C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Is *this an lvalue? Goto page 1, 2, 3, 4, 5, 6, 7  Next          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Matthias Hofmann Guest Posted: Sun Sep 14, 2003 12:02 am    Post subject: Is *this an lvalue? Hello! I have just consulted the standard about the typeid operator and I stumbled into something that strikes me as funny. It's included in the following passage: 5.2.8 Type identification (2) When typeid is applied to an lvalue expression whose type is a polymorphic class type (10.3), the result refers to a type_info object representing the type of the most derived object [...] to which the lvalue refers. (3) When typeid is applied to an expression other than an lvalue of a polymorphic class type, the result refers to a type_info object representing the static type of the expression. So when I apply the typeid operator to an rvalue, I get the static type. This is interesting because I have just learned to use the template pattern to implement virtual constructors in the following way: class MyClass { public: MyClass* Clone() { MyClass* p = DoClone(); ASSERT( typeid( *this ) == typeid( *p ); return p; } private: virtual MyClass* DoClone() = 0; }; The ASSERT uses the typeid operator to ensure that DoClone() is redefined in the most derived class. In order for this to work, typeid must return the dynamic type of both *this and *p. However, I found the following about the this pointer in the standard: 9.3.2 The this pointer (1) In the body of a nonstatic member function, the keyword this is a non-lvalue expression whose value is the address of the object for which the function is called. If the this pointer is an non-lvalue, I wonder wether *this is an lvalue - as otherwise the ASSERT will not work properly. Is this true? Do I always get an lvalue if I dereference a pointer? Best regards, Matthias Hofmann [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top John Potter Guest Posted: Sun Sep 14, 2003 1:39 am    Post subject: Re: Is *this an lvalue? On 13 Sep 2003 20:02:00 -0400, "Matthias Hofmann" <hofmann (AT) anvil-soft (DOT) com> wrote: Quote: 9.3.2 The this pointer (1) In the body of a nonstatic member function, the keyword this is a non-lvalue expression whose value is the address of the object for which the function is called. Since this is a non-lvalue, assignment is not allowed and its address may not be taken. Quote: If the this pointer is an non-lvalue, I wonder wether *this is an lvalue - as otherwise the ASSERT will not work properly. Is this true? Do I always get an lvalue if I dereference a pointer? Yes. 5.3.1/1. John [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gabriel Dos Reis Guest Posted: Sun Sep 14, 2003 11:24 am    Post subject: Re: Is *this an lvalue? "Matthias Hofmann" <hofmann (AT) anvil-soft (DOT) com> writes: Quote: If the this pointer is an non-lvalue, I wonder wether *this is an lvalue - yes, it is. See paragraph on unary *. Quote: Do I always get an lvalue if I dereference a pointer? yes. -- Gabriel Dos Reis <gdr (AT) integrable-solutions (DOT) net> [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Daniel Pfeffer Guest Posted: Tue Sep 16, 2003 12:00 pm    Post subject: Re: Is *this an lvalue? "Matthias Hofmann" <hofmann (AT) anvil-soft (DOT) com> wrote <snip> Quote: 9.3.2 The this pointer (1) In the body of a nonstatic member function, the keyword this is a non-lvalue expression whose value is the address of the object for which the function is called. If the this pointer is an non-lvalue, I wonder wether *this is an lvalue - as otherwise the ASSERT will not work properly. Is this true? Do I always get an lvalue if I dereference a pointer? It depends whether you are in a const method or not. For example: class X { public: // implementation details omitted void foo(const X &x) // X * const this; ('this' is a constant pointer to a variable 'X') { *this = x; // calls X::operator =(const X &); } void foo(const X &x) const // const X * const this; ('this' is a constant pointer to a const 'X') { *this = x; // error! } }; By extension, dereferencing a pointer to a variable object will give you an lvalue. Dereferencing a pointer to a const object will give you a non-lvalue. HTH Daniel Pfeffer [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Attila Feher Guest Posted: Tue Sep 16, 2003 8:32 pm    Post subject: Re: Is *this an lvalue? Daniel Pfeffer wrote: [SNIP] Quote: By extension, dereferencing a pointer to a variable object will give you an lvalue. Dereferencing a pointer to