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
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http://www.artima.com/cppsource/rvalue.html