表达式左值右值(C++学习)

左值右值是表达式的属性，该属性称为 value category。按该属性分类，每一个表达式属于下列之一：

lvalue	left value，传统意义上的左值
xvalue	expiring value, x值，指通过“右值引用”产生的对象
prvalue	pure rvalue，纯右值，传统意义上的右值(?)

而 xvalue 和其他两个类型分别复合，构成：

lvalue + xvalue = glvalue	general lvalue，泛左值
xvalue + prvalue = rvalue	右值

区分？

++x 与 x++ 假定x的定义为 int x=0;,那么前者是 lvalue，后者是rvalue。前者修改自身值，并返回自身；后者先创建一个临时对像，为其赋值，而后修改x的值，最后返回临时对像。

区分表达式的左右值属性有一个简便方法：若可对表达式用 & 符取址，则为左值，否则为右值。比如

&obj , &*ptr , &ptr[index] , &++x	有效
&1729 , &(x + y) , &std::string("meow"), &x++	无效

对于函数调用，根绝返回值类型不同，可以是lvalue、xvalue、prvalue:

The result of calling a function whose return type is an lvalue reference is an lvalue
The result of calling a function whose return type is an rvalue reference is an xvalue.
The result of calling a function whose return type is not a reference is a prvalue.

const vs non-const

左值和右值表达式都可以是const或non-const。

比如，变量和函数的定义为：

string one("lvalue");
const string two("clvalue");
string three() { return "rvalue"; }
const string four() { return "crvalue"; }

那么表达式：

表达式	分类
one	modifiable lvalue
two	const lvalue
three()	modifiable rvalue
four()	const rvalue

引用

Type&	只能绑定到可修改的左值表达式
const Type&	可以绑定到任何表达式
Type&&	可绑定到可修改的左值或右值表达式
const Type&&	可以绑定到任何表达式

重载函数

#include <iostream>
#include <string>
using namespace std;

string one("lvalue");
const string two("clvalue");
string three() { return "rvalue"; }
const string four() { return "crvalue"; }

void func(string& s)
{
    cout << "func(string& s): " << s << endl;
}

void func(const string& s)
{
    cout << "func(const string& s): " << s << endl;
}

void func(string&& s)
{
    cout << "func(string&& s): " << s << endl;
}

void func(const string&& s)
{
    cout << "func(const string&& s): " << s << endl;
}

int main()
{
    func(one);
    func(two);
    func(three());
    func(four());
    return 0;
}

结果：

func(string& s): lvalue
func(const string& s): clvalue
func(string&& s): rvalue
func(const string&& s): crvalue

如果只保留const string& 和 string&& 两个重载函数，结果为：

func(const string& s): lvalue
func(const string& s): clvalue
func(string&& s): rvalue
func(const string& s): crvalue

右值引用

C++0x第5章的第6段：

Named rvalue references are treated as lvalues and unnamed rvalue references to objects are treated as xvalues; rvalue references to functions are treated as lvalues whether named or not.

具名右值引用被视为左值
无名对对象的右值引用被视为x值
对函数的右值引用无论具名与否都将被视为左值

#include <iostream>
#include <string>

void F1(int&& a)
{
    std::cout<<"F1(int&&) "<<a<<std::endl;
}

void F1(const int& a)
{
    std::cout<<"F1(const int&) "<<a<<std::endl;
}

void F2(int&& a)
{
    F1(a);
}

int main()
{
    int && a=1;
    F2(a);
    F1(a);
    F2(2);
    F1(2);
    return 0;
}

结果

F1(const int&) 1
F1(const int&) 1
F1(const int&) 2
F1(int&&) 2

移动语义

在这之前，如果写一个交换两个值的swap函数：

template <class T> swap(T& a, T& b)
{
    T tmp(a);   // now we have two copies of a
    a = b;      // now we have two copies of b
    b = tmp;    // now we have two copies of tmp (aka a)
}

之后

template <class T> swap(T& a, T& b)
{
    T tmp(std::move(a));
    a = std::move(b);
    b = std::move(tmp);
}

std::move 接受左值或右值参数，并返回一个右值(其所作工作很简单)

template <class T>
typename remove_reference<T>::type&&
move(T&& a)
{
    return a;
}

要是的swap真正发挥作用，需要重载：

class T
{
public:
    T(T&& );
    T& operator = (T&& );
...

模板参数类型

为了对比左值引用和右值引用，一开始误打误撞，写了这样一个函数

template <typename Type> void Swap(Type&& sb1, Type&& sb2)
{
    Type sb(sb1);
    sb1 = sb2;
    sb2 = sb;
}

然后

int main()
{
    int a=1, b=2;
    Swap(a, b);
    std::cout<<a<<" "<<b<<std::endl;
    return 0;
}

结果却是

2 2

不用整数，换用一个自定义的类型试试看：

class A
{
public:
    A() {
        std::cout << "Default constructor." << std::endl;
        m_p = NULL;
    }

    ~A() {
        std::cout << "Destructor." << std::endl;
        delete m_p;
    }

    explicit A(const int n) {
        std::cout << "Unary constructor." << std::endl;
        m_p = new int(n);
    }

    A(const A& other) {
        std::cout << "Copy constructor." << std::endl;
        if (other.m_p) {
            m_p = new int(*other.m_p);
        } else {
            m_p = NULL;
        }
    }

    A(A&& other) {
        std::cout << "Move constructor." << std::endl;
        m_p = other.m_p;
        other.m_p = NULL;
    }

    A& operator=(const A& other) {
        std::cout << "Copy assignment operator." << std::endl;
        if (this != &other) {
            delete m_p;
            if (other.m_p) {
                m_p = new int(*other.m_p);
            } else {
                m_p = NULL;
            }
        }
        return *this;
    }

    A& operator=(A&& other) {
        std::cout << "Move assignment operator." << std::endl;
        if (this != &other) {
            delete m_p;
            m_p = other.m_p;
            other.m_p = NULL;
        }
        return *this;
    }

    int get() const {
        return m_p ? *m_p : 0;
    }

private:
    int * m_p;
};

int main()
{
    A a(1);
    A b(2);
    Swap2(a, b);
    std::cout<<a.get()<<" "<<b.get()<<std::endl;
    return 0;
}

结果

Unary constructor.
Unary constructor.
Copy assignment operator.
Copy assignment operator.
2 2
Destructor.
Destructor.

只出现了两个对象，那么Swap中的临时对象去哪儿了？

C++0x 14.8.2.1

If P is a cv-qualified type, the top level cv-qualifiers of P’s type are ignored for type deduction. If P is a reference type, the type referred to by P is used for type deduction. If P is an rvalue reference to a cv unqualified template parameter and the argument is an lvalue, the type “lvalue reference to A” is used in place of A for type deduction. 

template <class T> int f(T&&);
template <class T> int g(const T&&);
int i;
int n1 = f(i); // calls f<int&>(int&)
int n2 = f(0); // calls f<int>(int&&)
int n3 = g(i); // error: would call g<int>(const int&&), which
// would bind an rvalue reference to an lvalue

也就是前面提到的

template <typename Type> void Swap(Type&& sb1, Type&& sb2)

参数推导后

void Swap<int&>(int& sb1, int& sb1)

参考

http://blog.csdn.net/zwvista/article/details/5459774
http://blog.csdn.net/hikaliv/article/details/4541429
http://topic.csdn.net/u/20090706/16/514af7e1-ad20-4ea3-bdf0-bfe6d34d9814.html
http://www.artima.com/cppsource/rvalue.html