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Language: C++

April 30, 1999

What are Lvalues and Rvalues?

An object is a contiguous region of memory storage. An lvalue (pronounced: L value) is an expression that refers to such an object. The original definition of lvalue referred to "an object that can appear on the left-hand side of an assignment." However, const objects are lvalues, and yet they cannot appear on the left-hand side of an assignment. An expression that can appear in the right-hand side of an expression (but not in the left-hand side of an expression) is an rvalue. For example:

 
#include <string>
using namespace std;
int& f();

void func()
{
  int n;
  char buf[3]; 
  n = 5;   // n is an lvalue; 5 is an rvalue
  buf[0] = 'a';   //buf[0] is an lvalue, 'a' is an rvalue
  string s1 = "a", s2 = "b", s3 = "c";  // "a", "b", "c" are rvalues
  s1 =  // lvalue
      s2 +s3;   //s2 and s3 are lvalues that are implicitly converted to rvalues
  s1 =  
      string("z");    // temporaries are rvalues
  int * p = new int;    // p is an lvalue; 'new int' is an rvalue
  f() = 0;    // a function call that returns a reference is an lvalue
  s1.size();   // otherwise, a function call is an rvalue expression
}

An lvalue can appear in a context that requires an rvalue; in this case, the lvalue is implicitly converted to an rvalue. An rvalue cannot be converted to an lvalue. Therefore, it is possible to use every lvalue expression in the example as an rvalue, but not vice versa.