C ++ 11-- rvalue references

1. Left value and right value, lvalue references, rvalue references

In the usual sense, in C ++, it is desirable address, has a name that is left of value.

Desirable address, no name value to the right. Rvalue include literals, the function returns the value of the temporary variable expression and other temporary values.

Can accept the right of the right value of the reference value in itself, it was a left value

#include<iostream>

#include<utility>

#include<vector>

using namespace std;

int f();

int main() {

vector<int>vi(100);

int i=42;

int &&r1=i;//error不能把右值引用绑到左值上

int &&r2=10;

int &r3=i;

int &r4=10;//error非常量引用不能绑到右值上

const int &r5=10;//ok

int &&r5=r2;//error,r2是左值

int &&r6=std::move(r2);//ok,c++11std::move把左值转换为对应的右值引用类型。

int &&r4=f();//这个函数返回右值

int &r5=vi[1];//下标返回左值

int &r6=r4;//变量r4是左值

}

//左值引用表示一个对象的身份，右值引用表示对象的值

//返回左值引用的函数、赋值、下标、解引用和前置递增递减运算符都是返回左值。

//不能把右值引用绑定在它们上，但可以把左值引用绑在它们上

//返回非引用类型的函数、算术、关系、位以及后置递增递减运算符、取地址&都是返回右值

//不能把左值引用绑定它们上，但可以把const左值引用和右值引用绑定在它们上。

1, the concept of left and right values

Left value is the value placed on the left side of an assignment can be assigned; lvalue must have a physical in memory;
the right value when the value is assigned to the value of other variables out of the right side of an assignment; the right value can be in memory at CPU register.
When an object is used as the right value, using its contents (value), when it is left as a value, using its address.

2, reference

Reference is made to optimize C ++ syntax, or the nature of the referenced by a pointer to achieve. Equivalent to a reference variable alias.

Reference pointer may be changed, and can also change the value of the pointer points.
The basic rule of references:

When the statement quoted must be initialized, and once bound not to bind references to other objects; that is, reference must be initialized, a reference can not be redefined;
all references to the operation, the operation is equivalent to the original object.

3, left and right reference value reference value

3.1 lvalue references

The basic syntax lvalue references: type & reference name = lvalue expression;

3.2 rvalue references

Rvalue references of the basic syntax of the right type && quote name = value expressions;

Rvalue reference in corporate developers will be frequently used in the code optimization.

Rvalue references of "&&" middle can not have spaces.

#include <iostream>

using namespace std;

int main()

{

cout << "-------引用左值--------" << endl;

int a = 5;

int &add_a(a);

cout << " a =" << a <<" "<<" &a = "<<&a<< endl;

cout << "add_a =" << add_a<<" "<< "&add_a = " << &add_a << endl;

cout << "-----------------------" << endl;

cout << "-------引用右值--------" << endl;

int b = 10;

int &&add_b(b + 1);

cout << " b =" << b << " " << " &b = " << &b << endl;

cout << "add_b =" << add_b << " " << "&add_b = " << &add_b << endl;

add_b++;

cout << "add_b++ =" << add_b << " " << "&add_b++ = " << &add_b << endl;

cout << "-----------------------" << endl;

system("pause");

return 0;

}

其实对于左值还是很好理解的，主要是对于右值是不好理解的，特别是代码的16行处：右值的例子。C++之所以设计出右值引用的语法，主要是因为对于类似b+1；这样的运算是发生在CPU寄存器上的，就不能对其取地址、赋值等操作，所以这类运算只能放在等号的右边，将其赋给其他的变量。若等号右边出现：&b，这样的操作是，也是右值，因为取地址符的操作也是在寄存器中完成的。所以不能作为左值。

和右值引用相关的概念比较多，比如：右值、纯右值、将亡值、universal references、引用折叠、移动语义、move语义和完美转发等.

在C++11中所有的值必属于左值、将亡值、纯右值三者之一。比如，非引用返回的临时变量、运算表达式产生的临时变量、原始字面量和lambda表达式等都是纯右值。而将亡值是C++11新增的、与右值引用相关的表达式，比如，将要被移动的对象、T&&函数返回值、std::move返回值和转换为T&&的类型的转换函数的返回值等

2。std::move()与完善转发std::forward()

右值引用移动构造：相当于剪切

左值引用拷贝构造：相当于拷贝再删除

C ++ 11-- rvalue references

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