Question

I’ve build a class representing a matrix and I override all
the arithmetic operators. Here’s an example of overloading the unary minus

RegMatrix.h:

const RegMatrix operator - ();

RegMatrix.cpp

const RegMatrix RegMatrix::operator - ()
{
    RegMatrix newMatrix(*this);
    newMatrix *= -1;
    return newMatrix;
}

Now, this works perfect when I instantiate object on the stack like this: RegMatrix a(3,3,v) (v is a vector of values, doesn’t matter).
When I use the new keyword like this (in main.cpp):

RegMatrix* a = new RegMatrix(3,3,v);
RegMatrix* b = -a; //<---ERROR HERE

I get wrong type argument to unary minus
Any ideas why this happens? Thanks!

P.S.
Another question: the ‘=’ operator is automatically overridden by the copy constructor, right?

Answer 1

The type of a is RegMatrix *, not RegMatrix; if you want to apply operators on the object to which a points you have to dereference a (*a), apply the operator to it (-(*a)) and, if you want a separate instance of it on the heap, create a new copy of it on the heap with new and the copy constructor:

RegMatrix* a = new RegMatrix(3,3,v);
RegMatrix* b = new RegMatrix(-(*a));

Still, as @leftaroundabout pointed out in a comment, this is not a good way to work in C++, where, as a rule of thumb, you try to avoid dynamic allocation if you can (it’s slower and it requires smart pointers if you don’t want memory leaks).