Operator overload which permits capturing with rvalue but not assigning to

Question

Is it possible to design and how should I make overloaded operator+ for my class C to have this possible:

C&& c = c1 + c2;

but this not possible:

c1 + c2 = something;

Edit: I changed objects to small letters. c1, c2 and c are objects of class C. && is not the logical operator&&, but rather an rvalue reference.

For example writing:

double&& d = 1.0 + 2.0;

is 100% proper (new) C++ code, while

1.0 + 2.0 = 4.0;

is obviously a compiler error. I want exactly the same, but instead for double, for my class C.

Second edit: If my operator returns C or C&, I can have assignment to rvalue reference, but also assignment to c1 + c2, which is senseless. Giving const here disables it, however it disables assignment to rvalue too. At least on VC++ 2k10. So how double does this?

Please explain the semantics of your code samples (the intended results). — Emile Cormier
– Emile Cormier, Commented May 4, 2011 at 22:14
Wouldn't double&& d = 1. + 2.; create a dangling reference? Also, can't you just return by value? — Dennis Zickefoose
– Dennis Zickefoose, Commented May 4, 2011 at 22:30
As I said, if I return by value, then c1 + c2 = something will be possible which is not what I want. — Argbart
– Argbart, Commented May 4, 2011 at 22:33
@Fred: I was unaware they had extended the rule to include r-value references, but I suppose it makes sence that they would. — Dennis Zickefoose
– Dennis Zickefoose, Commented May 4, 2011 at 22:46

fredoverflow · Accepted Answer · 2011-05-04 22:34:00Z

14

Have the assignment operator be callable on lvalues only:

class C
{
    // ...

public:

    C& operator=(const C&) & = default;
};

Note the single ampersand after the closing parenthesis. It prevents assigning to rvalues.

answered May 4, 2011 at 22:34

fredoverflow

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7 Comments

Argbart Over a year ago

Thanks, it would be cool. However my VC++ 2k10 does not accept ampersand here. As I assume, it should and some day it will (or his never version ;)) Is there any way to do that in current VC++ 2k10?

Puppy Over a year ago

@Argbart: No. The language features do exist to enable this, but not within Visual Studio 2010. Remember that VS's implementation of rvalue references is relatively old and is only updated if there is a breaking problem. In future versions of the Standard, you can overload member functions depending on whether or not the class is an lvalue or rvalue.

Argbart Over a year ago

@DeadMG: Wow, that's cool, I didn't know about that! Thanks also to you, I'd also like to mark your comment as an answer and give you some points, but that's apparently not possible. Anyway, FredOverflow proposal was absolutely proper even if not useful to me. I am marking this as answer.

Argbart Over a year ago

@DeadMG: one more quick question. Does creating constructor which takes C&& as argument makes any sense? Isn't that needless because of return value optimization?

aschepler Over a year ago

@Argbart: Upvoted @DeadMG's comment for you. A constructor C(C&&); is a move constructor and does make sense, especially if it can be more efficient than the copy constructor by "stealing" things from the argument which is about to be destroyed.

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