std::move 移动对象资源

场景:

C++ 标准库使用比如vector::push_back 等这类函数时,会对参数的对象进行复制,连数据也会复制.这就会造成对象内存的额外创建, 本来原意是想把参数push_back进去就行了.

C++11 提供了std::move 函数来把左值转换为xrvalue, 而且新版的push_back也支持&&参数的重载版本,这时候就可以高效率的使用内存了.

对指针类型的标准库对象并不需要这么做.

参考:

1. Move Constructors and Move Assignment Operators (C++)
2. std::move

说明:

std::move(t) 用来表明对象t 是可以moved from的,它允许高效的从t资源转换到lvalue上.

注意,标准库对象支持moved from的左值在moved 之后它的对象原值是有效的(可以正常析构),但是是unspecified的,可以理解为空数据,但是这个对象的其他方法返回值不一定是0,比如size().所以,moved from 之后的对象最好还是不要使用吧?(如有不正确理解,请告知)

对本身进行move,并赋值给本身是undefined的行为.

std::vector v = {2, 3, 3};

v = std::move(v); // undefined behavior

std::move 的函数原型.

template<typename _Tp>
  constexpr typename std::remove_reference<_Tp>::type&&
  move(_Tp&& __t) noexcept
  { return static_cast<typename std::remove_reference<_Tp>::type&&>(__t); }

结构体 remove_reference 的原型,就是重载了多个结构体模板来获取原类型 type.

/// remove_reference
template<typename _Tp>
  struct remove_reference
  { typedef _Tp   type; };
template<typename _Tp>
  struct remove_reference<_Tp&>
  { typedef _Tp   type; };
template<typename _Tp>
  struct remove_reference<_Tp&&>
  { typedef _Tp   type; };

例子

以下用两个例子来说明std::move的用法.

例子1

– 原lvalue值被moved from之后值被转移,所以为空字符串.

– 摘录自cppreference

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";
}

输出:

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

例子2

– 自定义自己的类对象支持moved from 操作,需要实现 Move Constructors and Move Assignment Operators

#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";
}
void TestMyObjectWithoutUseMove()
{
   std::vector<MemoryBlock> v;
   MemoryBlock mb1(25);
   // MemoryBlock mb2(75);
   // MemoryBlock mb3(50);
   v.push_back(mb1);
   //v.push_back(mb2);
   //v.insert(v.begin() + 1, mb3);
}
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));
}
int main(int argc, char const *argv[])
{
    //TestSTLObject();
    TestMyObjectWithoutUseMove();
    std::cout << "......................................." << std::endl;
    TestMyObjectWithUseMove();
    return 0;
}

输出:
1. 注意,第一个函数每个对象多调用了拷贝构造函数,多创建了一次,而使用了move操作的只是移动了资源
2. 注意,vector即使 push_back 第二个对象时,会移动第一个对象,很奇怪,如果你把注释去掉的话,会发现资源 Moving 很多次,这是 vector 实现影响了, 这个是因为push_back引发vector长度增长导致其内存重新分配，原来vector中的对象都被移动到新分配的内存上，所以会多次调用move构造函数，如果事先reverse，就不会发生这种情况.比较清楚的看出来 Move 的特性的就是 push_back 一个参数.
3. 注意,g++ 4.8.1 的 vector push_back 多个对象时优化的没 vs 好,vs 是调用 Move 构造器,而 g++ 是调用 Copy 构造器,你会发现拷贝构造函数会调用很多次.

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.

.......................................

In MemoryBlock(size_t). length = 25.

In MemoryBlock(MemoryBlock&&). length = 25. Moving resource.

In ~MemoryBlock(). length = 0.

In ~MemoryBlock(). length = 25. Deleting resource.

**转自 https://blog.csdn.net/infoworld/article/details/50736633**