About destructor

Question

In the DirectX metro sample one can find a class named DirectXBase with roughly this header

ref class DirectXBase abstract
{
public:
    DirectXBase();

    virtual void Render() = 0;
    virtual void Present();
    /// ...............

protected:

    // Declare Direct2D Objects
    Microsoft::WRL::ComPtr<ID2D1Factory1>           m_d2dFactory;
    Microsoft::WRL::ComPtr<ID2D1Device>             m_d2dDevice;
    Microsoft::WRL::ComPtr<ID2D1DeviceContext>      m_d2dContext;
    Microsoft::WRL::ComPtr<ID2D1Bitmap1>            m_d2dTargetBitmap;
    //.............
};

What I find strange is that there is no destructor!

What about all those ComPtr? are they automatically set to null because it's a ref class?

is it a bug?

Answer 1

Whether or not you define a destructor, the compiler will ensure that any class members that you hold will have their destructors called when your class is destroyed.

You will find that Microsoft::WRL::ComPtr does have a destructor to release their COM handles.

Answer 2

Thanks for that!

In fact I'm a little bit of a C++ newbie as well. Anyway, to make things clear to me I tried the following test, created a header

Lang.h

#pragma once

ref class Lang
{
public:
	Lang(int n);
	~Lang();

	void Poo();

private:
	Lang^ sub;
	int n0;
};

Lang::Lang(int n)
: n0(n), sub(nullptr)
{
	if(n>0)
		sub = ref new Lang(n - 1);
	Platform::Console::WriteLine("Lang(" + n0.ToString() + ")");
}

Lang::~Lang()
{
	Platform::Console::WriteLine("~Lang(" + n0.ToString() + ")");
}

void Lang::Poo()
{
	Platform::Console::WriteLine("Lang::Poo(" + n0.ToString() + ")");
	if (sub != nullptr)
		sub->Poo();
}<br/>

And then I tested it by adding this in InitializeComponent() of my main window:

	{
		auto a = ref new Lang(3);
		a->Poo();
	}
	{
		Lang a(2);
		a.Poo();
	}

And used break points to follow control flow (as neither std::cout nor Platform::Console::WriteLine seemed to work)

I'm a little confused now.

I was glad to see that the destructors were automatically called.

However a.Poo() (in the second block) caused a violation exception!!!
Apparently a.sub->sub->sub->n0 is the source of the memory access problem!

What is wrong here??