std:move基本用法和理解

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<int> v = {2, 3, 3};
v = std::move(v); // undefined behavior

std::move 的函数原型.

/**
*  @brief  Convert a value to an rvalue.
*  @param  __t  A thing of arbitrary type.
*  @return The parameter cast to an rvalue-reference to allow moving it.
*/
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,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.