Welcome to OStack Knowledge Sharing Community for programmer and developer-Open, Learning and Share
Welcome To Ask or Share your Answers For Others

Categories

0 votes
375 views
in Technique[技术] by (71.8m points)

c++ - Is `new (this) MyClass();` undefined behaviour after directly calling the destructor?

In this question of mine, @DeadMG says that reinitializing a class through the this pointer is undefined behaviour. Is there any mentioning thereof in the standard somewhere?

Example:

#include <iostream>

class X{
  int _i;
public:  
  X() : _i(0) { std::cout << "X()
"; }
  X(int i) : _i(i) { std::cout << "X(int)
"; }

  ~X(){ std::cout << "~X()
"; }

  void foo(){
    this->~X();
    new (this) X(5);
  }

  void print_i(){
    std::cout << _i << "
";
  }
};

int main(){
  X x;
  x.foo();
  // mock random stack noise
  int noise[20];
  x.print_i();
}

Example output at Ideone (I know that UB can also be "seemingly correct behaviour").
Note that I did not call the destructor outside of the class, as to not access an object whose lifetime has ended. Also note, that @DeadMG says that directly calling the destructor is okay as-long-as it's called once for every constructor.

See Question&Answers more detail:os

与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
Welcome To Ask or Share your Answers For Others

1 Answer

0 votes
by (71.8m points)

That would be okay if it didn't conflict with stack unwinding.

You destroy the object, then reconstruct it via the pointer. That's what you would do if you needed to construct and destroy an array of objects that don't have a default constructor.

The problem is this is exception unsafe. What if calling the constructor throws an exception and stack is unwound and the destructor is called for the second time?

{
   X x;
   x.foo(); // here ~X succeeds, then construction fails
} //then the destructor is invoked for the second time.

That aspect specifically would be undefined behavior.


与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
Welcome to OStack Knowledge Sharing Community for programmer and developer-Open, Learning and Share
Click Here to Ask a Question

...