) Thus the return type is also int&. But in your case the operands are different category (123 is a prvalue, a is an lvalue). 68 initial value of reference to non-const must be an lvalue. int & a=fun (); does not work because a is a non-const reference and fun () is an rvalue expression. thus, this is legal: string&& s = foo (); // extends lifetime as before s += "bar"; baz (std::move (s)); // move the temporary into the baz function. That's my best guess anyway. I am still studying what is the reason in essence in compiler why a non-const reference can not be binded to a rvalue. col(0) is an rvalue, not an lvalue. Unlike a reference to non-const (which can only bind to modifiable lvalues), a reference to const can bind to modifiable lvalues, non-modifiable lvalues, and rvalues. So, when you call 'handle_ack_message ()' from this function, you're trying to pass an 'lvalue' to a function that only accepts an 'rvalue'. 3 -- Lvalue references ), we discussed how an lvalue reference can only bind to a modifiable lvalue. The Python-side. Since C++11, two kinds of references have existed - lvalue and rvalue references. By the way, don’t return const values from a function, because you make it impossible to use move semantics. Looks like an X-Y problem. This means the following is illegal: This is disallowed because it would allow us to modify a const variable ( x) through the non-const reference ( ref ). Share. Notes: A non-const or volatile lvalue reference cannot be bound to anrvalue of a built-in type. 3 Answers. A function lvalue; If an rvalue reference or a non-volatile const lvalue reference r to type T is to be initialized by the expression e, and T is reference-compatible with U, reference r can be initialized by expression e and bound directly toe or a base class subobject of e unless T is an inaccessible or ambiguous base class of U. My guess is that this restriction has historical roots in the C++98 standard where rvalues were limited to temporaries, that were fully managed by the compiler. You cannot do that with a non-member function that accepts an lvalue reference. The only difference (that I see) is that x2 knows it only has 3 rows, whereas x1 has a dynamic number of rows. If t were really an out-parameter, it would be passed by pointer: std::string *t. 19 tricky. Non-const reference may only be bound to an lvalue. (I'll comment on all the answers. A operator*(const A& a) const { A res; res. clang++ says: " error: non-const lvalue reference to type 'class foo' cannot bind to a temporary of type 'class foo'" Change foo. const A& x = 1; //compile x = 2; //error! A&& xxx = 1; //compile A& xx = 1; //does not compile. So the parameter list for a copy constructor consists of an const lvalue reference, like const B& x . Both const and non-const reference can be binded to a lvalue. Assume a variable name as a label attached to its location in memory. i have a player class in which i have a function to return a SDL_Rect for the dimensions and the position of my player object: SDL_Rect Player::pos () { return SDL_Rect { mPosX, mPosY, PLAYER_WIDTH, PLAYER_HEIGHT }; } i get the error: "initial value of. decltype(fun()) b=1; Then, your code initializes a const reference with a prvalue of a different (non-reference-related) type. . e. In function 'int main()': Line 15: error: invalid initialization of non-const reference of type 'std::string&' from a temporary of type 'std::string' compilation terminated due to -Wfatal-errors. "The temporary to which the reference is bound or the temporary that is the complete object of a sub-object to which the reference is bound persists for the lifetime of the reference. [3] Finally, this temporary variable is used as the value of the initializer. – The outcome is that the code compiles and works when using MSVC, but doesnt on GCC and Clang, with respective errors: GCC: cannot bind non-const lvalue reference of type 'FuncPtr<bool ()>&' to an rvalue of type 'FuncPtr<bool ()>' Clang: no matching constructor for initialization of 'A'. In your default constructor, you try to assign a temporary value (the literal 0) to it, and since it's a reference type you can't give it a temporary value. 124 Non const lvalue references. This rule does not reflect some underlying. Hot Network QuestionsNon-const references cannot bind to rvalues, it's as simple as that. T and U) are never reference types. The non-const subscript operator returns a non-const reference, which is your way of telling your callers (and the compiler) that your callers are allowed to modify the Fred object. A non-const reference may only be bound to an lvalue? (too old to reply) George 15 years ago Hello everyone, I am debugging MSDN code from,. GetCollider(). E may not have an anonymous union member. const int x = 0; int&& r = x; Here, we don't have an exact match in types: the reference wants to bind to an int, but the initializer expression has type const int. Assuming standard data sizes, you have a reference to 2 bytes of data that you're trying to pass into a function that takes a reference to only 1 byte. Regarding the second question. Actually for simple types you should prefer to pass by value instead, and let the optimizer worry about providing the best implementation. The whole idea of forwarding is to accept any value category and preserve it for