C++ Value Categories | lvalue, prvalue, xvalue Explained

Classification

expression
├─ glvalue
│  ├─ lvalue
│  └─ xvalue
└─ rvalue
   ├─ prvalue
   └─ xvalue

lvalue

Has a name and can have its address taken (with exceptions).

int x = 10;
int* p = &x;
int& r = x;

prvalue

Pure rvalue: temporaries, literals, results of many operations.

int y = x + 5;  // x+5 is a prvalue

xvalue

“Expiring” value—typically from std::move, certain && returns, or subobjects of temporaries.

std::vector<int> v2 = std::move(v);  // move(v) is an xvalue

Reference binding

const int& cr = 10;  // OK: const& extends lifetime of temporary
int&& rr = 10;       // OK: binds to prvalue

decltype and categories

int x = 10;
decltype(x)    // int
decltype((x))  // int&  — extra parens matter

Practical guidelines

• Prefer return by value and let RVO/std::move rules apply; do not std::move local variables being returned unless you know NRVO cannot apply and you intend a move.
• Use forwarding references T&& plus std::forward to preserve value category through wrappers.

FAQ

See the Korean original for additional overload-resolution and if constexpr category-print examples.

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Related posts

• Perfect forwarding
• Move semantics

Keywords

C++, lvalue, prvalue, xvalue, value category