Lvalues and Rvalues
Every C++ expression is either an lvalue or an rvalue. An lvalue refers to an object that persists beyond a single expression. You can think of an lvalue as an object that has a name. All variables, including nonmodifiable (const) variables, are lvalues. An rvalue is a temporary value that does not persist beyond the expression that uses it. To better understand the difference between lvalues and rvalues, consider the following example:
// lvalues_and_rvalues1.cpp
// compile with: /EHsc
#include <iostream>
using namespace std;
int main()
{
int x = 3 + 4;
cout << x << endl;
}
In this example, x 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 does not persist beyond the expression that defines it.
The following example demonstrates several correct and incorrect usages of lvalues and rvalues:
// lvalues_and_rvalues2.cpp
int main()
{
int i, j, *p;
// Correct usage: the variable i is an lvalue.
i = 7;
// Incorrect usage: The left operand must be an lvalue (C2106).
7 = i; // C2106
j * 4 = 7; // C2106
// Correct usage: the dereferenced pointer is an lvalue.
*p = i;
const int ci = 7;
// Incorrect usage: the variable is a non-modifiable lvalue (C3892).
ci = 9; // C3892
// Correct usage: the conditional operator returns an lvalue.
((i < 3) ? i : j) = 7;
}
注意
The examples in this topic illustrate correct and incorrect usage when operators are not overloaded. By overloading operators, you can make an expression such as j * 4 an lvalue.
The terms lvalue and rvalue are often used when you refer to object references. For more information about references, see Lvalue Reference Declarator: & and Rvalue Reference Declarator: &&.