Each expression has some non-reference type, and each expression belongs to exactly one of the three primary value categories: xvalue, and lvalue . Open the project's Property Pages dialog box. Convert any type to void, evaluating and discarding the value. If t returns by lvalue reference, the code does not compile because a rvalue reference cannot bind to it. Values return by functions/methods and expression are temporary values, so you do have to use std::move to move them (C++ standard to convert to rvalue) when you pass them to functions/methods. Officially, C++ performs an lvalue-to-rvalueconversion. static_cast<X &&> Once we have an expression of a value category, we can convert it to an expression of a different value category. However, if the value is const than the compiler can convert the rvalue to an lvalue duringThe title of the question you linked is a little misleading. Operationally, the difference among these kinds of expressions is this:std::function can be move-constructed from rvalue of a functor object. – T. This is already done in some places. It can appear only on the right-hand side of the assignment operator. In the previous question, I asked how this code should work: void f (const std::string &); //less efficient void f (std::string &&); //more efficient void g (const char * arg) { f (arg); } It seems that the move overload should probably be called because of the. thus, this is legal: string&& s = foo (); // extends lifetime as before s += "bar"; baz (std::move (s)); // move the temporary into the baz function. However, there is no reason why converting from one reference type to another as a cast should do anything at run time. 23. And let’s define our storage to be either one of those cases: template<typename T> using Storage = std::variant<Value<T>, ConstReference<T>, NonConstReference<T>>; Now we need to give access to the underlying value of our variant, by providing a reference. 255 How come a non-const reference cannot bind to a temporary object? 1 Why the code doesn't work on CodeBlocks,but on. That's an exception to the general rule that it is impossible for lvalues to be bound to rvalue. Radius: 2 2 4. 5, then the R-value is 2. static_cast can do other things, as listed in 5. ref], a reference can be bound directly to the result of applying a conversion function to an initializer expression. It's actually a cast. foo now is null. 18. c++11标准基本上是通过举例来说明一个表达式是否是一个lvalue还是rvalue的。. cv]/4. 2, and 4. rvalue references are marked with two ampersands (&&). 1 Answer. key here is Key&& key - this is an lvalue! It has a name, and you can take its address. Overload resolution is usually done in terms of a strict partial. In the introduction to "Effective Modern C++" it says: A useful heuristic to determine whether an expression is an lvalue is to ask if you can take its address. Until IBM's implementation of all the features of the C++11 standard is. The reference declared in the above code is lvalue. When I discovered this, it seemed odd to me, so I tried. Let's think of the addition +. This differs from ISO C, in. Even though the object in question is a temporary object, its lifetime has been extended. The expression *this is an lvalue; A {} is an rvalue (prvalue) even though they designate the same temporary object. The right constructors for the first two cases are called. 2 indicates the behavior of lvalues and rvalues in other significant contexts. void f1(int& namedValue){. This assignment uses the lvalueexpression nas an rvalue. When you typecast an expression, the result of that expression is an rvalue rather than an lvalue. You would then need to add a destructor to AttrDec and delete the pointer in it and add a copy constructor. " So an rvalue is any expression that is not an lvalue. However, a (prvalue). The list of languages that are currently supported includes C++, C#, Go, Java, Kotlin, PHP, Python, Ruby, Rust, TypeScript, and more. A glvalue of a non-function, non-array type T can be converted to a prvalue. Yes, rvalues are moved, lvalues are copied. c++11 decltype returns reference type. r can be bound to the conversion result of e or a base class of e if the following conditions are satisfied. Whenever a glvalue expression. Converts between types using a combination of explicit and implicit conversions. However, Microsoft compiler does accept it meaning that. Perhaps the most significant new feature in C++11 is rvalue references; they’re the foundation on which move semantics and perfect forwarding are built. The first constructor is the default one. 3. i is named object, so it is lvalue. test prep. Now an lvalue reference is a reference that binds to an lvalue. When you use "Hello, World" in a context in which it is implicitly converted to a const char* pointing to its initial element, the resulting pointer is an rvalue (because it is a temporary object resulting from an implicit. Therefore the usage of const rvalue references communicates thatAn lvalue is an expression representing an object that can have its address taken. Hence, values bound to an rvalue reference can be moved from (not