What are rvalues, lvalues, xvalues, glvalues, and

Question

In C++03, an expression is either an rvalue or an lvalue.

In C++11, an expression can be an:

1. rvalue
2. lvalue
3. xvalue
4. glvalue
5. prvalue

Two categories have become five categories.

• What are these new categories of expressions?
• How do these new categories relate to the existing rvalue and lvalue categories?
• Are the rvalue and lvalue categories in C++0x the same as they are in C++03?
• Why are these new categories needed? Are the WG21 gods just trying to confuse us mere mortals?

Answer 1

I have struggled with this for a long time, until I came across the cppreference.com explanation of the value categories.

It is actually rather simple, but I find that it is often explained in a way that's hard to memorize. Here it is explained very schematically. I'll quote some parts of the page:

Primary categories

The primary value categories correspond to two properties of expressions:

• has identity: it's possible to determine whether the expression refers to the same entity as another expression, such as by comparing addresses of the objects or the functions they identify (obtained directly or indirectly);

• can be moved from: move constructor, move assignment operator, or another function overload that implements move semantics can bind to the expression.

Expressions that:

• have identity and cannot be moved from are called lvalue expressions;
• have identity and can be moved from are called xvalue expressions;
• do not have identity and can be moved from are called prvalue expressions;
• do not have identity and cannot be moved from are not used.

lvalue

An lvalue ("left value") expression is an expression that has identity and cannot be moved from.

rvalue (until C++11), prvalue (since C++11)

A prvalue ("pure rvalue") expression is an expression that does not have identity and can be moved from.

xvalue

An xvalue ("expiring value") expression is an expression that has identity and can be moved from.

glvalue

A glvalue ("generalized lvalue") expression is an expression that is either an lvalue or an xvalue. It has identity. It may or may not be moved from.

rvalue (since C++11)

An rvalue ("right value") expression is an expression that is either a prvalue or an xvalue. It can be moved from. It may or may not have identity.

Answer 2

One addendum to the excellent answers above, on a point that confused me even after I had read Stroustrup and thought I understood the rvalue/lvalue distinction. When you see

int&& a = 3,

it's very tempting to read the int&& as a type and conclude that a is an rvalue. It's not:

int&& a = 3;
int&& c = a; //error: cannot bind 'int' lvalue to 'int&&'
int& b = a; //compiles

a has a name and is ipso facto an lvalue. Don't think of the && as part of the type of a; it's just something telling you what a is allowed to bind to.

This matters particularly for T&& type arguments in constructors. If you write

Foo::Foo(T&& _t) : t{_t} {}

you will copy _t into t. You need

Foo::Foo(T&& _t) : t{std::move(_t)} {} if you want to move. Would that my compiler warned me when I left out the move!

Answer 3

INTRODUCTION

ISOC++11 (officially ISO/IEC 14882:2011) is the most recent version of the standard of the C++ programming language. It contains some new features, and concepts, for example:

• rvalue references
• xvalue, glvalue, prvalue expression value categories
• move semantics

If we would like to understand the concepts of the new expression value categories we have to be aware of that there are rvalue and lvalue references. It is better to know rvalues can be passed to non-const rvalue references.

int& r_i=7; // compile error
int&& rr_i=7; // OK

We can gain some intuition of the concepts of value categories if we quote the subsection titled Lvalues and rvalues from the working draft N3337 (the most similar draft to the published ISOC++11 standard).

3.10 Lvalues and rvalues [basic.lval]

1 Expressions are categorized according to the taxonomy in Figure 1.

• An lvalue (so called, historically, because lvalues could appear on the left-hand side of an assignment expression) designates a function or an object. [ Example: If E is an expression of pointer type, then *E is an lvalue expression referring to the object or function to which E points. As another example, the result of calling a function whose return type is an lvalue reference is an lvalue. —end example ]
• An xvalue (an “eXpiring” value) also refers to an object, usually near the end of its lifetime (so that its resources may be moved, for example). An xvalue is the result of certain kinds of expressions involving rvalue references (8.3.2). [ Example: The result of calling a function whose return type is an rvalue reference is an xvalue. —end example ]
• A glvalue (“generalized” lvalue) is an lvalue or an xvalue.
• An rvalue (so called, historically, because rvalues could appear on the right-hand side of an assignment expression) is an xvalue, a
  temporary object (12.2) or subobject thereof, or a value that is not
  associated with an object.
• A prvalue (“pure” rvalue) is an rvalue that is not an xvalue. [ Example: The result of calling a function whose return type is not a
  reference is a prvalue. The value of a literal such as 12, 7.3e5, or
  true is also a prvalue. —end example ]

