strange problem with operator overloading

**misterowakka** · 01-17-2008

Using visual studio 2003

In a header file I have as follows

Code:

class Vector2_1
{
private:
	double a, b;
public:
	Vector2_1() {a=0; b=0;}
	Vector2_1(double vec1, double vec2):a(vec1), b(vec2) {}
	void SetSpecificValue(double value, int pos);
	double GetSpecificValue(int pos) const;
};


class Vector1_2:public Vector2_1
{
public:
	Vector1_2* Vector1_2::operator * (const Matrix2_2& M);  //This is the problematic line
};

class Matrix2_2
{
private:
	double M11, M12, M21, M22;
public:
	Matrix2_2(double UPLEFT, double UPRIGHT, double DOWNLEFT, double DOWNRIGHT):M11(UPLEFT), M12(UPRIGHT), M21(DOWNLEFT), M22(DOWNRIGHT) {}
	Matrix2_2() {M11=0; M12=0; M21=0; M22=0;}
	void SetSpecificValue(double value, int row, int column);
	double Getspecificvalue(int row, int column) const;

	Matrix2_2* operator * (const Matrix2_2&);
	Vector2_1* operator * (const Vector2_1&);
};

for some reason I get the following error
error C2143: syntax error : missing ',' before '&'

I tried to remove the CPP from the solution and leave the header only. The error disappeared. Adding the CPP with no contents but #include "Matrix.h" returns the error.

Help?

**anon** · 01-17-2008

I think you are supposed to put a forward declaration of Matrix2_2 before Vector1_2. The compiler doesn't know what Matrix2_2 is when it gets to the error line.

Also, why do you return a pointer from Vector1_2 * Matrix2_2 and exactly what would it be pointing at?

**misterowakka** · 01-17-2008

Originally Posted by anon

I think you are supposed to put a forward declaration of Matrix2_2 before Vector1_2. The compiler doesn't know what Matrix2_2 is when it gets to the error line.

Also, why do you return a pointer from Vector1_2 * Matrix2_2 and exactly what would it be pointing at?

1) Thank you!!! :-) It worked
2)

Code:

Vector1_2* Vector1_2::operator * (const Matrix2_2& M)
{
	Vector2_1* vec2=new Vector2_1;
	vec2->SetSpecificValue(a*M.Getspecificvalue(1,1)+b*M.Getspecificvalue(2,1),1);
	vec2->SetSpecificValue(a*M.Getspecificvalue(1,2)+b*M.Getspecificvalue(2,2),2);
	return vec2;
}

I thought of doing so in order to pass a pointer and not by value. But it is problematic.
For instance, I will have an operator in Matrix2_2

Matrix2_2* operator + (const Matrix2_2&);

So I will have a problem doing

Matrix2_2 mat1, mat2;
mat2=mat1+mat1+mat1;

Because the returned value from the first sum is a pointer and not const Matrix2_2&.

What do you think I should do instead?

**anon** · 01-17-2008

Not a good idea.

1) Unusual syntax. When overloading operators do like integers do. The usage is not

Code:

int a, b;
int* p = a * b;

2) Who is going to deallocate the new objects or even know which objects were allocated dynamically?

3) You may be underestimating the optimizing capabilities of compilers. Any temporary copies may be optimized away, if not otherwise then if you coded it like this:

Code:

Vector1_2 Vector1_2::operator * (const Matrix2_2& M) const
{
    return Vector1_2(a*M.Getspecificvalue(1,1)+b*M.Getspecificvalue(2,1),
        a*M.Getspecificvalue(1,2)+b*M.Getspecificvalue(2,2));
}

(The const is added for const-correctness.)

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