a const object will give you a non-lvalue. Is it so? So far I thought it is an lvalue (object has memory - at least by the C definition) only a const one. Where is it in the standard where it says that it is not an lvalue? What does "non-lvalue" mean here? Isn't that rvalue? Or there is something in between? If yes: what is that called? -- Attila aka WW [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gabriel Dos Reis Guest Posted: Tue Sep 16, 2003 8:36 pm    Post subject: Re: Is *this an lvalue? "Daniel Pfeffer" <pfefferd (AT) hotmail (DOT) co.il> writes: Quote: If the this pointer is an non-lvalue, I wonder wether *this is an lvalue - as otherwise the ASSERT will not work properly. Is this true? Do I always get an lvalue if I dereference a pointer? It depends whether you are in a const method or not. No. "lvalue" does not mean "modifiable". There can be non-modifiable lvalues and there can be modifiable rvalues. Quote: By extension, dereferencing a pointer to a variable object will give you an lvalue. Dereferencing a pointer to a const object will give you a non-lvalue. Wrong. Dereferencing a pointer to "const T" gives an lvalue, albeit non-modifiable. -- Gabriel Dos Reis <gdr (AT) integrable-solutions (DOT) net> [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Falk Tannhäuser Guest Posted: Tue Sep 16, 2003 8:52 pm    Post subject: Re: Is *this an lvalue? Daniel Pfeffer wrote: Quote: [...] dereferencing a pointer to a variable object will give you an lvalue. Yes (a non-const one). Dereferencing a pointer to a const object will give you a non-lvalue. No - it gives a 'const' lvalue. Unlike a non-const lvalue, it can't be assigned to or otherwise modified (except using const_cast or mutable members ...) but it still has an address and an identity. Falk [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top John Potter Guest Posted: Wed Sep 17, 2003 9:45 am    Post subject: Re: Is *this an lvalue? On 16 Sep 2003 08:00:01 -0400, "Daniel Pfeffer" <pfefferd (AT) hotmail (DOT) co.il> wrote: Quote: "Matthias Hofmann" <hofmann (AT) anvil-soft (DOT) com> wrote in message news:bjvta6$7ir$1 (AT) news1 (DOT) nefonline.de... If the this pointer is an non-lvalue, I wonder wether *this is an lvalue - as otherwise the ASSERT will not work properly. Is this true? Do I always get an lvalue if I dereference a pointer? It depends whether you are in a const method or not. For example: class X { public: // implementation details omitted void foo(const X &x) // X * const this; ('this' is a constant pointer to a variable 'X') No, this is an rvalue, not a pointer, with type X*. Pointers are fundamental types where rvalues are never const. &this; // invalid X* const& that(this); // creates and binds to a temporary Quote: { *this = x; // calls X::operator =(const X &); } Yes, *this is an lvalue of type X&. &*this; // valid Quote: void foo(const X &x) const // const X * const this; ('this' is a constant pointer to a const 'X') An rvalue of type X const* as above. Quote: { *this = x; // error! Yes, *this is an lvalue of type X const&. &*this; // valid Quote: } }; By extension, dereferencing a pointer to a variable object will give you an lvalue. Yes. Quote: Dereferencing a pointer to a const object will give you a non-lvalue. No. It gives an lvalue. A reference to const. Lvalue/rvalue and const/non-const are orthogonal. int i; int const& r(i); // An lvalue not usable on left of = X x; X() = x; // An rvalue used on the left of = John [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ron Natalie Guest Posted: Wed Sep 17, 2003 9:46 am    Post subject: Re: Is *this an lvalue? "Attila Feher" <attila.feher (AT) lmf (DOT) ericsson.se> wrote Quote: Daniel Pfeffer wrote: [SNIP] By extension, dereferencing a pointer to a variable object will give you an lvalue. Dereferencing a pointer to a const object will give you a non-lvalue. Is it so? So far I thought it is an lvalue (object has memory - at least by the C definition) only a const one. Where is it in the standard where it says that it is not an lvalue? What does "non-lvalue" mean here? Isn't that rvalue? Or there is something in between? If yes: what is that called? It is an lvalue. 5.3.1 clearly says so (doesn't mention const anywhere). Despite the unfortunate name, not all lvalues are legal left hand side of assignment expressions. