Lvalues vs Rvalues, Value Categories (glvalue, prvalue, xvalue)

Advanced13 min read•Updated: 2026-10-22

🧭 Lvalues vs Rvalues and C++ Value Categories

Introduction

Before C++11, expressions were simply classified as either lvalues (entities with identifiable memory addresses) or rvalues (temporary values). Modern C++ introduced a formal taxonomy of value categories that underpins move semantics and perfect forwarding.

🏛️ The ISO C++ Value Categories Taxonomy

               Expressions
             ┌──────┴──────┐
          glvalue       rvalue
        ┌────┴────┬────────┴────┐
      lvalue    xvalue       prvalue

1
glvalue (Generalized lvalue): An expression whose evaluation determines the identity of an object, bit-field, or function.
2
lvalue: An expression with an identifiable identity that is not movable by default (e.g., named variables, references).
3
prvalue (Pure rvalue): An expression whose evaluation computes a value that is not an object, or initializes an object (e.g., literals 42, 3.14, temporary returned values x + y).
4
xvalue ("eXpiring" value): An object that has an identity, but whose resources can be reused / moved because its lifetime is nearing expiration (e.g., the result of std::move(var)).
5
rvalue: The union of prvalue and xvalue.

💻 Identifying Lvalues and Rvalues in Code

1#include <iostream>
2#include <string>
3 
4int getConstant() { return 100; }
5 
6int main() {
7    int a = 10;          // 'a' is an lvalue (has memory address: &a is valid)
8    int b = 20;          // 'b' is an lvalue
9    int c = a + b;       // 'a + b' is a prvalue (temporary calculation)
10 
11    // &a;               // Valid: lvalue has persistent address
12    // &(a + b);         // 💥 Compiler Error: cannot take address of rvalue temporary
13    // &getConstant();   // 💥 Compiler Error: returned by value prvalue has no persistent identity
14 
15    std::string str = "MSK";
16    // std::move(str) yields an xvalue:
17    std::string destination = std::move(str); // str's buffer is pilfered!
18 
19    return 0;
20}

💡 The Practical Mental Model

If you can take its address with &, it is an lvalue.
If it is a temporary value, literal, or explicitly wrapped in std::move(), it is an rvalue whose memory buffers can be transferred with zero copying overhead.

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🏛️ The ISO C++ Value Categories Taxonomy

               Expressions
             ┌──────┴──────┐
          glvalue       rvalue
        ┌────┴────┬────────┴────┐
      lvalue    xvalue       prvalue

glvalue (Generalized lvalue): An expression whose evaluation determines the identity of an object, bit-field, or function.

lvalue: An expression with an identifiable identity that is not movable by default (e.g., named variables, references).

prvalue (Pure rvalue): An expression whose evaluation computes a value that is not an object, or initializes an object (e.g., literals 42, 3.14, temporary returned values x + y).

xvalue ("eXpiring" value): An object that has an identity, but whose resources can be reused / moved because its lifetime is nearing expiration (e.g., the result of std::move(var)).

rvalue: The union of prvalue and xvalue.

💻 Identifying Lvalues and Rvalues in Code

1#include <iostream>
2#include <string>
3 
4int getConstant() { return 100; }
5 
6int main() {
7    int a = 10;          // 'a' is an lvalue (has memory address: &a is valid)
8    int b = 20;          // 'b' is an lvalue
9    int c = a + b;       // 'a + b' is a prvalue (temporary calculation)
10 
11    // &a;               // Valid: lvalue has persistent address
12    // &(a + b);         // 💥 Compiler Error: cannot take address of rvalue temporary
13    // &getConstant();   // 💥 Compiler Error: returned by value prvalue has no persistent identity
14 
15    std::string str = "MSK";
16    // std::move(str) yields an xvalue:
17    std::string destination = std::move(str); // str's buffer is pilfered!
18 
19    return 0;
20}

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