future calls. Anything that is capable of returning a constant expression or value. The concepts of lvalue expressions and rvalue expressions are sometimes brain-twisting, but rvalue reference together with lvalue reference gives us more flexible options for programming. Use a const reference, which can be bound to rvalues. rvalue reference versus non-const lvalue. –And I want to make sure template parameter F&& f only accept a non-const lvalue reference. The code above is also wrong, because it passes t by non-const reference. The advantage of rvalue references over lvalue references is that with rvalue references you know that the object referred to is an rvalue. e. The int* needs to be converted to void* firstly, which is a temporary object and could be bound to rvalue-reference. So if the function binds to a rvalue reference, what is seen at the end by the compiler for a certain type T is: std::is_rvalue_reference<T>::value. Just as if you had done: typedef long long type; const type& x = type(l); // temporary! Contrarily an rvalue, as you know, cannot be bound to a non-const reference. Non-compliant compilers might allow a non-const or volatile lvalue reference to be bound to an rvalue. Taking a constant reference to a temporary extends the life of that temporary to as long as the reference lives, allowing you to access any readable state. a nonconst reference could only binded to lvalue. Non-const reference may only be bound to an lvalue. View Site LeadersThe result is an lvalue if T is an lvalue reference type or an rvalue reference to function type (8. I believe the relevant Standard paragraph is 8. I'll try paraphrasing it: In compiler version 2002, if the compiler had the choice if it would transform an object returned by a function to a non-const-reference or another type, it would have chosen the non-const-reference. */ } And called the function with: foo (createVector ()); It'd work fine. In the above code, getStr(); on line 12 is an rvalue since it returns a temporary object as well as the right-hand side of the expression on line 13. Non-explicit constructors have their uses. Of course the left value of an assignment has to be non-const. VS2008 is not too bad as at least it gives a compile warning: warning C4239: nonstandard extension used : 'initializing' : conversion from std::string to std::string & A non-const reference may only be bound to an lvalue A non-const reference may only be bound to an lvalue. It's not against the rules in C++ to use a non-const reference but I think it lends to massive confusion and potential bugs. an lvalue, this constructor cannot be used, so the compiler is forced to use. Reference-compatibility allows extra cv-qualifications in the reference type. You can correct the cases where the message is emitted so that your code is standard compliant. It seems a little inconsistent that adding const to a reference does more than just ban modification. This can only bind to a const reference, and then the objec's lifetime will be extended to the lifetime of the const reference it is bound to (hence "binding"). if a regular constant can be passed like this: In that example, you have an lvalue reference to const. push_back (std::move (obj)); } If caller passes an lvalue, then there is a copy (into the parameter) and a move (into the vector). 4 — Lvalue references to const. Oct 10, 2013 at 22:07. Alex November 11, 2023 In the previous lesson ( 12. @YueZhou Function lvalues may be bound to rvalue references. It got me quite curious. You have two options, depending on your intention. C++ prohibits passing a temporary object as a non-const reference parameter. There is no such thing as a const rvalue, since an rvalue permits a "destructive read". Only local const references prolong the lifespan. Follow edited Apr 5, 2021 at 12:41. An rvalue may be used to initialize a const lvalue [ rvalue] reference, in which case the lifetime of the object identified by the rvalue is extended until the scope of the reference ends. Since the temporary B that's returned by source () is not. Sometimes even for the original developer, but definitely for future maintainers. v = this->v*a. struct Foo{}; { const auto & r = Foo{}; // Foo object not destroyed at semicolon. a. (Binding to a const reference is allowed. Moreover, taking the value string by mutable lvalue reference in the call operator of your MapInserter is not a good idea: you don't want the argument to be modified, so you should either take it by const& or - my advice - take it by value and then move it into the returned pair, like so:A conversion is something like "An lvalue/xvalue/prvalue expression of type T may be converted to an lvalue/xvalue/prvalue expression of type U. My guess is that this restriction has historical roots in the C++98 standard where rvalues were limited to temporaries, that were fully managed by the compiler. . So naming kInt is not deemed an odr-use as long as it. thanks in advance, George. Jun 17, 2016 at 3:16. 3 The initialization of non-const reference. 