C++11新特性之 Move semantics(移动语义)

按值传递的意义是什么？
当一个函数的参数按值传递时，这就会进行拷贝。当然，编译器懂得如何去拷贝。
而对于我们自定义的类型，我们也许需要提供拷贝构造函数。

但是不得不说，拷贝的代价是昂贵的。

所以我们需要寻找一个避免不必要拷贝的方法，即C++11提供的移动语义。
上一篇博客中有一个句话用到了：

#include <iostream>

void f(int& i) { std::cout << "lvalue ref: " << i << "\n"; }
void f(int&& i) { std::cout << "rvalue ref: " << i << "\n"; }

int main()
{
    int i = 77;
    f(i);    // lvalue ref called
    f(99);   // rvalue ref called

    f(std::move(i));  // 稍后介绍

    return 0;
}

实际上，右值引用注意用于创建移动构造函数和移动赋值运算。

移动构造函数类似于拷贝构造函数，把类的实例对象作为参数，并创建一个新的实例对象。
但是 移动构造函数可以避免内存的重新分配，因为我们知道右值引用提供了一个暂时的对象，而不是进行copy，所以我们可以进行移动。

换言之，在设计到关于临时对象时，右值引用和移动语义允许我们避免不必要的拷贝。我们不想拷贝将要消失的临时对象，所以这个临时对象的资源可以被我们用作于其他的对象。

右值就是典型的临时变量，并且他们可以被修改。如果我们知道一个函数的参数是一个右值，我们可以把它当做一个临时存储。这就意味着我们要移动而不是拷贝右值参数的内容。这就会节省很多的空间。

说多无语，看代码：

#include <iostream>
#include <algorithm>

class A
{
public:

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

    // Destructor.
    ~A()
    {
    std::cout << "~A(). length = " << mLength << ".";

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

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

    // Copy constructor.
    A(const A& other)
        : mLength(other.mLength), mData(new int[other.mLength])
    {
    std::cout << "A(const A&). length = "
        << other.mLength << ". Copying resource." << std::endl;

    std::copy(other.mData, other.mData + mLength, mData);
    }

    // Copy assignment operator.
    A& operator=(const A& other)
    {
    std::cout << "operator=(const A&). length = "
             << other.mLength << ". Copying resource." << std::endl;

    if (this != &other) {
        delete[] mData;  // Free the existing resource.
        mLength = other.mLength;
            mData = new int[mLength];
            std::copy(other.mData, other.mData + mLength, mData);
    }
    return *this;
    }

    // Move constructor.
    A(A&& other) : mData(NULL), mLength(0)
    {
        std::cout << "A(A&&). length = " 
             << other.mLength << ". Moving resource.\n";

        // Copy the data pointer and its length from the 
        // source object.
        mData = other.mData;
        mLength = other.mLength;

        // Release the data pointer from the source object so that
        // the destructor does not free the memory multiple times.
        other.mData = NULL;
        other.mLength = 0;
    }

    // Move assignment operator.
    A& operator=(A&& other)
    {
        std::cout << "operator=(A&&). length = " 
             << other.mLength << "." << std::endl;

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

          // Copy the data pointer and its length from the 
          // source object.
          mData = other.mData;
          mLength = other.mLength;

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

    // Retrieves the length of the data resource.
    size_t Length() const
    {
        return mLength;
    }

private:
    size_t mLength; // The length of the resource.
    int* mData;     // The resource.
};

移动构造函数
语法：

A(A&& other) noexcept    // C++11 - specifying non-exception throwing functions
{
  mData =  other.mData;  // shallow copy or referential copy
  other.mData = nullptr;
}

最主要的是没有用到新的资源，是移动而不是拷贝。
假设一个地址指向了一个有一百万个int元素的数组，使用move构造函数，我们没有创造什么，所以代价很低。

// Move constructor.
A(A&& other) : mData(NULL), mLength(0)
{
    // Copy the data pointer and its length from the 
    // source object.
    mData = other.mData;
    mLength = other.mLength;

    // Release the data pointer from the source object so that
    // the destructor does not free the memory multiple times.
    other.mData = NULL;
    other.mLength = 0;
}

移动比拷贝更快！！！

移动赋值运算符
语法：

A& operator=(A&& other) noexcept
{
  mData =  other.mData;
  other.mData = nullptr;
  return *this;
}

工作流程这样的：Google上这么说的：

Release any resources that *this currently owns.
Pilfer other’s resource.
Set other to a default state.
Return *this.

// Move assignment operator.
A& operator=(A&& other)
{
    std::cout << "operator=(A&&). length = " 
             << other.mLength << "." << std::endl;

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

      // Copy the data pointer and its length from the 
      // source object.
      mData = other.mData;
      mLength = other.mLength;

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

让我们看几个move带来的好处吧！
vector众所周知，C++11后对vector也进行了一些优化。例如vector::push_back()被定义为了两种版本的重载，一个是cosnt T&左值作为参数，一个是T&&右值作为参数。例如下面的代码：

std::vector<A> v;
v.push_back(A(25));
v.push_back(A(75));

上面两个push_back()都会调用push_back(T&&)版本，因为他们的参数为右值。这样提高了效率。

而 当参数为左值的时候，会调用push_back(const T&) 。

#include <vector>

int main()
{
    std::vector<A> v;
    A aObj(25);       // lvalue
    v.push_back(aObj);  // push_back(const T&)
}

但事实我们可以使用 static_cast进行强制：

// calls push_back(T&&)
v.push_back(static_cast<A&&>(aObj));

我们可以使用std::move完成上面的任务：

v.push_back(std::move(aObj));  //calls push_back(T&&)

似乎push_back(T&&)永远是最佳选择，但是一定要记住：
push_back(T&&) 使得参数为空。如果我们想要保留参数的值，我们这个时候需要使用拷贝，而不是移动。

最后写一个例子，看看如何使用move来交换两个对象：

#include <iostream>
using namespace std;

class A
{
  public:
    // constructor
    explicit A(size_t length)
        : mLength(length), mData(new int[length]) {}

    // move constructor
    A(A&& other)
    {
      mData = other.mData;
      mLength = other.mLength;
      other.mData = nullptr;
      other.mLength = 0;
    }

    // move assignment
    A& operator=(A&& other) noexcept
    {
      mData =  other.mData;
      mLength = other.mLength;
      other.mData = nullptr;
      other.mLength = 0;
      return *this;
    }

    size_t getLength() { return mLength; }


    void swap(A& other)
    {
      A temp = move(other);
      other = move(*this);
      *this = move(temp);
    }

    int* get_mData() { return mData; }

  private:
    int *mData;
    size_t mLength;
};

int main()
{
  A a(11), b(22);
  cout << a.getLength() << ‘ ‘ << b.getLength() << endl;
  cout << a.get_mData() << ‘ ‘ << b.get_mData() << endl;
  swap(a,b);
  cout << a.getLength() << ‘ ‘ << b.getLength() << endl;
  cout << a.get_mData() << ‘ ‘ << b.get_mData() << endl;
  return 0;
}