Overview

In C++, every expression has two properties: a type and a value category.
Value categories describe what kind of value an expression yields.

C++11 introduced 3 primary categories:

lvalue
xvalue
prvalue

And two broader categories:

glvalue = lvalue or xvalue
rvalue = xvalue or prvalue

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Value Category Hierarchy Diagram

                    expression
                        |
           +------------+------------+
           |                         |
       glvalue                    rvalue
           |                         |
      +----+----+              +-----+-----+
      |         |              |           |
   lvalue    xvalue        xvalue      prvalue

Explanation:

glvalue (generalized lvalue): Has identity
rvalue: Can be moved from
lvalue: Has identity, cannot be moved from (unless explicitly cast)
xvalue: Has identity AND can be moved from
prvalue: No identity, temporary value

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prvalue (pure rvalue)

A prvalue is a temporary value that does not refer to an existing object.

Examples:

int x = 5;          // 5 is a prvalue  
int y = x + 10;     // (x + 10) is a prvalue  
std::string s("hi"); // temporary std::string → prvalue

Characteristics:

❌ Not addressable (no identity)
Creates a new temporary object
Result of most operators and literals

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lvalue

An lvalue refers to an identifiable, persistent object in memory.

Examples:

int a = 10;  // a is an lvalue  
int &r = a;  // r is also an lvalue  
struct S { int m; };
S s;
s.m = 5;     // s.m is an lvalue

Characteristics:

✅ Addressable (has identity)
Persists beyond a single expression
Can appear on the left side of assignment

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xvalue (expiring value)

An xvalue is a special glvalue that refers to an object whose resources can be reused (i.e., movable).

Examples:

std::string s = "hello";
std::string s2 = std::move(s);  // std::move(s) → xvalue  

struct S { std::string name; };
S getS();
getS().name = "Alice"; // getS() is prvalue, getS().name is xvalue

Characteristics:

✅ Addressable (has identity)
About to expire (resources can be moved)
Result of std::move() or member access on rvalue

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Summary Table

Category	Meaning	Example	Addressable?
prvalue	Temporary / non-object expression	`5`, `"hi"`, `x+1`	❌
lvalue	Persistent object with identity	variables, members	✅
xvalue	Expiring object suitable for move	`std::move(obj)`	✅
glvalue	lvalue or xvalue	—	✅
rvalue	prvalue or xvalue	—	depends

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Quick Examples

int a = 10;        // a → lvalue  
int b = a + 5;     // a + 5 → prvalue  

int &r = a;        // r → lvalue  

int &&rr = 20;     // 20 → prvalue; rr → lvalue (named ref)  

std::string s = "hello";          
std::string s2 = std::move(s);    // xvalue  

struct S { int m; };
S get();
get().m = 1;        // get() → prvalue, .m → xvalue

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Intuition

prvalue → makes a new temporary object
lvalue → refers to an existing object
xvalue → refers to a disposable/expiring object
glvalue → has identity
rvalue → temporary or movable

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Visual Relationships

Properties Matrix:

                    Has Identity    No Identity
                    ────────────    ───────────
Can Move From       │  xvalue   │   prvalue  │
                    ────────────────────────────
Cannot Move From    │  lvalue   │     N/A    │
                    ────────────────────────────

Groupings:

glvalue = { lvalue, xvalue }  ← Things with identity
rvalue  = { xvalue, prvalue } ← Things you can move from

Key Insight:

xvalue is the intersection: has identity AND can be moved from
Think of value categories as answering two questions:
1. Does it have an identity (address)?
2. Can we steal its resources (move)?

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