necessarily always going to be moved from, but it is allowed), and lvalues can be bound to lvalue references and can't be moved from. 3 Pointer Types): All function pointer types shall have the same representation as the type pointer to void. C++98 assigning a value to a volatile variable might result in an unnecessary read due to the lvalue-to-rvalue conversion applied to the assignment result introduce discarded-value expressions and exclude this case from the list of cases that require the conversion CWG 1343: C++98 sequencing of destructor calls inExcept for an implicit object parameter, for which see 13. Both of these options are user-defined conversion functions, so neither is better in terms of overload resolution, thus an ambiguity. Taking it by rvalue reference would cause a headache to a user who has an existing lvalue or const reference to a function; they would need to std::move it (in. This is why you will see the C++ library provide what appears to be a single template, that works in both lvalue and rvalue contexts. It matches arguments of any value category, making t an lvalue reference if the supplied argument was an lvalue or an rvalue reference if the supplied argument was an rvalue. Every expression is either an lvalue or an rvalue, so, an rvalue is an expression that does not represent an object occupying. 0. –6. An rvalue can be bound to an rvalue reference (T&&) to prolong its lifetime, and to lvalue references to const (const T&), but not to plain lvalue references (T&). But i=3; is legal if i is an integer. If type is an lvalue reference type or an rvalue reference to a function type, the cast result is an lvalue. When being passed an lvalue, the template parameter would be deduced as lvalue-reference, after reference. c++ base constructor lvalue to parameter. auto (* p) [42] = & a; is valid if a is an lvalue of type int [42]. The value category of a compound literal is lvalue (its address can be taken). Like this: template <typename T> void foo (T &&value) { f (std::forward<T> (value)); } Here, T &&value is called a forwarding reference (as long T is deduced by the compiler. Lvalue-to-rvalue conversion C++. G. In C++, non-const references can bind to lvalues and const references can bind to lvalues or rvalues, but there is nothing that can bind to a non-const rvalue. cast (this is applicable from C++11 and later). Without lvalue-to-rvalue conversion, it cannot read it's value. C++ 中有两种类型的表达式：. 区分左值和右值是很重要的，这是使用C++11 move语义的基础。. Radius: 2 2 4. 1 Answer. Yes it's possible, just add a const to your second overload: template<typename T> void overloaded (const T& x); template<typename T> void overloaded (const T&& x); // ^^^^^. The following diagram illustrates the relationships between the. @whY because for an rvalue a const reference is not an exact match for template deduction. I expect that when using a temporary instance of a Wraper object, the conversion operator defined for rvalue will always be used. The "my target must be copy-constructable" requirement of std::function is due to its own requirement of being copy-constructable. Nothing is being turned into a lvalue. [3] Finally, this temporary variable is used as the value of the initializer. @MikeMB the standard rarely prevents compilers from inserting for (int i = 0; i < 1 billion; ++i) at arbitrary points. Lvaluesand Rvalues Lvalues and rvalues aren’t really language features. This is a helper function to allow perfect forwarding of arguments taken as rvalue references to deduced types, preserving any potential move semantics involved. "When the function parameter type is of the form T&& where T is a template parameter, and the function argument is an lvalue of type A, the type A& is used for template argument deduction. 14159, are rvalues. What makes rvalue references a bit difficult to grasp is that when you first look at them, it is not clear what their purpose is or what problems they solve. Zero or one conversion from the following set: lvalue-to-rvalue conversion, array-to-pointer conversion, and function-to-pointer conversion. Under the conditions specified in [dcl. lval]/3. So in this case, this should be a prvalue B* and perfectly bindable to B*&&. ; // not legal, so no lvalue. Second (and you probably missed that), const char* is converted to a rvalue std::string via the const char* non-explicit constructor of std::string (# 5 in the link). (If you insist to know, the result of subscripting into an rvalue array used to be an lvalue in C++11, but is an xvalue in C++14 - issue 1213 . void f2(int&& namedValue){. Done. Unless encountered in unevaluated context (in an operand of sizeof, typeid, noexcept, or decltype), this conversion effectively copy-constructs a temporary object of type T using the original glvalue as the. Whenever an lvalue appears in a context where an rvalue is expected, the lvalue is converted to an rvalue; see 4. So you can write a couple of convert functions . The biggest difference between a C++03 reference (now called an lvalue reference in C++11) is that it can bind to an rvalue like a temporary without having to be const. g. The return of a new is a prvalue not an lvalue, because you cannot write: new T (arg) =. From reference - value categories. If t returns a local variable, then you get a dangling reference, since that variable is gone after the call. If you pass an argument to a reference type parameter (whether lvalue or rvalue reference), the object will not be copied. rvalue rvalue lvalue. 