Question

I have a class that holds an array of integers and to get a reference of this array the subscript operator [] is overloaded (this is a stripped down example with logic checks etc removed):

class Foo {
public:

Foo() {};

// and overload the [] operator
int& operator[](const int index);

private:
const int LEN = 7;
int arr[LEN];
};

int& Foo::operator[](const int index) {
    return arr[index];
}

A pointer of an instance of such class (named boo) is passed to a function. Now what I want to do is:

int val = boo[0];

but it fails “error: invalid cast from type ‘Foo’ to type ‘int’”.

My first idea was that Im passing a pointer to a class and I should bring a copy of the instance into scope and use that copy instead. This works. But Im curious if it would be possible to use the user defined type as a true built in type? Should I use a wrapper?

Answer 1

Because the overloaded operator is defined for objects, not pointers to objects. There’s two options:

int val = boo->operator[](0);

or

int val = (*boo)[0];

Or you can pass the object (by value or reference) instead of a pointer to it.

Your case is equivalent to:

Boo* b = ....;
b[0];           //this returns a Boo object

as opposed to:

Boo b;
b[0];           //calls operator[]