Re: returning a (const) reference to a temporary

From:

domachine <dominik.burgdoerfer@googlemail.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 22 Feb 2011 00:55:52 -0800 (PST)

Message-ID:

<a91ed939-a8d3-4076-9474-7b6105997b87@w6g2000vbo.googlegroups.com>

On Feb 22, 9:26 am, AdlerSam <Christof.Warl...@siemens.com> wrote:

Hi,

I wonder why the following two lines produce a warning:

class X {};
const X &f() {return X();}

$ g++ -c ref.cpp
ref.cpp: In function =91const X& f()':
ref.cpp:2: warning: returning reference to temporary

As far as I understand, a const reference _extends_ the lifetime of a
temporary until the very last reference instance that refers to the
temporary goes out of scope. Thus, where is the problem that justyfies
the warning?

Who told you, the thing with the lifetime? I think you misunderstood
something. The const does to a reference, is making it constant.
Comparable to constant pointers.

class Test
{
public:
    void non_const_method()
    {
        // Do something like writing to a member-variable
        // ...
    }

    void const_method() const
    {
        // Do something constant like reading a variable
        // ...
    }
}

int main()
{
    const Test& ref = Test();

    ref.non_const_method(); // This doesn't work
    ref.const_method(); // Yes this does
}

This is the only thing the const keyword does. We don't have a garbage
collector in C++.

Best regards Dominik