2) x is a variable of non-reference type that is usable in constant expressions and has no mutable subobjects, and E is an element of the set of potential results of an expression of non-volatile-qualified non-class type to which the lvalue-to-rvalue conversion is applied, or. ). My question is, why a non-const reference can not binded to a rvalue? I think the reason is rvalue is not addressable? And we can not change the rvalue through its reference?Warning: "A non-const reference may only be bound to an lvalue" I've encountered a very weird warning that, although compiles fine on windows, fails to. But result of such conversion is an rvalue, so your reference to non-const cannot be bound to it. x where s is an object of type struct S { int x:3; };) is an lvalue expression: it may be used on the left hand side of the assignment operator, but its address cannot be taken and a non-const lvalue reference cannot be bound to it. Within the body of a non-static member function of X, any id-expression e (e. All (lvalue, rvalue, const, non-const) -> const lvalue. In this context, binding an rvalue to the non-const reference behaves the same as if you were binding it to a const reference. int const&x = 42; // It's ok. is an xvalue, class prvalue, array prvalue or function lvalue and "cv1 T1" is reference-compatible with "cv2 T2", or. Change the declaration of the function GetStart like: Point & GetStart(); Also at least the function GetEnd should be changed like: Point & GetEnd(); You could overload the functions for constant and non-constant objects: You may say, "ha, that's allowed because we want to provide the programmer with some flexibility to do "stupid" things that are in fact not that stupid", which is the reason I can hardly buy because if I buy this excuse, I think that "binding temporary to non-const lvalue reference" can be justified using the same reason. Remember that an rvalue binds to a const lvalue reference, hence if you did: template <typename T> void foo (const T& bar) { /*. The literal 0 is still a poor choice for its default value, considering that 0 is an int, and your type is. There are several (very constrained) circumstances in which the compiler, with language extensions enabled, will still allow a non-const lvalue reference to bind to an rvalue expression. 0; // error: not an lvalue and reference not const int i = 2; double& rd3 = i; // error: type mismatch and reference not const —end example]A non-const reference may only be bound to an lvalue[/quote] 1 Reply Last reply Reply Quote 0. Understand the design first before you implement. Passing by reference, by a const reference wouldn't cost more than passing by value, especially for templates. " I really need some further explanations to solving this: #include "graph1. I dont know if its bug in compiler or is it intended. In fact, if the function returns a &, const& or &&, the object must exist elsewhere with another identity in practice. Consider also this: the language has no way of knowing that the lvalue reference returned by the iterator's operator * or the vector's operator[] refers to something whose lifetime is bound to that of. Sometimes even for the original developer, but definitely for future maintainers. Let's look at std::vector for example: reference at( size_type pos ); const_reference at( size_type pos ) const; Would you look at that. The rules about reference binding are that a non-const lvalue reference may only bind to an lvalue expression. Pass by reference can only accept modifiable lvalue arguments. One const and the other non-const. " I really need some further explanations to solving this: Non-const references cannot bind to rvalues, it's as simple as that. Your code has two problems. However, in VS2010 I seem to be able to do so:. – GManNickG. For details of the rvaluereferences feature, see Using rvaluereferences (C++11). A temporary can only bind to const lvalue references, or rvalue references. g. s. You are returning a copy of A from test so *c triggers the construction of a copy of c. My understanding is that this is largely to avoid breaking several enormous legacy codebases that rely on this "extension. 0f, c); The other similar calls need to be fixed too. c++; Share. –You may not bind a temporary object with a non-constant lvalue reference. @Nater The kind of reference (const/lvalue/rvalue) is irrelevant to the lifetime extension rules. However, since a reference acts identically to the object being referenced, when using pass by reference, any changes made to the reference parameter will affect the argument: #include <iostream. Share. Non-compliant compilers might allow a non-const or volatile lvalue reference to be bound to an rvalue. For non-const references, there is no such extension rule, so the compiler will not allow: bar(std::string("farewell")); because if it did, at the point foo starts, it would only have a reference to the destructed remnants of what was once the farewell string. No, "returning a reference" does not magically extend any lifetime. GetImage (iTileId, pulImageSize, a_pImage ); With the method defined as:This change is required by the C++ standard which specifies that a non-const. Note that for const auto& foo, const is qualified on the auto part, i. ref/6] ). 3 of the C++11 standard: It doesn't allow expressions that bind a user-defined type temporary to a non-const lvalue reference. ) Note that irr doesn't bind to iptr; so any modification on. error: invalid initialization of non-const reference of type 'int&' from an rvalue of type 'int' GCC complains about the reference not being const, namely a constant. 