Re: [eigen] Ref<> Problem

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 04.12.2014 10:41, Gabriel wrote:
> struct A{
>          // constructor does not generate a temporary:
>          template<class Derived>
>          A(const Eigen::MatrixBase<Derived> & in) : r(in) {}
>          // initializing r might create a temporary but that is stored in r
>
>
>          A  & operator=(const A & other)
>          {
>              new (&r) Eigen::Ref<const Eigen::MatrixXd>(other.r);
>
>              return *this;
>          }
>
>          Eigen::Ref<const Eigen::MatrixXd> r;
>      };
>
> Is that correct, it seems to work :-)?

That works for some parts, but there are still some pitfalls:

int main() {
  Eigen::MatrixXd M = Eigen::MatrixXd::Identity(3,3);

  A A1(3*M);   // creates temporary in A1
  {
    A A2(2*M); // creates temporary in A2
    A1 = A2;   // temporary in A1 is not destructed! --> LEAK
  } // temporary in A2 gets destructed, invalid reference in A1!

  std::cout << A1.r << std::endl; // PROBLEM!
}

So generally, like C++ references, Ref should be considered as 
initialize-once variables. And, like in C++, strange things can happen, 
if the variable by which it got initialized runs out of scope before the 
reference does.

Analogous example with C++ references:

struct B{
  const double& r;
  B(const double& r_) : r(r_) {}
};

B foo() {
  double x = 3.0;
  return B(x);
  // referenced x value will be invalid as soon as foo() exits
}

int main() {
  double y = 1.0;
  B b1(2*y);  // temporary created during construction but gets
              // out of scope before b is usable
  B b2 = foo(); // x in foo() is out of scope now

  std::cout << b1.r << "\n" << b2.r << std::endl;
}



Christoph

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Dipl.-Inf., Dipl.-Math. Christoph Hertzberg
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