Re: Help with constness and output operator<<

From:

"Balog Pal" <pasa@lib.hu>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 23 Sep 2009 18:26:28 CST

Message-ID:

<h9e15d$2aji$1@news.ett.com.ua>

<marco.guazzone@gmail.com>

The problem is that the "const" method is not called in the output
operator <<.
In fact when I execute the line:
std::cout << foo(1) << std::endl
Instead of getting zero as output (resulting from the call to the
const method) I get the exception "Not assignable".
To make it to work I have to explicitly cast to type "foobar const&".

Where I am wrong?

In understanding the overload rules.

They work exactly as you see happening. The function that is called is
selected based on the *call parameters* and NEVER does it take in account a
return type, or what the return is used for. So if called on a non-const
instance of your class, the nonconst overloads are called, always. On a
const instance or a const& as you created, the const ones.

To have what you want you need to desing another kind of interface, i.e. a
function assign(T newval) or for_assign(void) .

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