Move Semantics

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RValue and LValues

If we can take the address of something then it is a lvalue else it is a rvalue. If it has a name then it is a lvalue. Usually things on the left side of an assignment operator are called lvalues and the ones on the right side are called rvalues.However this is not always the case. We can also have lvalues on the right hand side of a statement. The below program shows some example of this concept.

File: values1.cpp

#include <iostream>


using namespace std ;



//Returns a rvalue
int setValue()
{
    return 6;
}

int global = 100;

int& setGlobal()
{
    return global;
}




int main()
{
    int x1 = 100 ;
    //x1 is lvalue because it has an address
    //100 is a rvalue because it is temporary and does
    //not have a permanent address in memory. It may be stored
    //in a register but after the statement,
    //it's value is lost.

    // 100 = x1 ;
    //This produces a compiler error.
    //values1.cpp: In function ‘int main()’:
    //values1.cpp:20:6: error: lvalue required as left operand of assignment
    //   20 |      100 = x1 ;
    //The "100" does not have an address and is a rvalue and we cannot
    //put it to the left as it makes no sense.
    //The left of an assignment must be a lvalue .

    //setValue() = 100 ;
    //Compiler error as the function does not return a lvalue


    setGlobal() = 100 ;
    //The above is all right because the function
    // "setGlobal" returns a lvalue .

     int x2 = x1 ;
     //Here x2 is a lvalue and x1 is also a lvalue.
     // Because x1 is a name and anything that has a name has an address.

    int& x3 = x1 ;
    //x3 is a lvalue. There is a name.
    // Right hand side x1 is also a lvale

    string str1 = "Te" ;
    string str2 = "st" ;

    //Left hand side is not a lvalue
    str1 + str2 = "Test" ;



}

Exercise:
File: values1_ex.txt

1) In the expression

 int x1 = 10 ; 
 
Which is the lvalue and which is the rvalue . 

2)
int y1 = x1 + 1 ; 

Is y1 a lvalue or rvalue. 


3)
 int* ptr1 = &x1 ; 
 
 Is ptr1 a lvalue or rvalue ; 
 Is x1 a lvalue or rvalue ; 
 
4) 
  int x1 = 5 ; 
  int y1 = 6 ; 
  
  Is the expression ( x1 + y1 ) a lvalue or rvalue ?
  
5)   
 int x1 = 5 ; int b1 = 6 ; 
 int* ptr = &(a1 + b1);. 
 
 Is this a valid statement and why not ?