std::move example

Guide

copy constructors and copy assignment operators

MemoryBlock(const MemoryBlock& other)
MemoryBlock& operator=(const MemoryBlock& other)

move constructors and move assignment operators

MemoryBlock(MemoryBlock&& other)  
MemoryBlock& operator=(MemoryBlock&& other)  

example_move

#include <iostream>
#include <stdio.h>

#include <utility>
#include <vector>
#include <string>

class MemoryBlock
{
public:

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

   // Destructor.
   ~MemoryBlock()
   {
      std::cout << "In ~MemoryBlock(). length = "
                << _length << ".";

      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])
   {
      std::cout << "In MemoryBlock(const MemoryBlock&). length = "
                << other._length << ". Copying resource." << std::endl;

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

   // Copy assignment operator.
   MemoryBlock& operator=(const MemoryBlock& other)
   {
      std::cout << "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 constructor.
    MemoryBlock(MemoryBlock&& other)
       : _data(nullptr)
       , _length(0)
    {
       std::cout << "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)
    {
       std::cout << "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.
};

void TestSTLObject()
{
    std::string str = "Hello";
    std::vector<std::string> v;

    // uses the push_back(const T&) overload, which means
    // we'll incur the cost of copying str
    v.push_back(str);
    std::cout << "After copy, str is \"" << str << "\"\n";

    // uses the rvalue reference push_back(T&&) overload,
    // which means no strings will be copied; instead, the contents
    // of str will be moved into the vector.  This is less
    // expensive, but also means str might now be empty.
    v.push_back(std::move(str));
    std::cout << "After move, str is \"" << str << "\"\n";

    std::cout << "The contents of the vector are \"" << v[0]
                                         << "\", \"" << v[1] << "\"\n";

    /*
    After copy, str is "Hello"
    After move, str is ""
    The contents of the vector are "Hello", "Hello"
    */
}

void TestMyObjectWithoutUseMove()
{
   std::vector<MemoryBlock> v;
   MemoryBlock mb1(25);
   //MemoryBlock mb2(75);
   //MemoryBlock mb3(50);

   v.push_back(mb1);
   //v.push_back(mb2);
   //v.push_back(mb3);

   /*
    In MemoryBlock(size_t). length = 25.
    In MemoryBlock(const MemoryBlock&). length = 25. Copying resource.
    In ~MemoryBlock(). length = 25. Deleting resource.
    In ~MemoryBlock(). length = 25. Deleting resource.
   */
}

void TestMyObjectWithUseMove()
{
   std::vector<MemoryBlock> v;

   MemoryBlock mb1(25);
   // MemoryBlock mb2(75);
   // MemoryBlock mb3(50);

   v.push_back(std::move(mb1));
   //v.push_back(MemoryBlock(75));
   //v.insert(v.begin() + 1, MemoryBlock(50));

   /*
    In MemoryBlock(size_t). length = 25.
    In MemoryBlock(MemoryBlock&&). length = 25. Moving resource.
    In ~MemoryBlock(). length = 0.
    In ~MemoryBlock(). length = 25. Deleting resource.
   */
}

int main(int argc, char const *argv[])
{
    //TestSTLObject();
    //TestMyObjectWithoutUseMove();
    TestMyObjectWithUseMove();
    return 0;
}

rewrite move constructor

// Move constructor.  
MemoryBlock(MemoryBlock&& other)  
   : _data(nullptr)  
   , _length(0)  
{  
   *this = std::move(other);  
}  

Reference

• move
• Move Constructors and Move Assignment Operators (C++)
• move example

History

• 20180201: created.