1. — Otherwise, the reference shall be an lvalue reference to a non-volatile const type (i. const unsigned int&), (and its lifetime is extended to the lifetime of the reference,) but can't be bound to lvalue-reference to non-const (i. 9,096 1 33 54. Data members: Never const. Of course since methods can be called on rvalue (and thus prvalue) and those methods may return a reference to the objects they were called on we can easily bypass the silly (1) a reference is only allowed to bind to a lvalue restriction. But since it's a non-const reference, it cannot bind to an rvalue. Its . 3) non-const lvalues can be passed to the parameter. – You may not bind a temporary object with a non-constant lvalue reference. Apr 13, 2017 at 13:00. If P is a forwarding reference and the argument is an lvalue, the type “lvalue reference to A ” is used in place of A for type deduction. There are exceptions, however. Visual C++ is non-compliant with the standard in allowing binding of temporaries to non-const lvalue references. png", 560, 120); int x2 = 560 + 54; int x1 = 560; int y1 = 120; int y2 = 291 + 120; const int * xSolv2 = &x2. The binding rules for rvalue references now work differently in one. operator[] is - either change the return type of the function from Value* to const Value&, or return *cleverconfig[name];The C++ Standard (2003) indicates that an rvalue can only be bound to a const non-volatile lvalue reference. Temporary objects cannot be bound to non-const references; they can only. Fibonacci Series in C++. Overload resolution is usually done in terms of a strict. In the second case, fun () returns a non-const lvalue reference, which can bind to another non-const reference, of course. The compiler will generate it for you. Just like how we don't want the first example to create a temporary int object (a copy of x) and then bind r to that, in the. . Create_moneys () is a function that takes a mutable reference to a pointer. Then you should not have used a forwarding reference. Otherwise. i. @relent95 Yes, whether the id-expression refers to a variable of reference or non-reference type doesn't matter because of what you quoted. If it is not immediately obvious, we can try to check: Therefore, you can opt to change your getPtr ()'s return to a non-const lvalue reference. cannot bind non-const lvalue reference of type to an rvalue of type. What "r-value reference for this` does is allow you to add another alternative: void RValueFunc () &&; This allows you to have a function that can only be called if the user calls it through a proper r-value. i have a player class in which i have a function to return a SDL_Rect for the dimensions and the position of my player object: SDL_Rect Player::pos () { return SDL_Rect { mPosX, mPosY, PLAYER_WIDTH, PLAYER_HEIGHT }; } i get the error: "initial value of. This may sound like a silly question, but I was confused about this following behaviour:. Both const and non-const reference can be binded to a lvalue. Calling a non-static member function of class X on an object that is not of type X, or of a type derived from X invokes undefined behavior. init. In other words, in your first example the types actually do match. There are exceptions, however. It reflects the old, not the new. You switched accounts on another tab or window. A modifiable lvalue is any lvalue expression of complete, non-array type which is not const-qualified, and, if it's a struct/union, has no members that are const-qualified, recursively. For lvalue references, T is deduced to be an lvalue reference, and for rvalue references, T is deduced to be a non-reference. h(418) : warning C4239: nonstandard extension used : 'argument' : conversion from 'XUTIL::xList<T>::iterator' to. Values are fine: auto refInstance = m_map. To reduce template instantiation overhead, I would recommend a more direct implementation:will result in output: Constructor called 42. So you want x to be either an. Good article to understand both lvalue and rvalue references is C++ Rvalue References Explained. Expression like a+b will return some constant. Follow edited May 23, 2017 at 11:55. 3. The page is trying to say that you can write m. If the initializer expression. initial value of reference to non-const must be an lvalue when calling a function. Since there are some non-modifiable lvalues (so we do not always need to modify values through its reference). reference (such as the B& parameter in the B::B (B&) constructor) can only. However, lvalue references to const forbid any change to the object and thus you may bind them to an rvalue. (2) (since C++11) 1) Lvalue reference declarator: the declaration S& D; declares D as an lvalue reference to the type determined by decl-specifier-seq S. GetCollider(); platform1. Add a comment. Now, when printValue(x) is called, lvalue reference parameter y is bound to argument x. C++. So your reference would be referring to the copy of the pointer which wouldn't be modified if you change the Player object. init. A non-const reference may only be bound to an lvalue[/quote] 1 Reply Last reply Reply Quote 0. As a reader pointed out, if g() returned const int instead of const T, the output would be different. Both const and non-const reference can be binded to a lvalue. Furthermore, we don't know if somefunc2 modifies the referenced byte, and if it does then we don't know what should happen to the other byte. Jan 8, 2015 at 8:51. If you are trying to modify the variable 'pImage' inside the method 'GetImage ()' you should either be passing a pointer or a reference to it (not doing both). (Binding to a const reference is allowed. 