C ++ lvalue (left value) and rvalue (rvalue)

lvalue (left value) and rvalue (rvalue)

Yesterday met a write code like the following: "cannot bind non-const lvalue reference of type ‘int&’ to an rvalue of type ‘int’",Code similar to the following

class MyClass {
    int data;
public:
    MyClass(int& e):data(e){};
};

int main(){
    MyClass c(10);
    return 0;
}

The compiler tells us: "can not be non-const lvalue and rvalue reference binding", inside there are two key words: lvalueand rvalue.

What is lvalue (left value)

A simple definition: the memory had to determine the memory address value of the expression object, which can be a variable name: int a;can be an assignment expression a=b, or string constants "cnblogs", in short, the easiest way is to compile the following identification of this sentence &(需要鉴别的表达式), can by compiling value is left.

Strictly defined: C ++ Document

What is the nature of value left?

1. Can be taken address (&), above us is to use this method to identify the value left.

2. It can be assigned, or copy assignment;

   //赋值
   int a = 10;
   //复制赋值
   int a(10);

3. Left can initialize a reference value, which is our common references;

   int a=10;
   int& reference = a;

   Here are some common properties see more properties: C ++ Document

What is the rvalue (rvalue)

: Non-lvalue.

Common are literal (constant except string), post-increment-decrement expressiona++;a--;

What is the nature of the right value?

1. The address can not be taken

   int a = 1;
   int* p = &(a++); //报错，a++是右值，不能被取地址，而++a是左值

2. It can not be assigned or copy assignment

   42 = 10;//很明显会报错

3. You can initialize const lvalue reference , which is why I begin with the program being given! , The document does not say you can initialize a non-const lvalue reference!

   class MyClass {
       int data;
   public:
       MyClass(int const& e):data(e){};  //这里的形参引用用const修饰就可以编译通过了！！！
   };
   
   int main(){
       MyClass c(10);
       return 0;
   }

4. You can initialize rvalue reference; reference value because the right is beyond my scope of knowledge, and come back later learned supplement.