Matrix class operator overloading,destructor problem

问题

I was trying to write a matrix class which would be able to find inverse,adjoint,etc. of a square matrix of any order. The constructor initializes an identity matrix of order n(passed to it).

class Matrix
{
int** elements;
int order;

public:
Matrix& operator=(const Matrix& second_inp)
{
    if(this->order!=second_inp.order)
        cout<<"The matrix cannot be assigned!!!\n"<<this->order<<"\n"<<second_inp.order;

    else
    {
        for(int i=0;i<this->order;i++)
            for(int j=0;j<this->order;j++)
                this->elements[i][j] = second_inp.elements[i][j];

    }

    return *this;
}

Matrix operator*(const Matrix& a)const
{
    Matrix c(a.order);

    for(int i=0;i<c.order;i++)                      
        for(int j=0;j<c.order;j++)
            c.elements[i][j]=0;

    if (this->order!=a.order)
    {
        cout<<"The 2 Matrices cannot be multiplied!!!\n";
        return Matrix();
    }

    else
    {
        for(int i=0;i<a.order;i++)
            for(int j=0;j<a.order;j++)
                for(int k=0;k<a.order;k++)
                    c.elements[i][j] += (this->elements[i][k])*(a.elements[k][j]);

        return c;
    }
}
};

~Matrix()
{
    for(int i=0;i<this->order;i++)
        delete[] *(elements+i);
    delete[] elements;
    elements=nullptr;
}

If i were to run the following code using this class:

Matrix exp1(2),exp2(2),exp3(2);
exp1.get_matrix();
exp3=exp1*exp2;
exp3.show_matrix();

I get a run-time error, while debugging i found out that, after the multiplication(exp1*exp2) the =operator was not able to access the data if the result of the *operator.

But if i were to use a manual destructor like this one at the end of the main() to free all allocated memory, the program works fine.

void destroctor()
{
  for(int i=0;i<order;i++)
    delete[] *(elements+i);
  delete[] elements;
}

how can i edit the destructor or the operator overloads to correct this problem?

The constructor i used:

Matrix(int inp_order):order(inp_order)
{
    elements=new int*[order];

    for(int i=0;i<order;i++)
        *(elements+i)=new int[order];

    for(int i=0;i<order;i++)
        for(int j=0;j<order;j++)
        {
            if (i==j)
                *(*(elements+j)+i)=1;
            else
                *(*(elements+j)+i)=0;
        }
}

回答1:

It is hard to tell what is going wrong, since you have not posted your constructors.

In the exp3=exp1*exp2; a lot of things happen:

First a new matrix c is constructed in the operator* function. Then the return c; statement calls the copy constructor and then the destructor. After that operator= is called and after that the destructor for the temporary matrix again.

I think what happens is that you are using the default copy constructor which does not make a deep copy. That way the destructor being called at the time of return c deletes the data that still shared between the matrices.

回答2:

I get a run-time error, while debugging i found out that, after the multiplication(exp1*exp2) the =operator was not able to access the data if the result of the *operator.

You didn't show us your constructor, so there is no way to tell why you are getting this errors.

I suspect that the cause is that you aren't allocating the memory needed to contain your matrix. You declared it as an int**, so you need to allocate an array of int* pointers, and for each of those you need to allocate an array of int.

Edit
While I was typing this you posted code for your constructor.

You are not returning a value from your overload of operator*, and you don't have a copy constructor (rule of three).

Do you have compiler warnings enabled? Any compiler worth its salt would have complained about the missing return statement in the operator overload.

回答3:

You have not defined a copy constructor, so the compiler will generate one for you. This constructor will be called in order to copy the return value of operator*(const & Matrix a) into the result.

As the generated copy constructor only performs a shallow memberwise copy, it will not allocate a new array of elements, hence the error.

来源：https://stackoverflow.com/questions/7460984/matrix-class-operator-overloading-destructor-problem