关于c++中的左值和右值

Lvalues and Rvalues (Visual C++)

Visual Studio 2015

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The latest version of this topic can be found at Lvalues and Rvalues (Visual C++).

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;  
}  

Note
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: &&.