Introduction to C++11 Value Categories

lvalues in the C Programming Language [2]

Todo

Merge what is in new-vlau-rvalue.rst into this file. Have idex.rst reference this file instead of value-categories.rst. then re-listen to Ben Saks lecture.

The value categories that categorize expresssions aren’t really language features, rather they are semantic properties of expressions. Understanding value categories helps in deciphering compiler message, understanding reference types and how reference types help in defining user-defined operators.

Prior to C++, in C expressions were categorized as lvalue expressions, where lvalue meant an expression that identifies an object, a region of data storage, a defined location in memory, that may have a value, a “locator value” in memory, and can appear on the left hand side of an assignment statement; for example,

int n;
n = 1;

In the expression n = 1 above, the subexpression n refers to an integer object, a specific location in memory. Thus n is an lvalue, a storage location that can hold a value. In C an lvalue can appear on the left hand side of an assignment and can be assigned to, but in C++11 the “l” in lvalue is no longer of any real significance, and lvalues occur in contexts outside of assignment.

Anything that is not an lvalue is termed an rvalue. The subexpression 1 above is an rvalue.

Why are the concepts of lvalues and rvalues important? Why both to distinguish what is an lvalue or an rvalue?

Todo

resume Ben Saks at 5:20 Minutes.

References are implemented using pointers

References are implemented using pointers. References act like a const pointer that’s dereferenced automatically. And like a dereferenced pointer *p, which yields an lvalue, a reference, too, is an lvalue expression. The real strength of references comes out in operator overloading.

Advantages of Referecnes over Pointers

References allow classes to overload built in operators while still allowing objects to be passed as arguments. When reference-to-const arguments are used, they work identical, in terms of usage and syntax, to pass-by-value arguments. For example,

class complex {
    complex operator+(const complex& param);
    //....
};

The complex binary + operator takes a “reference to const”. This parameter will bind to both const of non-const objects. If we had written complex operator+(complex& param), then param would only bind to non-cost lvalue arguments. A “reference to const” of type T binds to both const and non-const lvalues expressions involving objects of type T, and also to any rvalue, say x, if x is convertible to T. For example

int const &intref = 3;
const double& dref = intref;

Here 3 is obviously “convertable” to an int, and so the compiler creates a temporary to hold 3 and binds intref to it. The same thing occurs on the second line. intref is an lvalue that is convertable to a double. The compiler creates a temporary and stores the converted-to double value in it, and, then, binds dref to it.

Only const references can bind to rvalues. If intref were a non-const reference to a int, then it would not bind to 3 and no temporary would be created.

Why does C++ create the temporary? C++ does this to mimic the behavaior of pass by value, so pass by value and pass by reference look the same in all cases. For example, the pass-by-value function f and the pass-by-const-reference function fref below behave the same way.

long double x;
void f(long double ld);
//...
f(x); // passes a copy of x
f(1); // passes a copy of 1 converted to a double.
void fref(const long double& ld);
fref(x); // ld is const reference to the double x.
fref(1); // passes a reference to a temporary, a copy of 1, converted to a double.

Todo

resume the merging of new-lvalue-rvalue.rst into this document here.

lvalues, rvalues and references in C++03

Pre-2011 C++ followed the C model, but assigned the name rvalue to non-lvalue expressions. In the expression n = 1;, for example, 1 is an rvalue because it is not an object, not a location in memory, and thus not an lvalue. C++03 added the rule that references can bind to lvalues, but only references-to-const can bind to rvalues (in addition to both const and non-const lvalue expressions). Several non-lvalue C expressions also became lvalue (<– do I mean rvalue?Listen again to Ben Saks at https://www.youtube.com/watch?v=XS2JddPq7GQ ) expressions in C++.

Distinguishing rvalues from lvalues allows the compiler to improve the efficiency of the code it generates. The compiler does not need to place rvalues in storage (although this does not apply to class instances as will be discussed).