3 Answers. at returns a proxy (of type std::vector<bool>::reference) that allows you to write the element. Reference is always constant, you can't change reference. However, lvalue references to const forbid any change to the object and thus you may bind them to an rvalue. C++/SDL "initial value of reference to a non-const must be an lvalue". The conversion produces an rvalue (i. New rvalue reference rules were set by the C++ specification. It isn't "hard to spell type"; the compiler will prevent you from using the type explicitly. I have to think for a while-_-!. it is explained that an lvalue is when you can take its address. And this is precisely what the compiler is telling you:. For lvalue-references (that is, the type T&) there isn't. One const and the other non. The ability you're talking about is exactly why forwarding references exist: so that the copy/move aspects. at member function does not return a reference to bool, but a proxy object that can be assigned to and converted to bool. e. , temporary) double but a temporary cannot be bound to a non-const reference. 3 -- Lvalue references ), we discussed how an lvalue reference can only bind to a modifiable lvalue. The simplest fix is to simply store the temporary object somewhere, first: Collider c=player. Sorted by: 6. 3/5, [dcl. As to why using const & or even rvalue && is a bad idea, references are aliases to an object. rval] is not applied (i. "non-const lvalue reference to type 'QByteArray' cannot bind to a temporary of type 'QByteArray'". (non const) lvalue reference and rvalue that also means that you can convert the rvalue into an lvalue and therefore. Improve this question. For non-static member functions, the type of the implicit object parameter is — “lvalue reference to cv X” for functions declared without a ref-qualifier or with the & ref-qualifier — “rvalue reference to cv X” for functions declared with the && ref. a nonconst reference could only binded to lvalue. add (std::move (ct)); } A forwarding reference can bind to both lvalues and rvalues, but. next);. This won't work. I get tired of writing a pair of iterators and make a View class. There are exceptions, however. 2. Actor & actor = get_actor_ref_from_ped (PLAYER::PLAYER_PED_ID ()); ^^^^^^^ reference. However, you don't have double && in your code, you have U && for a deduced U. g. When I discovered this, it seemed odd to me, so I tried. An lvalue reference (commonly just called a reference since prior to C++11 there was only one type of reference) acts as an alias for an existing lvalue (such as a variable). This function receives as a second parameter a const lvalue reference, this is an lvalue and then it calls to copy assignment. This constness can be cast away with a const_cast<>. reference (such as the B& parameter in the B::B (B&) constructor) can only. non-const lvalue reference to type 'const int *' cannot bind to a. The Standard says no. Some older compilers couldn't support the latter in proper way. This program outputs: value = 5 value = 5. 7. non-const reference of type from an rvalue. Basically, VS will allocate the space somewhere and just let the reference point to it, as if it was a reference-to- const without the constness (or in C++11 an rvalue reference). References to non-pointer values make more sense. In such cases: [1] First, implicit type conversion to T is applied if necessary. But since it's a non-const reference, it cannot bind to an rvalue. r can be bound to the conversion result of e or a base class of e if the following conditions are satisfied. In contrast you can bind const references to temporary values as in: std::string const & crs1 = std::string (); However the following is illegal: std::string & rs1 = std::string (); Don't pass int&, it can't be bound to a constant or temporary because those can't be modified - use const int& instead. The rules were already more complex than "if it has a name it's an lvalue", since you have to consider the references. Since the temporary B that's returned by source () is not. If t returns by rvalue reference, you obtain a reference to whatever was returned. CheckCollision (0. I am aware that a non-const reference can't bind to a temporary, but I really don't see why x2 can be considered as one and not x1. Actor actor = get_actor_ref_from_ped (PLAYER::PLAYER_PED_ID ()); Is going to make a copy of the value returned from the function as it calls the copy constructor. lvalue references are marked with one ampersand (&). 