Visual c move semantics

This topic describes how to write a move constructor and a move assignment operator for a C++ class. A move constructor enables you to implement move semantics, which can significantly improve the performance of your applications. For more information about move semantics, see .

This topic builds upon the following C++ class, MemoryBlock, which manages a memory buffer.

// MemoryBlock.h #pragma once #include #include 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 != NULL) { 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; } private: size_t _length; // The length of the resource. int* _data; // The resource. };

The following procedures describe how to write a move constructor and a move assignment operator for the example C++ class.

To create a move constructor for a C++ class

1. Define an empty constructor method that takes an rvalue reference to the class type as its parameter, as demonstrated in the following example:

   MemoryBlock(MemoryBlock&& other) : _data(NULL) , _length(0) { }

2. In the move constructor, assign the class data members from the source object to the object that is being constructed:

   _data = other._data; _length = other._length;

3. Assign the data members of the source object to default values. This prevents the destructor from freeing resources (such as memory) multiple times:

   other._data = NULL; other._length = 0;