c++: overloading + operator for a sparse matrix

Question

void add(sparseMatrix<T> &b, sparseMatrix<T> &c); // c is output

sparseMatrix<T> operator+(sparseMatrix<T> &b);

I'm creating a sparse matrix which is made up of an arrayList of singly linked lists of matrix terms (matrix terms contain the row, column, and value). I'm having trouble overloading the + operator. I have an add method which works fine, but when I try to use it to overload the + operator I get the following errors:

sparseMatrix.cpp: In function ‘int main()’:
sparseMatrix.cpp:268: error: no match for ‘operator=’ in ‘c = sparseMatrix<T>::operator+(sparseMatrix<T>&) [with T = int](((sparseMatrix<int>&)(& b)))’
sparseMatrix.cpp:174: note: candidates are: sparseMatrix<T>& sparseMatrix<T>::operator=(sparseMatrix<T>&) [with T = int]
make: *** [sparseMatrix] Error 1

Here is my implementation for the overloaded + operator:

sparseMatrix<T> sparseMatrix<T>::operator+(sparseMatrix<T> &b) 
{
        sparseMatrix<T> c;

 add(b, c);
 return c;

}

The line in main that gives the error is c = a + b (a, b, c are all sparse matrices). Note that if I do a.add(b,c) everything works fine. I have also overloaded the = operator which works when I do a = b etc. but it seems to be complaining about it in the error message I posted. I'm really not sure what the problem is. Any ideas?

Answer 1

note: candidates are: sparseMatrix& sparseMatrix::operator=(sparseMatrix&)

Your operator= should take a const reference.

If the reference isn't const, it can't be bound to a temporary, so the assignment operator can't be used for the temporary created by a + b.

(The same is true for operator+, here also the argument should be const sparseMatrix<T> &. Additionally this method should be declared as const, since it doesn't modify the object it is called on.)

Answer 2

sth: has correctly diagnosed the problem:

But I would make your operators more standard.

class sparseMatrix
{
   sparseMatrix(sparseMatrix const& copy);
   sparseMatrix& operator=(sparseMatrix const& copy);

   sparseMatrix& add(sparseMatrix const& value) // Add value to the current matrix
   {
       // Do Work.
       return *this;
   }

   // Re-use add to implement the += operator.
   sparseMatrix& operator+=(sparseMatrix const& rhs)
   {
       return add(rhs);
   }

   // Two things here:
   //
   // Implement the operator + in terms of the operator +=
   //
   // This basically means that you make a copy of one parameter then add the other
   // value two it. Because + is symmetric it does not matter which you copy and which
   // you add to the copy.
   //
   // So we pass the parameter by value this will provide an implicit copy
   // generated by the compiler. This will also help the compiler with NRVO
   // Then we just add (*this) to the copy of the rhs.
   sparseMatrix operator+(sparseMatrix rhs)
   {
       return rhs += *this; 
   }
}