c++11 右值引用和移动语义

vector<int> get_all_scores()
{
	vector<int> vec;
	vec.push_back(1);
	vec.push_back(2);
	return vec;
}
vector<int> vec =get_all_scores();//这里实际上调用了三次构造函数

#include <stdio.h>
#include <string.h>
#include <iostream>
#include <vector>
#include <string>
using std::cout;
using std::endl;
using std::vector;
using std::string;


class String
{
public:
	String()
	: _pstr(new char[1]())
	{}

	String(const char * pstr)
	: _pstr(new char[strlen(pstr) + 1]())
	{
		cout << "String(const char *)" << endl;
		strcpy(_pstr, pstr);
	}

	//复制构造函数
	String(const String & rhs)
	: _pstr(new char[strlen(rhs._pstr) + 1]())
	{
		cout << "String(const String & rhs)" << endl;
		strcpy(_pstr, rhs._pstr);
	}
　　　　　//如果传递的是右值，而复制构造函数和移动构造函数同时存在，此时移动构造函数优先执行。
	//移动构造函数 C++11	
	String(String && rhs)
	: _pstr(rhs._pstr)
	{
		cout << "String(String && rhs)" << endl;
		rhs._pstr = NULL;
	}
	//移动赋值运算符函数
	String & operator=(String && rhs)
	{
		cout << "String & operator=(String && )" << endl;
		if(this != &rhs)
		{
			delete [] _pstr;
			_pstr = rhs._pstr;
			rhs._pstr = NULL;
		}
		return *this;
	}

	//赋值运算符函数
	String & operator=(const String & rhs)
	{
		cout << "String & operator=(const String&)" << endl;
		if(this != &rhs)
		{
			delete [] _pstr;
			_pstr = new char[strlen(rhs._pstr) + 1]();
			strcpy(_pstr, rhs._pstr);
		}
		return *this;
	}


	~String()
	{
		delete [] _pstr;
		cout << "~String()" << endl;
	}

	const char * c_str() const
	{	return _pstr;	}

	friend std::ostream & operator<<(std::ostream & os, const String & rhs);
	
private:
	char * _pstr;
};
std::ostream & operator<<(std::ostream & os, const String & rhs)
{
	os << rhs._pstr;
	return os;
}

 
int test0(void)
{
	//vector<String> vec;
	//vec.push_back("hello,world");

	String s1("hello,world");
	cout << "s1 = " << s1 << endl;

	s1 = String("shenzhen");
	cout << "s1 = " << s1 << endl;
	printf("s1‘s address = %p\n", s1.c_str());
	cout << endl;

	String s2("wangdao");
	cout << "s2 = " << s2 << endl;
	s2 = std::move(s1);//显式的将一个左值转换成右值来使用
	cout << "s2 = " << s2 << endl;
	printf("s2‘s address = %p\n", s2.c_str());

	cout << "s1 = " << s1 << endl;
	
	cout << "......" << endl;

	return 0;
}


void test1(void)
{
	int a = 1;
	int b = 2;
	&a;
	&b;
	//&(a+b);// error, 右值
	//&(a++);// error, 右值
	&(++a);
	int * pFlag = &a;
	&pFlag;
	&(*pFlag);

	//&100;//error,字面值，右值
	//&string("hello");//error, 右值，匿名对象，
	string s1("hello");
	string s2("world");
	//&(s1 + s2);//error, 右值
	
	const int & m = 1;
	&m;//左值

	int && n = 1;//右值引用绑定到右值
	&n;

	//int && x = a;//error 右值引用无法绑定到左值
}


void test2(void)
{
	//const引用不仅可以绑定到左值，也可以绑定到右值，
	//const引用无法区分出传递过来的参数到底是左值还是右值
	//
	//C++11引入右值引用，解决该问题
	//
	String && ref1 = String("hello,world");	
	String s1("hello");
	cout << ref1 << endl;

	const String & ref2 = s1;
	cout << ref2 << endl;
}

int main(void)
{
	test0();
	//test1();
	//test2();
	return 0;
}