3. If T is a non-class type, the type of the prvalue is the cv-unqualified version of T. void func (unsigned int& num) this function need quote type. There is no implicit conversion as suggested in the title, the reference binds directly to the. The expression 0 is. An lvalue (locator value) represents an object that occupies some identifiable location in memory (i. An lvalue is an expression that designates (refers to) an object. The r-value reference is a reference to the original object, so converting it to a l-value reference will just make a reference to the original object. Assignment involving scalar types requires a modifiable lvalue on the left hand side of the assignment operator. const foo&& can only bind to an rvalue, but const foo& can bind to both lvalues and rvalues. An lvalue (pronounced “ell-value”, short for “left value” or “locator value”, and sometimes written as “l-value”) is an expression that evaluates to an identifiable object or function (or bit-field). Note that there is one exception: there can be lvalue const reference binding to an rvalue. The right constructors for the first two cases are called. In any assignment statement “lvalue” must have the capability to store the data. Conversion operators are treated inconsistentlyAn lvalue can be converted to a value of an expression through lvalue conversion. B. begin(), dataBlock. Sorted by: 17. HI Enlico, Thank's for the awesome answer, now I have a clearer idea of how to use RValue and LValue references. 2) If target-type is an rvalue reference type, static_cast converts the value of glvalue, class prvalue, or array prvalue (until C++17) any lvalue (since C++17) expression to xvalue referring to the same object as the expression, or to its base sub-object (depending on target-type). Compiled with "g++ -std=c++0x". The third constructor is called move constructor. But I do not see how it is related to the warning, please explain. int a =5; int b = 3; int c = a+b; the operator + takes two rvalues. I think it's reasonable to call print_stream like this:. Another example of conversion: int c = 6; &c = 4; //ERROR: &c is an rvalue On the contrary you cannot convert an rvalue to an lvalue. The goal was providing a function that both accepts lvalue and rvalue references, I did not want to write two functions or to really care about lvalue/rvalue on the caller's side. Therefore, I thought of providing some macro/function that wraps a parameter so it can be passed whether it's an l/rvalue - in this case get_addr. For details, see Set C++ compiler and build properties in Visual Studio. In C, (time_t) { time (NULL) } is a compound literal C99, initialized by the return value of time. 1. You need to pass in an rvalue, and for that you need to use std::move: Insert(std::move(key), Value()); // No compiler error any more I can see why this is. universal reference. @banana36 With that function, calling foo(std::move(my_ptr_var)) wont actually pass ownership. Per paragraph 8. Or the compiler could convert said references to pointers, push a pointer on the stack, pop the identical pointer off, and call it std::move. Assuming C++11 or later:. 3. Rvalues of type int cannot bind to int& (aka an lvalue reference to int) so the compiler rejects your code. int a = 2, b = 3; // lvalues int && temp = a + b; // temp is constructed in-place using the result of operator+(int,int) The case with func. I think I'm missing something basic regarding the lvalue-to-rvalue standard conversion. "3" is an integer, and an rvalue. Thus, if the thickness is 1 inch, and the K-value is 0. 2. ; The value of i is implicitly converted to integer by constructor. As well as the potentially dangling lvalue references you've identified, this led in C++03 to the situation where operator<< on a temporary ostream could be called with a char (member function operator) but not with a string (free operator); C++11 fixes this with free operator overloads for rvalue references and rvalue *this overload for member. When you pass a string literal a temporary std::string will be constructed from the string literal. There is no lvalue-to-rvalue conversion in this scenario. 1) Is actually not so arbitrary. The choice of copy or move constructor only occurs when passing an object by value. Arrays can only be lvalues, and whenever they are used in an lvalue they decay to a pointer to the first element. int rVal () { return 0; }. the name of a variable, a function, a template parameter object (since C++20), or a data member, regardless of type, such as std::cin or std::endl. b is just an alternative name to the memory assigned to the variable a. The reference could be bound to the result of the implicit conversion if it wasn't non-const because the result of that implicit conversion is an rvalue i. Here's what happens when we call move with lvalue: Object a; // a is lvalue Object b = std::move (a); and corresponding move instantiation:3. 