[文档]移动构造函数和移动赋值运算符 (C++)

2020/12 21 12:12
// MemoryBlock.h
#pragma once
#include <iostream>
#include <algorithm>

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;
   }

private:
   size_t _length; // The length of the resource.
   int* _data; // The resource.
};

为 C++ 创建移动构造函数

定义一个空的构造函数方法,该方法采用一个对类类型的右值引用作为参数,如以下示例所示:

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

在移动构造函数中,将源对象中的类数据成员添加到要构造的对象:

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

将源对象的数据成员分配给默认值。 这可以防止析构函数多次释放资源(如内存):

other._data = nullptr;
other._length = 0;

为 C++ 类创建移动赋值运算符

定义一个空的赋值运算符,该运算符采用一个对类类型的右值引用作为参数并返回一个对类类型的引用,如以下示例所示:

MemoryBlock& operator=(MemoryBlock&& other)
{
}

在移动赋值运算符中,如果尝试将对象赋给自身,则添加不执行运算的条件语句。

if (this != &other) { }

在条件语句中,从要将其赋值的对象中释放所有资源(如内存)。

以下示例从要将其赋值的对象中释放 _data 成员: delete[] _data;

执行第一个过程中的步骤 2 和步骤 3 以将数据成员从源对象转移到要构造的对象:

// 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;

返回对当前对象的引用,如以下示例所示: return *this;

示例:完成移动构造函数和赋值运算符

MemoryBlock 类的完整移动构造函数和移动赋值运算符:

// Move constructor.
MemoryBlock(MemoryBlock&& other) noexcept
   : _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) noexcept
{
   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;
}

示例使用移动语义提高性能

以下示例演示移动语义如何能提高应用程序的性能。 此示例将两个元素添加到一个矢量对象,然后在两个现有元素之间插入一个新元素。 vector类使用移动语义,通过移动矢量的元素而不是复制矢量来有效地执行插入操作。

// rvalue-references-move-semantics.cpp
// compile with: /EHsc
#include "MemoryBlock.h"
#include <vector>

using namespace std;

int main()
{
   // Create a vector object and add a few elements to it.
   vector<MemoryBlock> v;
   v.push_back(MemoryBlock(25));
   v.push_back(MemoryBlock(75));

   // Insert a new element into the second position of the vector.
   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(size_t). length = 75.
In MemoryBlock(MemoryBlock&&). length = 75. Moving resource.
In MemoryBlock(MemoryBlock&&). length = 25. Moving resource.
In ~MemoryBlock(). length = 0.
In ~MemoryBlock(). length = 0.
In MemoryBlock(size_t). length = 50.
In MemoryBlock(MemoryBlock&&). length = 50. Moving resource.
In MemoryBlock(MemoryBlock&&). length = 25. Moving resource.
In MemoryBlock(MemoryBlock&&). length = 75. Moving resource.
In ~MemoryBlock(). length = 0.
In ~MemoryBlock(). length = 0.
In ~MemoryBlock(). length = 0.
In ~MemoryBlock(). length = 25. Deleting resource.
In ~MemoryBlock(). length = 50. Deleting resource.
In ~MemoryBlock(). length = 75. Deleting resource.

在 Visual Studio 2010 之前,此示例生成以下输出:
In MemoryBlock(size_t). length = 25.
In MemoryBlock(const MemoryBlock&). length = 25. Copying resource.
In ~MemoryBlock(). length = 25. Deleting resource.
In MemoryBlock(size_t). length = 75.
In MemoryBlock(const MemoryBlock&). length = 25. Copying resource.
In ~MemoryBlock(). length = 25. Deleting resource.
In MemoryBlock(const MemoryBlock&). length = 75. Copying resource.
In ~MemoryBlock(). length = 75. Deleting resource.
In MemoryBlock(size_t). length = 50.
In MemoryBlock(const MemoryBlock&). length = 50. Copying resource.
In MemoryBlock(const MemoryBlock&). length = 50. Copying resource.
In operator=(const MemoryBlock&). length = 75. Copying resource.
In operator=(const MemoryBlock&). length = 50. Copying resource.
In ~MemoryBlock(). length = 50. Deleting resource.
In ~MemoryBlock(). length = 50. Deleting resource.
In ~MemoryBlock(). length = 25. Deleting resource.
In ~MemoryBlock(). length = 50. Deleting resource.
In ~MemoryBlock(). length = 75. Deleting resource.

使用移动语义的此示例版本比不使用移动语义的版本更高效,因为前者执行的复制、内存分配和内存释放操作更少。

std::move函数将 lvalue 转换 other 为右值。

--转载请注明: http://blog.coolcoding.cn/?p=2967