Re: Trying to understand "&&"

From:

SG <s.gesemann@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Sat, 1 Jan 2011 12:02:38 -0800 (PST)

Message-ID:

<3b09c5ec-e6af-4925-af11-b608930dfe20@l24g2000vby.googlegroups.com>

On 1 Jan., 14:42, Chameleon <cham_...@hotmail.com> wrote:

class A
{
public:
    A();
    A(const char *string);
    ~A();
    A(A &a);

The copy constructor usually takes a reference to a CONST.

    A && operator+(A &a) {
        A b;
        b.str = new char[strlen(this->str) + strlen(a.str) + 1];

This this->str as well as a.str could be null pointers couldn't they?

        strcpy(b.str, this->str);
        strcpy(b.str + strlen(this->str), a.str);
        cout << "A + A = " << b.str << endl;
        return std::move(b);
    }

A&& is a reference type. b is a function-local object. You're
returning a reference to a function-local object. This produces a
dangling reference. Using this dangling reference invokes undefined
behaviour. When in doubt, avoid declaring functions that return rvalue
references. This is almost always the wrong thing to do. Your operator
+ should "return an object by value". Also, if you return a function-
local object you should not use std::move as this inhibits a potential
copy/move elision.

Cheers!
SG