1 for an lvalue-to-rvalue conversion. If the function argument is an rvalue, the compiler deduces the argument to be an rvalue reference. In short: every named object is Lvalue, and even if v is reference to Rvalue you need to use move to force move ctor to be called. For example, this means, that when rvalue reference is passed to a function, an lvalue reference overload will be chosen: T&& x=T(); f(x); Links: C++ lvalue rvalue xvalue glvalue prvalue Value categories in C++ 17 Value categories. rvalue (until C++11) / prvalue (since C++11)Since you are giving your rvalue reference a name in the parameter list, it indeed becomes an lvalue. The reference declared in the above code is lvalue. The type after conversion is not qualified by either const or volatile. When you have a named value, as in . This is a follow-on question to C++0x rvalue references and temporaries. 3. Lvalues and xvalues can be of incomplete types, but (prvalue) rvalues must be of complete types or void types. This is a follow-on question to C++0x rvalue references and temporaries. 1) does not accept such code (makes perfect sense). using g++. e. Here is a silly code that doesn't compile: int x; 1 = x; // error: expression must be a modifyable lvalue. 3) If new_type is an rvalue reference type, static_cast converts the value of expression to xvalue. In the case of object constructing is true but in the case of object assigning is false. Lvalue to rvalue conversion A glvalue of any non-function, non-array type T can be implicitly converted to a prvalue of the same type . Lvalue to rvalue conversion. A modifiable lvalue may be used as the first (left) argument of the built-in assignment operator. Say we want to steal from an lvalue: int main() { Holder h1(1000); // h1 is an lvalue Holder h2(h1); // copy-constructor invoked (because of lvalue in input) } This will not work: since h2 receives an lvalue in input, the copy constructor is being triggered. Abbreviations of constructors, operators and destructors: Dc — Default constructorA{} is always an rvalue per [expr. An lvalue is an expression that yields an object reference, such as a variable name, an array. They are declared using the ‘&’ before the name of the variable. That is the whole point of references. The result of std::move is an xvalue [1], which is a type of glvalue; and converting a glvalue to an lvalue reference with reinterpret_cast appears to be allowed by the wording. std::move performs a static_cast to an rvalue reference type and returns the rvalue reference. ASCII defines a set of characters for encoding text in computers. 23. To answer the titular question, "rvalue reference" is a kind of type, while "xvalue" is a kind of expression. 2. @user2308211: I think what I might have meant to say (back when I didn't know any C++!) was that vec4(). However, it's type will be const std::string or std::string depending on the choice of const in the MyPair type. 2 Answers. e. When such a binding occurs to a prvalue, a temporary object is materialized. You can't assign to an object that is const. Every lvalue is, in turn, either modifiable or non-modifiable. The object identified by an xvalue expression may be a nameless temporary, it may be a named object in scope, or any other kind of object, but if used as a function argument, xvalue will always bind to the rvalue reference overload if available. The lvalue is. Move semantics relies on a new feature of C++11, called rvalue references, which you'll want to understand to really appreciate what's going on. int&& x = 3; x is now an lvalue. A nice feature of this heuristic is that it helps you remember that the type of an expression is independent of. 5. std::move doesn't move anything, it just converts the type of the expression to an rvalue reference. But one important rule is that: one can. Temporary lifetime extension does not pass through functions so there is no way to get a lvalue from the rvalue you pass to the function. it is a reference only to rvalues. It is a forwarding reference. Lvalue references and rvalue references are syntactically and semantically similar, but. An obvious example of an lvalue expression is an identifier with suitable type and storage class. 3. Visual Studio warning disappears if one removes std::move. about undefined behaviorIf T is a reference an lvalue-reference type, the result is an lvalue; otherwise, the result is an rvalue and the lvalue-to-rvalue (conv. 16. Therefore it makes sense that they are mutable. An lvalue or xvalue is an expression that refers to such an object. Allowing both rvalues and lvalues to be bound to an lvalue reference makes that impossible. An lvalue-to-rvalue conversion (converting the name of the object x to its value 2. This type of static_cast is used to implement move semantics in std::move. A reference (“lvalue reference” since C++11) is a type of C++ variable that can act as an alias to another value. 