C++ Move constructor for class containing vector

Question

I have written a move constructor for a class in the following way:

class A
{

    std::vector<double> m;

    A(A&& other)
        : m{other.m}
    {
    }
}

Is this the correct way to move other.m to m?

Should I be doing this instead?

A(A&& other)
    : m{std::move(other.m)}
{
}

Or perhaps I should be doing something else entirely?

Answer 1

/******************************************************************************
Below program demonstrates how to use move constructor and move assignment operator

*******************************************************************************/

#include <iostream>
#include <algorithm>
#include <vector>

class MemoryBlock
{
public:

 MemoryBlock()
   {
       this->id++;
      std::cout << "Default Constructor"<<std::endl;
   }

   // Simple constructor that initializes the resource.
   explicit MemoryBlock(size_t length)
      : _length(length)
      , _data(new int[length])
   {
       this->id++;
      std::cout << "Constructor In MemoryBlock(size_t). length = and id =" 
                << _length << "." <<id<< std::endl;
   }

   // Destructor.
   ~MemoryBlock()
   {
       this->id--;
      std::cout << "Destructor In ~MemoryBlock(). length = and id ="
                << _length << "."<<id;

      if (_data != nullptr)
      {
         std::cout << " Deleting resource.";
         // Delete the resource.
         delete[] _data;
      }

      std::cout << std::endl;
   }

   // Copy constructor.
   MemoryBlock(const MemoryBlock& other)
      : _length(other._length)
      , _data(new int[other._length])
   {
       this->id++;
      std::cout << " Copy Constructor MemoryBlock(const MemoryBlock&). length = and id ="
                << other._length << "." <<id<<"Copying resource." << std::endl;

      std::copy(other._data, other._data + _length, _data);
   }

   // Copy assignment operator.
   MemoryBlock& operator=(const MemoryBlock& other)
   {
      std::cout << "Assignment operator In operator=(const MemoryBlock&). length = "
                << other._length << ". Copying resource." << std::endl;

      if (this != &other)
      {
         // Free the existing resource.
         delete[] _data;

         _length = other._length;
         _data = new int[_length];
         std::copy(other._data, other._data + _length, _data);
      }
      return *this;
   }

   // Retrieves the length of the data resource.
   size_t Length() const
   {
      return _length;
   }
   
  //Move copy constructor 
  MemoryBlock(MemoryBlock&& other) noexcept
   : _data(nullptr)
   , _length(0)
{
   std::cout << "Move Constructor In MemoryBlock(MemoryBlock&&). length = "
             << other._length << ". Moving resource." << std::endl;

   // Copy the data pointer and its length from the
   // source object.
   _data = other._data;
   _length = other._length;

   // Release the data pointer from the source object so that
   // the destructor does not free the memory multiple times.
   other._data = nullptr;
   other._length = 0;
}

// Move assignment operator.
MemoryBlock& operator=(MemoryBlock&& other) noexcept
{
   std::cout << "Move assignment operator In operator=(MemoryBlock&&). length = "
             << other._length << "." << std::endl;

   if (this != &other)
   {
      // Free the existing resource.
      delete[] _data;

      // Copy the data pointer and its length from the
      // source object. 
      _data = other._data;
      _length = other._length;

      // Release the data pointer from the source object so that
      // the destructor does not free the memory multiple times.
      other._data = nullptr;
      other._length = 0;
   }
   return *this;
}

private:
   size_t _length; // The length of the resource.
   int* _data; // The resource.
   static int id;
};
int MemoryBlock::id=0;
int main()
{
    std::vector<MemoryBlock> v1;
    MemoryBlock m1(100);
    MemoryBlock m2(100);
    MemoryBlock m3(100);
    v1.push_back(m1);
    v1.push_back(m2);
    v1.push_back(m3);
    return 0;
}

Answer 2

The second snippet is the correct way to move other.m since it's a lvalue that needs to be turned into r-value-reference for std::vector move constructor to kick in.

even though, in this very specific example, it will be enough to simply write

A(A&& rhs) = default;

the compiler will generate a constructor that moves each member of rhs to the corresponing member of *this.

p.s. you also probably meant to make the constructor public.