Lvalues and RValues

lvalues and rvalues
----------------------------------

An object is a contiguous region of storage. An lvalue 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 that cannot be used in the left-hand side of an assignment. Similarly, 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

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 = //lvalue

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.