object. Instead of copying, the object's resources are assigned to the new object, and the original object is left in a valid but empty state. This approach prevents potential conflicts and ensures the OpenGL object remains valid.

In the modified BufferObject class below, the copy constructor and copy assignment operator are deleted, and move semantics are implemented:

class BufferObject {
private:
  GLuint buff_;

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

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

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

  BufferObject& operator=(BufferObject&& other) {
    if (this != &other) {
      Release();
      buff_ = other.buff_;
      other.buff_ = 0;
    }

    return *this;
  }

  ~BufferObject() { Release(); }

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

By implementing move semantics, it's possible to create move-only RAII wrappers for OpenGL objects. This approach allows for safe and efficient handling of OpenGL objects in C .

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