C++11新特性之 Move semantics(移动语义)
#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。
#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)
{
cout<<"右值引用"<<endl;
mData = other.mData;
mLength = other.mLength;
other.mData = nullptr;
other.mLength = 0;
}
// move assignment
A& operator=(A&& other) noexcept
{
cout<<"move"<<endl;
mData = other.mData;
mLength = other.mLength;
other.mData = nullptr;
other.mLength = 0;
return *this;
}
size_t getLength() { return mLength; }
void swap(A& other)
{
cout<<other.getLength()<<endl;//11
A temp = move(other);
cout<<other.getLength()<<endl;//0
cout<<temp.getLength()<<endl;//11
cout<<this->getLength()<<endl;//22
other = move(*this);
cout<<other.getLength()<<endl;//22
cout<<this->getLength()<<endl;//0
cout<<"first"<<endl;
cout<<temp.getLength()<<endl;//11
*this = move(temp);
cout<<temp.getLength()<<endl;//0
cout<<this->getLength()<<endl;//11
cout<<"second"<<endl;
}
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;
a.swap(b);
cout << a.getLength() << ' ' << b.getLength() << endl;
cout << a.get_mData() << ' ' << b.get_mData() << endl;
return 0;
}
运行结果:
11 22
0xb11420 0xb11790
鍙冲€煎紩鐢
move
move
22 11
0xb11790 0xb11420
11
鍙冲€煎紩鐢
0
11
22
move
22
0
first
11
move
0
11
second
11 22
0xb11420 0xb11790
自己在使用过程中发现,新变量的地址和旧变量是不一致的。 move实际上是将旧内存复制到了新内存,然后在旧内存的地方开头写入0x00(字符串是这样的,简单int不受影响),使其失效。
所以move之前一定要确认之前的东西不用了,保证他被销毁了没人再使用了。
下面这份测试代码中,有把数据从一个map move到另一个map的操作,并且之后还访问之前的map,这是不安全的,最好move完把之前的销毁。
#include <iostream>
#include <string>
#include <map>
#include <utility>
#include <vector>
#include <stdio.h>
using namespace std;
// check if stuct have move constructor
struct StructMove {
StructMove(){};
StructMove(int n): _n(n) { cout<<"construct "<<n<<endl;};
int _n;
};
class ClassMove {
public:
ClassMove():_n(0){cout<<"default construct"<<endl;};
ClassMove(int n):_n(n){cout<<"construct"<<endl;};
// Copy assignment operator is implicitly deleted because 'ClassMove' has a user-declared move constructor
// 当你自定义了copy construct的时候,记得同时写一下移动赋值运算符的重载
ClassMove(const ClassMove& classMove) {_n=classMove._n; cout<<"copy construct"<<endl;}
// ClassMove(const ClassMove&& classMove) {_n=classMove._n; cout<<"move construct"<<endl;}
// Move constructor.
ClassMove(ClassMove&& other) noexcept { *this = std::move(other); }
//移动赋值运算符重载
ClassMove& operator = (ClassMove&& other) {
this->_n = other._n;
return *this;
}
int _n;
};
int main() {
cout << "structMove" << endl;
map<int, StructMove> m1;
map<int, StructMove> m2;
m1[0] = StructMove(10);
printf("%x\n", &m1[0]);
printf("%x\n", &(m1[0]._n));
m2[0] = std::move(m1[0]);
cout << m2[0]._n << endl;
printf("%x\n", &m2[0]);
printf("%x\n", &(m2[0]._n));
cout << m1[0]._n << endl;
cout << m2[0]._n << endl;
cout << "end structMove" << endl;
cout << endl;
cout << "start ClassMove" << endl;
map<int, ClassMove> mc1;
map<int, ClassMove> mc2;
mc1[0] = ClassMove(11);
printf("%x\n", &(mc1[0]._n));
mc2[1] = std::move(mc1[0]);
cout << mc1[0]._n << endl;
cout << mc2[1]._n << endl;
printf("%x\n", &(mc1[0]._n));
printf("%x\n", &(mc2[1]._n));
cout << "end ClassMove" << endl;
cout << endl;
cout << "test base string" << endl;
std::string str = "Hello";
printf("%x\n", &(str[0]));
printf("%x\n", &(str));
std::vector<std::string> v;
//调用常规的拷贝构造函数,新建字符数组,拷贝数据
v.push_back(str);
//调用移动构造函数,掏空str,掏空后,最好不要使用str
v.push_back(std::move(str));
cout << "*" << v[0] << "*" << v[1] << "*" << str << "*" << endl;
printf("%x\n", &(v[0]));
printf("%x\n", &(v[0][0]));
printf("%x\n", &(v[1]));
printf("%x\n", &(v[1][0]));
cout << "end test base string" << endl;
cout << endl;
map<int, string> ms1;
ms1[1] = "abc";
printf("%x\n", &(ms1[1]));
map<int, string> ms2;
ms2[2] = std::move(ms1[1]);
cout << ms1[1] << endl;
cout << ms2[2] << endl;
printf("%x\n", &(ms2[2]));
return 0;
}