When an lvalue is used on the right hand side of an assignment as below

int n, m;
n = 1;
m = n; // m and n are both lvalues. n undergoes lvalue-to-rvalue conversion.

it is said to undergo lvalue-to-rvalue conversion. In m = n, n undergoes lvalue-to-rvalue conversion. When we talk about something being an lvalue, we are concerned with where the object lives, but when we only need to know the value it holds, we can view the object, the lvalue in this example, as an rvalue.

lvalues and rvalues are relevant in contexts other than assignment. Take, for example, the binary operator +. It operands can be either rvalues or lvalues.

x + 2   // lvalue + rvalue
5 + 2   // rvalue + rvalue
x + y   // lvalue + lvalue

The result of the + operation is a temporary object that the compiler may place in a register. Since this temporary object is not guaranteed to be in memory, it is therefore an rvalue. Another of example of an lvalue is the result of the derefence operator

int a[N];
int *p = a;
char *s = nullptr;
*p = 3;       // *p is an lvalue
*s = '\0';    // *s is an lvalue even though s is null and even though *s causes undefined behavior.

*s is an lvalue even though s is nullptr and even though *s causes undefined behavior.

In C++, rvalues of class type do occupty data storage. Why? Consider this example

struct S {int x; int y;};
S foo();
int j = foo().y; // foo() is an rvalue that occupies storage.

To get the y member of foo().y, the compiler first needs the base address of the struct S returned by foo(), and any object with an address occupies storage.

Recapping lvalues and rvalues so far

Value Category Can Take Address Of Can Assign To
lvlaue yes yes
const lvlaue yes no
rvalue no no

While conceptually rvalues don’t occupy storage, rvalues of class type do, and “const references to a temporary” also causes the temporary to be placed in storage. For example

const int& int_ref1 = 10;
int& int_ref2 = 11;      // Error: int_ref2 is not const

in the code above, the temporary 10 is place in storage so that the const refernence to int can bind to it. Without ‘const’ the compiler issues an error.

lvalues and rvalues in C++11

What were previously called “references” in C++03 are now called lvalue references in C++11. This was done to distinguishes them from the new rvalue references of C++11. lvalue references in C++11 behave just like references did in C++03. On the other hand, rvalue refernces in C++11 are used primarily as parameter declarations of move constructors and move assignment operators, and as a function return type, primarily the return type of std::move(). Move construction and move assignment significantly improve performance. when the compiler detects an rvalue by “stealing” instead of copying resources.

What were previously called rvalues in C++03 are now called prvalues in C++11, and another new rvalue subcategory, xvalues or “expiring values”, was introduced. xvalues result when a lvalue is cast to an rvalue reference or when a method returns an rvalue reference. “Pure rvalues” abbreviated prvalues don’t occupy data storage. “Expiring values” abbreviated xvalues that do occupy storage.

Todo

Read C++ Move Semantics eDocument by Nikolai Josuttis that I purchased.

Note

Keep in mind that as a programmer you don’t need to worry about the distinction prvalues and xvalues. These terms exist in the C++ standard so compiler authors know what needs to be done.

lvalue references are declared using single & and rvalue reference are declared using a double &&. rvalue references can be used as function parameters and return types, for example

int&& ri = 10; // rvalue reference to int.
double &&f(int &&rint);

const int&& rci = 20;  // A const rvalue reference is not really of any use.

The primary use of rvalue references is as functon parameters and return types. Their purpose is not primarily to allow us to delcare variables like ri above.

rvalue references can only bind to rvalues. This is true even for a “rvalue reference to const”, as in the example below

int n = 10;
int &&ri = n;       // error: n is an lvalue.
const int &&rj = n; // error: n is an lvalue.

temporary materialization conversion

When a temporary is created due to binding to a const reference, it undergoes what is called a “temporary materialization conversion” that converts a prvalue into an xvalue. This places the pure rvalue, the prvalue, that is not in storage, into storage, and making it an xvalue. For example, in the code below

 class string {
   public:
      string(const string&);
      string(const char *); // converting construcotr
      string& operator=(const string&);
      //...
 };

string operator+(const string& lo, const string& ro); // lvalue reference to const will bind to both lvalues and rvalues.
string s{"hello"};
string t{"world"};

s = s + ", " t;

the compiler implicitly invokes the converting constructor string::string(const char*) to convert the character string “, ” into a string object:

s = s + string(", ") + t; // lvalue + rvalue + lvalue

The binary operator operator+(const string& lo, const string& ro) returns an rvalue. Since we can’t do something like

string *p = &(s + t); // error: can't take address of rvalue.

the result of operator+(const string& lo, const string& ro) must be an rvalue.