1. A reference to the container element is obtained from the iterator with the indirection operator: *hand_it. @KerrekSB: Binding a temporary to a const reference can cause a copy construction. (I) An rvalue had been bound to an lvalue reference to a non-const or volatile type. Am getting cannot bind non-const lvalue reference of type ‘Type&’ to an rvalue of type 'Type'The function returns a pointer, which you are trying to bind to a reference. The unary & operator gets a pointer to a variable. The reference is. RVO may explain this particular quark, but you cannot return a reference to something that doesn't exist. Unless an object is created in the read-only section of a program, it is open for modifiction without adverse consequences. rvalues can be residing on read-only memory spaces where changing them might not be allowable and hence the compiler prohibits them. You can change the parameter type to const char* in or const char* const & in if in won't be modified in UTF8toWide() , or use a named variable instead. Find more info here. The first variant returns a reference to the actual value associated with the key test, whereas the second one returns a reference to the map element, which is a pair<const key_type, mapped_type>, i. A function lvalue; If an rvalue reference or a nonvolatile const lvalue reference r to type T is to be initialized by the expression e, and T is reference-compatible with U, reference r can be initialized by expression e and bound directly to e or a base class subobject of e unless T is an inaccessible or ambiguous base class of U. of the Microsoft compiler. Neither the proxy object, nor the converted bool (which is a prvalue) can be bound to a bool& as you try to do in the return statement. ; T is not reference-related to U. Non-const references cannot bind to rvalues, it's as simple as that. name. In the previous lesson ( 12. The standard has a concept of two types being reference-related. bind to an lvalue. I have looked elsewhere on this site and read similar postings about this error: "initial value of reference to a non-const must be lvalue. So the first fix is to not use the wrong technique here, and accept by an lvalue reference instead:The simple answer is that you are right in essence. Are there specific scenarios where binding temporary to non-const reference is allowed. Overload between rvalue reference and const lvalue reference in template. It can appear only on the right-hand side of the assignment operator. It seems perfectly reasonable for the standard to have been that a temporary is created, and dropped at the end of the function's execution (as you currently have to manually do). A rvalue can be used as const T&, however, the compiler complains about binding a non-const lvalue to a rvalue. Non-const reference may only be bound to an lvalue. e. ctor] A non-template constructor for class X is a copy constructor if its first parameter is of type X&, const X&, volatile X& or const volatile X&, and either there are. Share. The term “identity” is used by the C++ standard, but is not well-defined. an rvalue could bind to a non-const lvalue reference, then potentially many modifications could be done that would eventually be discarded (since an rvalue is temporary), this being useless. Your conclusion happens to be correct, but it doesn't follow from your premise. rvalue references are marked with two ampersands (&&). Only expressions have values. r-value simply means, an object that has no identifiable location in memory (i. The most likely explanation is that the programmer meant to pass by const reference and just forgot the const. You can implement a method and have one "version" for a const object, and one for a non-const object. I recently filed a bug against MSVC which relates to this, where the non-standard behavior caused standard-compliant code to fail to compile and/or compile with a deviant behavior. Case 3: binding to data members. e. -hg. You can call a non-const member function on a temporary because this does not involve binding of a reference. const foo&& can only bind to an rvalue, but const foo& can bind to both lvalues and rvalues. I have looked elsewhere on this site and read similar postings about this error: "initial value of reference to a non-const must be lvalue. However, since Visual C++ allows this as an extension, how does it work? From what I've gathered, the standard does not allow this since you're getting a reference to a temporary variable, which can cause issues. There are two overloads. Other situations call for other needs, but today we will focus on constant references. And an rvalue reference is a reference that binds to an rvalue. 4) const lvalues can be passed to the parameter. , cv1 shall be const), or the reference shall be an rvalue reference. The second version is only allowed non-const rvalues because you can't implicitly strip const from the referencee and rvalue references don't allow lvalues to bind. Because a reference to a non-const value can only bind to a modifiable lvalue (essentially a non. I agree with the commenter 康桓瑋 that remove_rvalue_reference is a good name for this. Saturday, December 15, 2007 4:49 AM. The lifetime extension is not transitive through a. , cv1 shall be const), or the reference shall be an rvalue reference. 3 -- Lvalue references ), we discussed how an lvalue reference can only bind to a modifiable lvalue. Rvalue references should be unconditionally cast to rvalues when forwarding them to other functions: void sink (ConcreteType&& ct) // can only be called on rvalues { collection. However, getPlayer is returning a copy of that pointer. nik7. An expression that designates a bit-field (e. Lvalue reference to const. C++. 71. not an rvalue reference, everything under the sun can be bound by a forwarding reference – Piotr Skotnicki. 255 (i. Similar rationale is applied to the const qualifier.