Tatarinho - 2 months ago 26

C++ Question

I wrote a class which represent Qubit. So object has only one value, state, with 0 or 1 (bool). To make needed calculations I overloaded operators like +,*,^.

It seems that everything is ok with + and *, also with ^, but only if I won't use it with std::ostream operator.

`Qubit x5, x6;`

cout << x5^x6; !ERROR!

but with

`Qubit x5, x6;`

Qubit z = x5^x6;

cout << z;

it's working. My std:operator

`std::ostream & operator <<(std::ostream & os, const Qubit & qubit)`

{

os << qubit.GetState();

return os;

}

and my XOR operator

`Qubit & Qubit::operator ^(const Qubit & qubit)`

{

Qubit *q = new Qubit;

((this->state == 1 && qubit.state == 0) ||

(this->state == 0 && qubit.state == 1)) ? q->SetState(1) : q->SetState(0);

return *q;

}

Answer

`cout << x5 ^ x6`

is evaluated as `(cout << x5) ^ x6`

due to *operator precedence*.

Since you have not provided an overloaded XOR operator for an `ostream&`

and a `Qubit`

(or `const Qubit&`

etc.), compilation fails.

The solution is to write `cout << (x5 ^ x6);`

(Note that the `+`

and `*`

operators have higher precedence than `<<`

which is why they work as you describe).

Finally, you have a serious memory leak in the XOR operator (who is going to `delete`

the allocated memory?). Fix that by changing the function to return a value copy:

`Qubit Qubit::operator^(const Qubit& qubit) const`

and use `Qubit q;`

in the function body. *Named Return Value Optimisation* will obviate a value copy. For more details, see http://en.cppreference.com/w/cpp/language/operator_arithmetic