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Asked: 2020-12-12 01:25:38 +0800 CST 2020-12-12 01:25:38 +0800 CST

Differences between Rvalue and Lvalue

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For some time now, two terms have been appearing on some blogs that I don't quite understand: Rvalueand Lvalue. Without being clear about the meaning of both terms, the content of certain examples becomes complicated to understand.

  1. What differences are there between a Rvalueand a Lvalue?
  2. How can each be identified?
  3. What can and cannot be done with each of them?
c++

1 Answers

  1. Best Answer

    There is a short and fairly clear explanation in this MSDN article . I translate (emphasis mine):

    All C++ expressions are either an lvalue or an rvalue . An lvalue is an object that persists beyond an expression . You can think of an lvalue as an object that has a name. All variables, including hard-coded (const) variables, are lvalues ​​.

    An rvalue is a temporary value that does not persist beyond the expression in which it is used .

    To better understand the difference between lvalue and rvalue , consider the following example:

    // lvalues_y_rvalues1.cpp
// compilar con: /EHsc
#include <iostream>
using namespace std;
int main()
{
   int x = 3 + 4;
   cout << x << endl;
}

    Here xit is an lvalue because it persists beyond the expression that defines it. The expression 3+4 is an rvalue because it evaluates to a temporary value that doesn't persist beyond the expression.

    Here are some examples of incorrect uses of lvalues ​​and rvalues ​​:

    // lvalues_and_rvalues2.cpp
int main()
{
   int i, j, *p;

   // Uso correcto: la variable i es un lvalue.
   i = 7;

   // Uso incorrecto: el operando a la izquierda debe ser un lvalue (C2106).
   7 = i; // C2106
   j * 4 = 7; // C2106

   // Uso correcto: el puntero desreferenciado es un lvalue.
   *p = i; 

   const int ci = 7;
   // Uso incorrecto: la variable es un valor no modificable (C3892).
   ci = 9; // C3892

   // Uso correcto: el operador condicional devuelve un lvalue.
   ((i < 3) ? i : j) = 7;
}

    TL;DR (not part of original): You can assign a value to an lvalue (if it's not const). Not to an rvalue , but you can use it as the value you assign to an lvalue .

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