1.1.8.2 lvalue rvalue and xvalue

• Statement and expression are two important conceptions, you can see their definition in cppreference.com to see academic explanation. In C/C++ langauge, there are many expressions, and there are if, switch, return, block, iteration statements. they are all end with semi-comma. And function call is a expression, because it can yield a value.
• Each C++ expression (an operator with its operands, a literal, a variable name, etc.) is characterized by two independent properties: a type and a value category. Each expression has some non-reference type, and each expression belongs to exactly one of the three primary value categories: lvalue, prvalue, and xvalue;
• lvalue is not defined "can be put on the left of =". Because const int a is also a lvalue, and you can’t put a on the left side of = . It has three characteristics:
  1. lvalue has Identity, (can be get address by & operator),
  2. lvalue can be persist beyond the expression.
  3. lvalue can’t be move(stolen), because you need to keep original one intact.
• On the contrary, prvalue has no identity, and will not persist beyond expression and can be move(stolen)
• xvalue has identity, and will persist beyond expression and can be move(stolen)
• xvalue is just rvalue reference. Why you need xvalue, because in C++, it introduce right reference and move semantics.
  int a = 7;  // a is lvalue, It has Identity 
  int && r1 = 13,  //r1, r2 and r3 are xvalue 
  int && r2= x+y; //x+y is prvalue 
  // but the whole expression is xvalue 
   
  cont int & lv = x+y; //lv is lvalue 
  int && r3 = std::sqrt(2.0); 
   
  int & r = x+y; //error, 
  // lvalue reference can not bind with an rvalue. 
   
  a+b // a+b are pure right value.
• You need to know, previous codes and explanations just give the basic idea of rvalue reference. Just like function pointer, we never use function pointer in simple way, instead, we use it as a function argument to implement call back. For rvalue reference, We just use it in the move ctor and move assignment. I will explain it in the next section.