Every expression belongs to exactly one of the fundamental classifications in this taxonomy: lvalue, xvalue, or prvalue. This property of an expression is called its value category.

But I am not quite sure about that this subsection is enough to understand the concepts clearly, because "usually" is not really general, "near the end of its lifetime" is not really concrete, "involving rvalue references" is not really clear, and "Example: The result of calling a function whose return type is an rvalue reference is an xvalue." sounds like a snake is biting its tail.

PRIMARY VALUE CATEGORIES

Every expression belongs to exactly one primary value category. These value categories are lvalue, xvalue and prvalue categories.

lvalues

The expression E belongs to the lvalue category if and only if E refers to an entity that ALREADY has had an identity (address, name or alias) that makes it accessible outside of E.

#include <iostream>

int i=7;

const int& f(){
    return i;
}

int main()
{
    std::cout<<&"www"<<std::endl; // This address ...
    std::cout<<&"www"<<std::endl; // ... and this address are the same.
    "www"; // The expression "www" in this row is an lvalue expression, because it refers to the same entity ...
    "www"; // ... as the entity the expression "www" in this row refers to.

    i; // The expression i in this row is an lvalue expression, because it refers to the same entity ...
    i; // ... as the entity the expression i in this row refers to.

    int* p_i=new int(7);
    *p_i; // The expression *p_i in this row is an lvalue expression, because it refers to the same entity ...
    *p_i; // ... as the entity the expression *p_i in this row refers to.

    const int& r_I=7;
    r_I; // The expression r_I in this row is an lvalue expression, because it refers to the same entity ...
    r_I; // ... as the entity the expression r_I in this row refers to.

    f(); // The expression f() in this row is an lvalue expression, because it refers to the same entity ...
    i; // ... as the entity the expression f() in this row refers to.

    return 0;
}

xvalues

The expression E belongs to the xvalue category if and only if it is

— the result of calling a function, whether implicitly or explicitly, whose return type is an rvalue reference to the type of object being returned, or

int&& f(){
    return 3;
}

int main()
{
    f(); // The expression f() belongs to the xvalue category, because f() return type is an rvalue reference to object type.

    return 0;
}

— a cast to an rvalue reference to object type, or

int main()
{
    static_cast<int&&>(7); // The expression static_cast<int&&>(7) belongs to the xvalue category, because it is a cast to an rvalue reference to object type.
    std::move(7); // std::move(7) is equivalent to static_cast<int&&>(7).

    return 0;
}

— a class member access expression designating a non-static data member of non-reference type in which the object expression is an xvalue, or

struct As
{
    int i;
};

As&& f(){
    return As();
}

int main()
{
    f().i; // The expression f().i belongs to the xvalue category, because As::i is a non-static data member of non-reference type, and the subexpression f() belongs to the xvlaue category.

    return 0;
}

— a pointer-to-member expression in which the first operand is an xvalue and the second operand is a pointer to data member.

Note that the effect of the rules above is that named rvalue references to objects are treated as lvalues and unnamed rvalue references to objects are treated as xvalues; rvalue references to functions are treated as lvalues whether named or not.