1, a standard conversion sequence cannot be formed if it requires binding an lvalue reference other than a reference to a non-volatile const type to an rvalue or binding an rvalue reference to an lvalue other than a function lvalue. We provide you with easy how-to’s and step-by-step instructions that provide understanding and guidance for a successful installation process, ensuring professional results. Here is a silly code that doesn't compile: int x; 1 = x; // error: expression. If an l-value could bind to an r-value reference, that would mean the detection I was talking about. That's the pass-by-value case. e. 3. Unscopedenumeration values implicitly convert to integer. So the parameter list for a copy constructor consists of an const lvalue reference, like const B& x . This example might clarify it: 16. You are returning a copy of A from test so *c triggers the construction of a copy of c. There is no implicit conversion as suggested in the title, the reference binds directly to the. When you create a std::reference_wrapper<int> and pass it in, rvalues of that type can convert to int&. int & a = b * 5 is invalid. That being said, and assuming you don't want to overload doStuff (otherwise see Hinnant's answer), you can write a utility. I would respect the first compiler more, it is at least honest with its inefficiency. 1/2 (your. arg the expression (it is an expression at lines 3 and 4) has type int and value category "lvalue". (for user-defined types): rvalue or lvalue?. An lvalue may be used to initialize an lvalue reference; this associates a new name with the object identified by the expression. — Otherwise, the reference shall be an lvalue reference to a non-volatile const type (i. The addition operator + (and all other binary operators) requires both operands to be rvalue, and the result is rvalue. 6) An lvalue (until C++11) glvalue (since C++11) expression of type T1 can be converted to reference to another type T2. lvalueとrvalueとは いずれもオブジェクトだ 。. 1 is rvalue, it doesn't point anywhere, and it's contained within lvalue x. If t returns by rvalue reference, you obtain a reference to whatever was returned. In C++, an rvalue is a temporary object that does not have a stable location in memory. func) standard conversions are performed on the the expression v. Related reference: “Pointers” on page 114. This distinction is very important and seems to be overlooked by most when introduced to the topic. rvalue — The expression that refers to a. Rvalue references work in principle similarly to Lvalue references: We declare them by writing the data type of the rvalue followed by && and an identifier. But an rvalue reference can't bind to an lvalue because, as we've said, an rvalue reference refers to a value whose contents it's assumed we don't need to preserve (say, the parameter for a move constructor). The purpose of r-value reference parameters is to detect specifically when an object is an r-value. Then I use rvalue reference of class B's this pointer to pass it to A's constructor. @YueZhou Function lvalues may be bound to rvalue references. Write a function template to convert rvalues to lvalues: template<typename T> T &as_lvalue (T &&val) { return val; } Now, use it: deref (&as_lvalue (42)); Warning: this doesn't extend the lifetime of the temporary, so you mustn't use the returned reference after the end of the full-expression in which the temporary was. The compiler will synthesize a move constructor only for such class that doesn't define any of its own copy-control members (copy-constructor, copy-assignment, or destructor), and if all the non- static members can be moved. User-defined conversion function and casting to reference. Otherwise, the type of the prvalue is T. 2. Correct, the epxression T() is always an rvalue for scalar and user-defined types T. Now an lvalue reference is a reference that binds to an lvalue. In return w, the implicitly movable entity w is treated as an rvalue when the return type of the function is RRefTaker as in example three, but it is treated as an lvalue when the return type of the function is Widget && as in example four. 3 -- Lvalue references ), we discussed how an lvalue reference can only bind to a modifiable lvalue. 2) Lvalue of any type T may be converted to an lvalue or rvalue. Forwarding references are very greedy, and if you don't pass in the. Used to move the resources from a source object i. L-Values are locations, R-Values are storable values (i. , with extensions: pointer or reference to a is additionally allowed to be cast to pointer or reference to unambiguous base class (and vice versa) even if the base class is (that is, this cast ignores the private inheritance specifier). If x is a type, then it may be any fundamental, object , or compound type. In the previous lesson ( 12. undefined behavior: The lvalue or xvalue is a nonclass type, qualified by either const or volatile. Both of g and h are legal and the reference binds directly. b is just an alternative name to the memory assigned to the variable a. by unconditionally casting its argument—which might be an lvalue—to an