CODEWITHSUNDEEP
c++c++11move-semantics
thehouse
thehouse

Reputation: 8074

Move-assignment and reference member

Copy-assignment for a class with a reference member variable is a no-no because you can't reassign the reference. But what about move-assignment? I tried simply moveing it but, of course, that destroys the source object when I just want to move the reference itself:

class C
{
public:
    C(X& x) : x_(x) {}
    C(C&& other) : x_(std::move(other.x_)) {}
    C& operator=(C&& other)
    {
        x_ = std::move(other.x_);
    }
private:
    X& x_;
};

X y;
C c1(y);

X z;
C c2(z);

c2 = c1; // destroys y as well as z

Should I just not be implementing move-assignment and sticking with move-construction only? That makes swap(C&, C&) hard to implement.

Upvotes: 30

Views: 12705

Answers (2)

Yakk - Adam Nevraumont
Yakk - Adam Nevraumont

Reputation: 275310

A reference is, in some sense, a T *const with syntactic sugar on top to auto-derefence, auto-capture, and auto-lifetime extend temporaries. (note that this is not quite true, but often is in practice and practicality)

If you want a reseatable reference, C++ has those: they are called pointers. You can use an accessor to replace dereferencing with a function call if you like. The remaining feature (temporary lifetime extension) that is hard to emulate does not apply to struct members.

Upvotes: 2

Luis Machuca
Luis Machuca

Reputation: 1057

(Posted as an answer from comment as suggested by the OP)

In general if one wants to do non-trivial stuff with references in C++, one would be using reference_wrapper<T>, which is essentially a fancy value-semantic stand-in for a T&, then one would be done with it - it already provides (re)assignment and other operations. I'm sure that would make move constructor and assignment near-trivial, if not trivial (note not trivial as in per the is_trivially_* semantics).

"Reference wrapper" is added to C++03 as part of TR1, and is part of C++11.

Documentation: http://en.cppreference.com/w/cpp/utility/functional/reference_wrapper

Upvotes: 31

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