Re: Implicit move constructor rules in c++0x still badly broken?

Patrik Kahari <>
Wed, 23 Feb 2011 15:37:22 CST

On Feb 21, 5:40 pm, Howard Hinnant <> wrote:

Here's Dave's code, completed and instrumented:

#include <iostream>

class Y
    int state_;
    enum {destructed = -2, moved_from, default_constructed};

    Y() : state_(default_constructed) {}
    Y(const Y& a) : state_(a.state_) {}
    Y& operator=(const Y& a) {state_ = a.state_; return *this;}
    Y(Y&& a) : state_(a.state_)
        a.state_ = moved_from;
    Y& operator=(Y&& a)
        state_ = a.state_;
        a.state_ = moved_from;
        return *this;
        state_ = destructed;
    explicit Y(int s) : state_(s) {}

    operator<<(std::ostream& os, const Y& a)
        switch (a.state_)
        case Y::destructed:
            os << "Y is destructed\n";
        case Y::moved_from:
            os << "Y is moved from\n";
        case Y::default_constructed:
            os << "Y is default constructed\n";
            os << "Y = " << a.state_ << '\n';
        return os;

    friend bool operator==(const Y& x, const Y& y)
        {return x.state_ == y.state_;}
    friend bool operator<(const Y& x, const Y& y)
        {return x.state_ < y.state_;}


class InvariantChecker
    Y* v_;

    InvariantChecker(const InvariantChecker&);
    InvariantChecker& operator=(const InvariantChecker&);
    explicit InvariantChecker() : v_(0) {}
    explicit InvariantChecker(Y& v) : v_(&v) {}
        if (v_)
            std::cout << *v_ << '\n';


f(bool choose)
    Y a(1), b(2);
    InvariantChecker xa(a), xb(b);

    // Incorrect comment follows:

    // The use of ?: guarantees that there will be no copy elision.
    // Instead, either a or b will be *moved* into the return value.
    return choose ? a : b;

    // xb and xa get destroyed here
    // b and a get destroyed thereafter


int main()
    Y y = f(true);


Using a recent build of clang and libc++ this, at least for me

Y = 2

Y = 1


I ran your code on VC10 and got the same result as you. However if I
change the conditional to:

f(bool choose)
    Y a(1), b(2);
    InvariantChecker xa(a), xb(b);

      return a;
      return b;


Then I get the output I expected:

"Y = 2

Y is moved from"

Not sure why that makes a difference. Just goes to show how subtle
changes like this can change the semantics.

Thanks, Patrik

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