DevTut

# Value Categories

# Value Category Meanings

Expressions in C++ are assigned a particular value category, based on the result of those expressions. Value categories for expressions can affect C++ function overload resolution.

Value categories determines two important-but-separate properties about an expression. One property is whether the expression has identity. An expression has identity if it refers to an object that has a variable name. The variable name may not be involved in the expression, but the object can still have one.

The other property is whether it is legal to implicitly move from the expression's value. Or more specifically, whether the expression, when used as a function parameter, will bind to r-value parameter types or not.

C++ defines 3 value categories which represent the useful combination of these properties: lvalue (expressions with identity but not movable from), xvalue (expressions with identity that are moveable from), and prvalue (expressions without identity that are moveable from). C++ does not have expressions which have no identity and cannot be moved from.

C++ defines two other value categories, each based solely on one of these properties: glvalue (expressions with identity) and rvalue (expressions that can be moved from). These act as useful groupings of the prior categories.

This graph serves as an illustration:

Graph of value categories in the C++ language (opens new window)

# rvalue

An rvalue expression is any expression which can be implicitly moved from, regardless of whether it has identity.

More precisely, rvalue expressions may be used as the argument to a function that takes a parameter of type T && (where T is the type of expr). Only rvalue expressions may be given as arguments to such function parameters; if a non-rvalue expression is used, then overload resolution will pick any function that does not use an rvalue reference parameter. And if none exist, then you get an error.

The category of rvalue expressions includes all xvalue and prvalue expressions, and only those expressions.

The standard library function std::move exists to explicitly transform a non-rvalue expression into an rvalue. More specifically, it turns the expression into an xvalue, since even if it was an identity-less prvalue expression before, by passing it as a parameter to std::move, it gains identity (the function's parameter name) and becomes an xvalue.

Consider the following:

std::string str("init");                       //1
std::string test1(str);                        //2
std::string test2(std::move(str));             //3

str = std::string("new value");                //4 
std::string &&str_ref = std::move(str);        //5
std::string test3(str_ref);                    //6

std::string has a constructor which takes a single parameter of type std::string&&, commonly called a "move constructor". However, the value category of the expression str is not an rvalue (specifically it is an lvalue), so it cannot call that constructor overload. Instead, it calls the const std::string& overload, the copy constructor.

Line 3 changes things. The return value of std::move is a T&&, where T is the base type of the parameter passed in. So std::move(str) returns std::string&&. A function call who's return value is an rvalue reference is an rvalue expression (specifically an xvalue), so it may call the move constructor of std::string. After line 3, str has been moved from (who's contents are now undefined).

Line 4 passes a temporary to the assignment operator of std::string. This has an overload which takes a std::string&&. The expression std::string("new value") is an rvalue expression (specifically a prvalue), so it may call that overload. Thus, the temporary is moved into str, replacing the undefined contents with specific contents.

Line 5 creates a named rvalue reference called str_ref that refers to str. This is where value categories get confusing.

See, while str_ref is an rvalue reference to std::string, the value category of the expression str_ref is not an rvalue. It is an lvalue expression. Yes, really. Because of this, one cannot call the move constructor of std::string with the expression str_ref. Line 6 therefore copies the value of str into test3.

To move it, we would have to employ std::move again.

# xvalue

An xvalue (eXpiring value) expression is an expression which has identity and represents an object which can be implicitly moved from. The general idea with xvalue expressions is that the object they represent is going to be destroyed soon (hence the "eXpiring" part), and therefore implicitly moving from them is fine.

Given:

struct X { int n; };
extern X x;

4;                   // prvalue: does not have an identity
x;                   // lvalue
x.n;                 // lvalue
std::move(x);        // xvalue
std::forward<X&>(x); // lvalue
X{4};                // prvalue: does not have an identity
X{4}.n;              // xvalue: does have an identity and denotes resources
                     // that can be reused

# prvalue

A prvalue (pure-rvalue) expression is an expression which lacks identity, whose evaluation is typically used to initialize an object, and which can be implicitly moved from. These include, but are not limited to:

  • Expressions that represent temporary objects, such as std::string("123").
  • A function call expression that does not return a reference
  • A literal (except a string literal - those are lvalues), such has 1, true, 0.5f, or 'a'
  • A lambda expression

The built-in addressof operator (&) cannot be applied on these expressions.

# lvalue

An lvalue expression is an expression which has identity, but cannot be implicitly moved from. Among these are expressions that consist of a variable name, function name, expressions that are built-in dereference operator uses and expressions that refer to lvalue references.

The typical lvalue is simply a name, but lvalues can come in other flavors as well:

struct X { ... };

X x;         // x is an lvalue
X* px = &x;  // px is an lvalue
*px = X{};   // *px is also an lvalue, X{} is a prvalue

X* foo_ptr();  // foo_ptr() is a prvalue
X& foo_ref();  // foo_ref() is an lvalue

Additionally, while most literals (e.g. 4, 'x', etc.) are prvalues, string literals are lvalues.

# glvalue

A glvalue (a "generalized lvalue") expression is any expression which has identity, regardless of whether it can be moved from or not. This category includes lvalues (expressions that have identity but can't be moved from) and xvalues (expressions that have identity, and can be moved from), but excludes prvalues (expressions without identity).

If an expression has a name, it's a glvalue:

struct X { int n; };
X foo();

X x;
x; // has a name, so it's a glvalue
std::move(x); // has a name (we're moving from "x"), so it's a glvalue
              // can be moved from, so it's an xvalue not an lvalue

foo(); // has no name, so is a prvalue, not a glvalue
X{};   // temporary has no name, so is a prvalue, not a glvalue
X{}.n; // HAS a name, so is a glvalue. can be moved from, so it's an xvalue

← Preprocessor →