#include <functional>

struct As
{
    int i;
};

As&& f(){
    return As();
}

int main()
{
    f(); // The expression f() belongs to the xvalue category, because it refers to an unnamed rvalue reference to object.
    As&& rr_a=As();
    rr_a; // The expression rr_a belongs to the lvalue category, because it refers to a named rvalue reference to object.
    std::ref(f); // The expression std::ref(f) belongs to the lvalue category, because it refers to an rvalue reference to function.

    return 0;
}

prvalues

The expression E belongs to the prvalue category if and only if E belongs neither to the lvalue nor to the xvalue category.

struct As
{
    void f(){
        this; // The expression this is a prvalue expression. Note, that the expression this is not a variable.
    }
};

As f(){
    return As();
}

int main()
{
    f(); // The expression f() belongs to the prvalue category, because it belongs neither to the lvalue nor to the xvalue category.

    return 0;
}

MIXED VALUE CATEGORIES

There are two further important mixed value categories. These value categories are rvalue and glvalue categories.

rvalues

The expression E belongs to the rvalue category if and only if E belongs to the xvalue category, or to the prvalue category.

Note that this definition means that the expression E belongs to the rvalue category if and only if E refers to an entity that has not had any identity that makes it accessible outside of E YET.

glvalues

The expression E belongs to the glvalue category if and only if E belongs to the lvalue category, or to the xvalue category.

A PRACTICAL RULE

Scott Meyer has published a very useful rule of thumb to distinguish rvalues from lvalues.

• If you can take the address of an expression, the expression is an lvalue.
• If the type of an expression is an lvalue reference (e.g., T& or const T&, etc.), that expression is an lvalue.
• Otherwise, the expression is an rvalue. Conceptually (and typically also in fact), rvalues correspond to temporary objects, such as those returned from functions or created through implicit type conversions. Most literal values (e.g., 10 and 5.3) are also rvalues.

Answer 4

Why are these new categories needed? Are the WG21 gods just trying to confuse us mere mortals?

I don't feel that the other answers (good though many of them are) really capture the answer to this particular question. Yes, these categories and such exist to allow move semantics, but the complexity exists for one reason. This is the one inviolate rule of moving stuff in C++11:

Thou shalt move only when it is unquestionably safe to do so.

That is why these categories exist: to be able to talk about values where it is safe to move from them, and to talk about values where it is not.

In the earliest version of r-value references, movement happened easily. Too easily. Easily enough that there was a lot of potential for implicitly moving things when the user didn't really mean to.

Here are the circumstances under which it is safe to move something:

1. When it's a temporary or subobject thereof. (prvalue)
2. When the user has explicitly said to move it.

If you do this:

SomeType &&Func() { ... }

SomeType &&val = Func();
SomeType otherVal{val};

What does this do? In older versions of the spec, before the 5 values came in, this would provoke a move. Of course it does. You passed an rvalue reference to the constructor, and thus it binds to the constructor that takes an rvalue reference. That's obvious.

There's just one problem with this; you didn't ask to move it. Oh, you might say that the && should have been a clue, but that doesn't change the fact that it broke the rule. val isn't a temporary because temporaries don't have names. You may have extended the lifetime of the temporary, but that means it isn't temporary; it's just like any other stack variable.

If it's not a temporary, and you didn't ask to move it, then moving is wrong.

The obvious solution is to make val an lvalue. This means that you can't move from it. OK, fine; it's named, so its an lvalue.

Once you do that, you can no longer say that SomeType&& means the same thing everwhere. You've now made a distinction between named rvalue references and unnamed rvalue references. Well, named rvalue references are lvalues; that was our solution above. So what do we call unnamed rvalue references (the return value from Func above)?

It's not an lvalue, because you can't move from an lvalue. And we need to be able to move by returning a &&; how else could you explicitly say to move something? That is what std::move returns, after all. It's not an rvalue (old-style), because it can be on the left side of an equation (things are actually a bit more complicated, see this question and the comments below). It is neither an lvalue nor an rvalue; it's a new kind of thing.

What we have is a value that you can treat as an lvalue, except that it is implicitly moveable from. We call it an xvalue.

Note that xvalues are what makes us gain the other two categories of values:

• A prvalue is really just the new name for the previous type of rvalue, i.e. they're the rvalues that aren't xvalues.

• Glvalues are the union of xvalues and lvalues in one group, because they do share a lot of properties in common.

So really, it all comes down to xvalues and the need to restrict movement to exactly and only certain places. Those places are defined by the rvalue category; prvalues are the implicit moves, and xvalues are the explicit moves (std::move returns an xvalue).