Thread: Trouble with move semantics...

Hey guys, I have some code and I'm having trouble understanding why my default constructor call isn't being treated as an rvalue reference. The STL accepts it properly so it must be something with my implementation, I'm just having trouble seeing where...

Code:

template<class T>
class Element {
private:
  char buffer[sizeof(T)];
  bool alive_;


public:
  Element(void) {
    std::cout << "calling Element default constructor" << std::endl;
    alive_ = false;
    memset(&buffer, 0, sizeof(T));
  }


  Element(const T &t) {
    std::cout << "calling Element(T) copy constructor" << std::endl;
    new(buffer) T(t);
    alive_ = true;
  }


  Element(T &&t) {
    std::cout << "calling Element(T) move constructor" << std::endl;
    new(buffer) T(std::move(t));
    alive_ = true;
  }


  ~Element(void) {
    std::cout << "calling Element destructor" << std::endl;
    if (alive_) {
      T *tmp = (T* ) buffer;
      tmp->~T();
      alive_ = false;
    }
  }


  T& operator*(void) {
    T *tmp = reinterpret_cast<T*>(buffer);
    return *tmp;
  }
};

with test code:

Code:

struct MyStructure {
  int *data;
  int size;


  MyStructure(void) {
    std::cout << "calling MyStructure default constructor" << std::endl;
    size = 256;
    data = new int[size];
  }


  MyStructure(const MyStructure &other) {
    std::cout << "calling MyStructure copy constructor" << std::endl;
    size = other.size;
    data = new int[size];
  }


  MyStructure(MyStructure &&other) : data(other.data), size(other.size) {
    std::cout << "calling MyStructure move constructor" << std::endl;
    other.data = nullptr;
    other.size = 0;
  }


  ~MyStructure(void) {
    size = 0;
    delete[] data;
  }


  friend std::ostream& operator<<(std::ostream &os, MyStructure &a);
};


std::ostream& operator<<(std::ostream &os, MyStructure &a) {
  os << a.size;
  return os;
}


MyStructure test(void) {
  MyStructure tmp;
  return tmp;
}


int main(void) {
  MyStructure a;
  Element<MyStructure> el2(a);
  std::cout << *el2 << std::endl;


  MyStructure &&rvalue = MyStructure();
  Element<MyStructure> el3(std::move(rvalue));
  std::cout << *el3 << std::endl;


  Element<MyStructure> el4(std::move(MyStructure()));
  std::cout << *el4 << std::endl;

  // this is the problem one!
  Element<MyStructure> el5(MyStructure());
  std::cout << *el5 << std::endl;


  std::vector<MyStructure> x;
  x.push_back(MyStructure());
  std::cout << x[0].size << std::endl;


  return 0;
}

I've been getting this compilation error which leads me to believe that el5 is actually receiving a function pointer instead of the return from the default constructor call. But it works for std::vector!

Code:

warning: the address of ‘Element<MyStructure> el5(MyStructure (*)())’ will always evaluate as ‘true’ [-Waddress]
   std::cout << *el5 << std::endl;

Huh, I get the same thing. You can't move construct a default construction, can you? Does this mean I need the move assignment operator?

Holy poop, that works! Or at least, it works for the int example so I trust it.

Thank you so much, Elysia.