int i = 42;
int &r = i;     //r引用i
int &r2 = i*2;    //錯誤，i*2是一個右值
int &&rr = i；    //錯誤，不能將一個右值引用綁定到一個左值上
int &&rr2 = i*2;   //正確，將rr2綁定到一個乘法結(jié)果上
const int &r3 = i*2; //正確，將一個常量引用綁定到一個右值上

int &&rr3 = std::move(rr2);

#include <iostream> 
#include <algorithm> 

class MemoryBlock 
{ 
public: 
  // 構(gòu)造函數(shù)
  explicit MemoryBlock(size_t length) : _length(length) , _data(new int[length]) {} 

  // 析構(gòu)函數(shù) 
  ~MemoryBlock() 
  {
    if (_data != nullptr)  delete[] _data;
  } 

  // 拷貝賦值運算符 
  MemoryBlock& operator=(const MemoryBlock& other) 
  { 
    if (this != &other) 
    { 
      delete[] _data; 
      _length = other._length; 
      _data = new int[_length]; 
      std::copy(other._data, other._data + _length, _data); 
    } 
    return *this; 
  } 

  // 拷貝構(gòu)造函數(shù) 
  MemoryBlock(const MemoryBlock& other) 
    : _length(0) 
    , _data(nullptr) 
  { 
    *this = other; 
  } 

  // 移動賦值運算符，通知標(biāo)準(zhǔn)庫該構(gòu)造函數(shù)不拋出任何異常(如果拋出異常會怎么樣？)
  MemoryBlock& operator=(MemoryBlock&& other) noexcept
  {
    if (this != &other) 
    {  
      delete[] _data; 
      // 移動資源
      _data = other._data; 
      _length = other._length; 
      // 使移后源對象處于可銷毀狀態(tài)
      other._data = nullptr; 
      other._length = 0; 
    } 
    return *this; 
  }

  // 移動構(gòu)造函數(shù)
  MemoryBlock(MemoryBlock&& other) noexcept
    _data(nullptr) 
    , _length(0) 
  { 
    *this = std::move(other); 
  } 

  size_t Length() const 
  { 
    return _length; 
  } 

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

class Foo{
public:
  Foo() = default;
  Foo(const Foo&);
  // 為定義移動構(gòu)造函數(shù)
}；
Foo x;
Foo y(x);         //調(diào)用拷貝構(gòu)造函數(shù)
Foo z(std::move(x));   //調(diào)用拷貝構(gòu)造函數(shù)，因為未定義移動構(gòu)造函數(shù)