We saw that binding an “lvalue reference to const” to an rvalue triggers a temporary materialization conversion, in which a prvalue that is not in storage is turned into a xvalue that is placed in storage. The temporary materialization conversion also occurs when we bind an “rvalue reference” to an rvalue. When we bind a rvalue reference to an rvalue, an xvalue is created.

What rvalue references offer

The main reason rvalue references are in C++11 is to provide more efficient move constructors and move assignment operator that the compiler can call whenever it detects an rvalue.

class string {
  public:
     string(const string&);
     string(const char *);                // converting construcotr
     string& operator=(const string&);

     string(string&&) noexcept;            // move constructor
     string& operator=(string&&) noexcept; // move assignment
     //...
};

string s1, s2, s3;
s1 = s2;         // Because s2 is not expiring, and it must be preserved, the copy constructor is invoked.

s1 = s2 + s3;    // Since the result of s1 + s2 expires at the end of the statement, it can be moved from.

The result of s2 + s3 is an rvalue that expires at the end of the statement. Since rvalues can be moved from, the more efficient move constructor will be called.

Note

rvalue reference parameters are considered lvalues within the body of the function.

Take, for example

string& string::operator=(string&& other) noexcept
{
    string temp(other); // invokes copy constructor
    //...
}

Because the rvalue reference parameter “other” has a name, it is an lvalue within string::operator=(string&&other).

Converting lvalues into xvalues, eXpiring values

std::swap() is an good example of where we would like to force the compiler to move an object’s state instead of copying it. Take, for example,

template<class T> void swap(T& a, T& b)
{
   temp t(a);
   a = b;
   b = t;
}

This code invokes the copy constructors for T. But since we know that the state of a does not need to preserved, it is therefore more efficient to move its state. But to do so, we need to tell the compiler that a does not need to be preserved by casting it from an lvalue to an xvalue. This is done by calling std::move(), which converts the input parameter into an xvalue, an unamed rvalue reference. std::move() could perhaps better have been named std::rvalue() or std::xvalue().

template<typename T> constexpr typename std::remove_reference<T>::type&& move(T&& t) noexcept
{
   return static_cast<typename std::remove_reference<T>::type&&>(t);
}

Since return values never have names, calling std::move() returns an unamed rvalue reference.

template<class T> void swap(T& a, T& b)
{
   temp t(std::move(a));
   a = std::move(b);
   b = std::move(t);
}

What Distinguishes Value Categories

The figure below show that the two key properties that distinguishes the value categories of C++11 are “has identity” and “move-able”:

value categories

Figure: value categories

An xvalue Example

void f(vector<string>& vs)
{
   vector<string>& v2 = move(vs);
   //...
}

move(vs) is an xvalue. It has identity. We can refer to it as vs, but we have cast it to an unamed rvalue reference. Since move(vs) is moveable and has identity it is an xvalue.

struct A { ... };
A a;             // a is an lvalue
static_cast<A&&>(a); // but this expression is an xvalue.

In the code example above, we haven’t moved anything yet. We’ve just created an xvalue by casting an lvalue to an unnamed rvalue reference. It can still be identified by its lvalue name; but, as an xvalue, it is now capable of being moved. But you can think of the “x” in “xvalue” as meaning “expert-only” if that helps. By casting an lvalue into an xvalue (a kind of rvalue), the value then becomes capable of being bound to an rvalue reference.

Further Explanantion and Examples

Microsoft’s Value categories, and references to them is an excellent explantion of the “has identity” and “move-able” properties that characterize and distinguish lvalues, xvalues and prvalues. The articles also contains examples of each of the value category.

Examples of lvalues, xvalues and prvalues can be found at:

Value Categories in C++17

Todo

Mention the important change in C++17 having to do with materialization and how this relates to value categories!!!!!