rvalue reference, it enables the compiler to subsequently move, rather than copy, the value passed in Arg if its type is. 2) non-modifiable lvalues, which are const. In k++, the expression k is an l-value (roughly speaking, it has a name), which is its value-category. Put simply, an lvalue is an object reference and an rvalue is a value. Assignment to an rvalue doesn't really make sense, so it should be forbidden. The implicitly defined copy constructor takes an lvalue reference (i. It satisfies the requirements in 4. r-value references are designed to be the subject of a move-constructor or move-assignment. Loosely speaking, think of lvalue as some sort of container, and rvalue as the value contained in the container. The difference between lvalues and rvalues plays a role in the writing and understanding of expressions. 1 Can't make a function accept both rvalue and lvalue references. All lvalues that aren't arrays, functions or of incomplete types can be converted to rvalues. str is a rvalue reference, i. This is what std::move is for. If you wanted to move an rvalue, you’re in luck!14. C Server Side Programming Programming. The output is: Copy constructor with lvalue reference. 1, 4. ) is characterized by two independent properties: a . The quote doesn't say anything about the result of &, which in fact is an rvalue. This is not an rvalue reference. Expressions Each expression in C (an operator with its arguments, a function call, a constant, a variable name, etc) is characterized by two independent. A move constructor and move assignment operator can now. You are returning a copy of A from test so *c triggers the construction of a copy of c. 1. Suppose r is an rvalue reference or non-volatile const lvalue reference to type T, and r is to be initialized by an expression e of type U. Category 4 used to be a bit different in C++11, but I believe this wording is correct for C++14. must error, because you're trying to pass an rvalue argument, std::move(n), to a lvalue reference parameter, T&. In C++03, Boost's Foreach, using this interesting technique, can detect at run-time whether an expression is an lvalue or an rvalue. – super. 11 for the exact listing what the cast can do; what that section doesn't list, it can't do. On the other hand lvalue references to const forbids any change to the object they reference and thus you may bind them to a rvalue. Conversion of a function pointer to void * shall not alter the representation. 「右辺値」「左辺値」というのは 誤訳だ (正確には時代遅れ)、もう一度言うが直ちに脳内から消去するべきである。. (Lvalue-to-rvalue conversions on class types are rare, but do occur in some places in the language, e. Being an lvalue or an rvalue is a property of an expression; that is, every expression is either an lvalue or an rvalue. 0. 3. e. An lvalue does not necessarily permit modification of the object it designates. To convert an lvalue to an rvalue, you can also use the std::move() function. Convert to rvalue references. If you can't, it's usually an rvalue. 5. The terms are somewhat language-specific; they were first introduced in CPL. A pointer is not the kind of thing that can be an rvalue or an lvalue. 12. The compiler will synthesize a move constructor only for such class that doesn't define any of its own copy-control members (copy-constructor, copy-assignment, or destructor), and if all the non- static members. So we declare a variable x: int x = 42; An expression x in this scope is now an lvalue (so also a glvalue). The result is an lvalue if T is an lvalue reference type or an rvalue reference to function type (8. Yes, rvalues are moved, lvalues are copied. Correct. You would need const_cast<char*&> (a) in order to have an lvalue to assign to, and that brings up the next problem. In C++, each expression, such as an operator with its operands, literals, and variables, has type and value. As with all cast expressions, the result is: an lvalue if target-type is an lvalue reference type or an rvalue reference to function type(since C++11) ; an xvalue if target. std::string hello = "hello"; std::string planet. and some other people did a test on their C++ compiler ( please explain ) : says (time_t){time(NULL)} this will still be a rvalue which is opposite to the C. This function takes an lvalue reference and converts it to an rvalue reference. If the C-value is 0. you cannot change the integer 5, fact. There is no lvalue-to-rvalue conversion in this scenario. const A& ), and lvalue-to-rvalue conversion is suppressed when binding lvalue-reference. An rvalue is a prvalue or an xvalue. goo<int> is an lvalue of function type, but expressions of function type are. e. thus, this is legal: string&& s = foo (); // extends lifetime as before s += "bar"; baz (std::move (s)); // move the temporary into the baz function. You. An rvalue is any expression that has a value, but cannot have a value assigned to it. The result is an lvalue if T is an lvalue reference type or an rvalue reference to function type and an xvalue if T is an rvalue reference to object type; otherwise the result is a prvalue. An identifier that refers to an object is an lvalue, but an. 2, and 4.