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temporary objects get destroyed and corrupt program

JUANDENT (411)
I believe the following code has a bug related to temporary objects getting destroyed leaving dangling references... however I don't know how to find this in the code... could you please help me?

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struct plus;
struct minus;

using namespace std::literals;

// expression tree node
template<class L, class OpTag, class R>
struct Expression
{
	Expression(L l, R r) : l{ l }, r{ r } {}
	~Expression() {}

	auto operator[](unsigned index) const
	{
		return OpTag::template apply(l[index], r[index]);
	}

	L const& l;
	R const& r;
};


// addition operator
template<class L, class R>
Expression<L, plus, R> operator+(L const& l, R const& r)
{
	return Expression<L, plus, R>(l, r);
}
template<unsigned Cols>
struct Vector
{
	float data[Cols];

	Vector() = default;
	Vector(const Vector& rhs) = default;
	Vector(std::initializer_list<float> list)
	{
		auto p = std::begin(list);
		for (int i = 0; i < Cols; ++i, ++p) { data[i] = *p; }
	}
	~Vector() = default;

	float operator[](size_t index) const
	{
		return data[index];
	}

	float& operator[](size_t index)
	{
		return data[index];
	}

	Vector& operator=(const Vector<Cols>& rhs)
	{
		for (int i = 0; i < Cols; ++i)
		{
			data[i] = rhs[i];
		}
		return *this;
	}
};

template<size_t N>
Vector<N> operator+(const Vector<N>& lhs, const Vector<N>& rhs)
{
	Vector<N> res;
	for (int i = 0; i < N; ++i)
	{
		res[i] = lhs[i] + rhs[i];
	}
	return res;
}



template<unsigned Rows, unsigned Cols>
struct Array
{
	Vector<Cols> data[Rows];

	Array() {}
	Array(const std::initializer_list<Vector<Cols>>& list)
	{
		auto p = std::begin(list);
		for (int i = 0; i < Rows; ++i, ++p) { data[i] = *p; }
	}
	~Array() = default;
	Array(const Array& rhs) = default;

	template<typename Expr>
	Array& operator=(Expr const& x)
	{
		for (unsigned i = 0; i < Rows; ++i)
		{
			(*this)[i] = x[i];
		}
		return *this;
	}

	const Vector<Cols>& operator[](size_t index) const
	{
		return data[index];
	}
	Vector<Cols>& operator[](size_t index)
	{
		return data[index];
	}

};


struct plus
{
	template<size_t N>
	static auto apply(Vector<N> a, Vector<N> b)
	{
		return a + b;
	}
};
void useExpr()
{
	// Vector<2> v{ 1.3,2.3 };
	Array<2, 2> x;

	Vector<2> a0{ 1,2 };
	Vector<2> a1{ 3,4 };
	Array<2, 2> a{ a0, a1 };

	Vector<2> b0{ 2,5 };
	Vector<2> b1{ 5,8 };
	Array<2, 2 > b{ b0, b1 };

	Vector<2> c0{ 3,6 };
	Vector<2> c1{ 7,9 };
	Array<2, 2> c{ c0,c1 };

	x = a + b + c;
	for (int i = 0; i < 2; ++i)
		for (int j = 0; j < 2; ++j)
			std::cout << x[i][j];

}


The problem is that x should have a result of {6,13},{15,21} but it doesn't

upon debugging I noted that there were many temporary objects being destroyed corrupting the stack and the values of local variables

Please all I need is some guidance!

Regards,
Juan Dent
Ganado (6810)
What is the output for you?
My output is: 
6121418
(VS 2022)

Edit: Output when 
1
2
 
L const& l;
R const& r;
is replaced with 
1
2
 
L l;
R r;

is: 
6131521

which is what you're expecting.

So at least that narrows down the issue a bit, but sorry I don't have an answer.

May be related: https://stackoverflow.com/a/25845843
It won't compile if you add
Expression(const Expression&&) = delete; // prevents rvalue binding
Last edited on
JUANDENT (411)
Thank you!!!!
I was taking a reference to local objects in Expression ctor. So your solution builds copies of the arguments or I can make arguments const references to L and R in the ctor, like so:

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	Expression(L const& l, R const& r) : l{ l }, r{ r } {}
	L const& l;
	R const& r;


That's it!!


Solved!


Many thank yous....
Ganado (6810)
Haha, glad you got it; I didn't make the connection until you pointed it out. It's a shame the compiler doesn't see this class of bug.
Last edited on
coder777 (8444)
That's it!!
Not quite. Having references as member variables are always risky.

You can have this:
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...
	Expression(L const& l, R const& r) : l{ l }, r{ r } {}
	L const& l;
	R const& r;

...

Expression<int, int> e{1, 2}; // This will generate dangling references

int l = 1;
int r = 2;
Expression<int, int> e{l, r}; // Ok
The reason for the dangling references is that a temporary objects is created when no reference able object is provide but a suitable constructor exists.
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