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

Marc <>
Tue, 22 Feb 2011 16:26:08 CST
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_)

std::cout << "Moving out!\n";

       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

Using g++-4.4 with libc++ I get the identical output. To demonstrate
Dave's point I expect to see:

Y is moved from

in the output.

Using the -fno-elide-constructors flag in clang or gcc shows that the
move occurs after the InvariantChecker destructors.

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