Operator overloading for << and >> does not work stream object in argument is const &

Question

I have a confusion. I am making a set class and when I overload the << or >> operators with this prototype:

ostream& operator<<(const ostream & out, Set & argSet)
{
    //whatever my code does
    return out;
}

It does not work. But if I remove the "const" keyword in first argument, it works. I am confused why it is not working. Any help will be appreciated.

Think a bit about what's changing when you read/write from/to a string. The only constant is the written data. — zdf, Mar 13 '16 at 13:58
Also, when you return the first argument, you would remove the constness. Const can always be added, but never removed. — leemes, Mar 13 '16 at 14:02
@leemes that is not true. There is `const_cast` which can remove constness. — NathanOliver, Mar 13 '16 at 14:08
@NathanOliver but it is still undefined behaviour if you modify the object (or write through the pointer) that you used const_cast to remove const from. — Jesper Juhl, Mar 13 '16 at 16:00
@πάνταῥεῖ I am using a function from argSet so I cannot and do not want to make it constant. — Abdul Mateen, Mar 13 '16 at 17:43
@JesperJuhl Only if that object was `const` to begin with. see: http://stackoverflow.com/questions/357600/is-const-cast-safe — NathanOliver, Mar 13 '16 at 17:49
@NathanOliver You are correct. But that is often something that it can be very hard to know for certain. — Jesper Juhl, Mar 13 '16 at 18:09

πάντα ῥεῖ · Accepted Answer · 2016-03-13T14:17:57.230

3

But if I remove the "const" keyword in first argument, it works. I am confused why it is not working.

In both cases (overloading operator<<() and operator>>()) for the ostream& and the istream& references the stream state is usually changed by the operations made inside, thus these can't be const references.

One obvious reason you can't do so, is the required return type that can't be implicitly changed from const reference to non const reference.

edited Mar 13 '16 at 14:17

answered Mar 13 '16 at 14:10

πάντα ῥεῖ

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Hmm that's helpful. Thanks for help. – Abdul Mateen Mar 13 '16 at 17:44

score 1 · Answer 2 · answered Mar 13 '16 at 14:09

1

The stream cannot be const because its internal state has to be able to change upon insertion/extraction or other operations.

For instance, if it were to reach EOF, how would it set the relevant bit (eofbit)?

answered Mar 13 '16 at 14:09

edmz

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Operator overloading for << and >> does not work stream object in argument is const &

2 Answers2