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top thp@cs.ucr.edu Guest Posted: Thu Sep 18, 2003 10:08 am    Post subject: Re: Is *this an lvalue? Attila Feher <attila.feher (AT) lmf (DOT) ericsson.se> wrote: + Daniel Pfeffer wrote: + [SNIP] +> By extension, dereferencing a pointer to a variable object will give +> you an lvalue. Dereferencing a pointer to a const object will give +> you a non-lvalue. + + Is it so? So far I thought it is an lvalue (object has memory - at least by + the C definition) only a const one. Where is it in the standard where it + says that it is not an lvalue? What does "non-lvalue" mean here? IIRC, the C++ standard never actually defines "lvalue", and the C standard has blown two attempts at defining "lvalue": - C90 defined "lvalue" as "any expression that denotes an object", which eliminates "*p" when p is 0 -- an absurdity, since compilers must statically determine semantic correctness. - C99 attempted to correct this problem with the definition that an lvalue is "any expression of an object type", which makes "3" an lvalue -- another absurdity. Tom Payne [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gabriel Dos Reis Guest Posted: Fri Sep 19, 2003 10:13 am    Post subject: Re: Is *this an lvalue? [email]thp (AT) cs (DOT) ucr.edu[/email] writes: Quote: IIRC, the C++ standard never actually defines "lvalue", C++ standard defines lvalue by extension (as opposed to comprehension). -- Gabriel Dos Reis <gdr (AT) integrable-solutions (DOT) net> [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Attila Feher Guest Posted: Fri Sep 19, 2003 10:15 am    Post subject: Re: Is *this an lvalue? [email]thp (AT) cs (DOT) ucr.edu[/email] wrote: Quote: IIRC, the C++ standard never actually defines "lvalue", and the C standard has blown two attempts at defining "lvalue": - C90 defined "lvalue" as "any expression that denotes an object", which eliminates "*p" when p is 0 -- an absurdity, since compilers must statically determine semantic correctness. - C99 attempted to correct this problem with the definition that an lvalue is "any expression of an object type", which makes "3" an lvalue -- another absurdity. I guess you might have a different meaning for object than what the C standard means there. C99 3.14 object "region of data storage in the execution environment, the contents of which can represent values". According to this *p if p is zero does not denote an object. Neither does 3, since it is a literal, which does not necessarily have an associated data storage. At least AFAIK. -- Attila aka WW [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ron Natalie Guest Posted: Fri Sep 19, 2003 11:10 am    Post subject: Re: Is *this an lvalue? <thp (AT) cs (DOT) ucr.edu> wrote Quote: IIRC, the C++ standard never actually defines "lvalue", and the C standard has blown two attempts at defining "lvalue": C++ defines it (weakly) in 3.10. What is amusing is that C++ defines rvalue (as things that are not lvalues). C doesn't even use that term. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gabriel Dos Reis Guest Posted: Fri Sep 19, 2003 11:22 am    Post subject: Re: Is *this an lvalue? "Attila Feher" <attila.feher (AT) lmf (DOT) ericsson.se> writes: Quote: thp (AT) cs (DOT) ucr.edu wrote: IIRC, the C++ standard never actually defines "lvalue", and the C standard has blown two attempts at defining "lvalue": - C90 defined "lvalue" as "any expression that denotes an object", which eliminates "*p" when p is 0 -- an absurdity, since compilers must statically determine semantic correctness. - C99 attempted to correct this problem with the definition that an lvalue is "any expression of an object type", which makes "3" an lvalue -- another absurdity. I guess you might have a different meaning for object than what the C standard means there. C99 3.14 object "region of data storage in the execution environment, the contents of which can represent values". I think you missed Tom's point: "3" is an expression, which has type "int", an object type. Quote: According to this *p if p is zero does not denote an object. Neither does 3, since it is a literal, which does not necessarily have an associated data storage. At least AFAIK. lvaluness/rvalueness is a static notion, i.e. a notion derived from static analysis of a program text. Determining whether p is null, most of the time, is a dynamic notion, i.e. requires a dynamic analysis. Therefore, *p as an expression is an lvalue from a static analysis point of view, but is meaningless from a dynamic analysis point of view. -- Gabriel Dos Reis <gdr (AT) integrable-solutions (DOT) net> [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top thp@cs.ucr.edu Guest Posted: Sat Sep 20, 2003 10:48 pm    Post subject: Re: Is *this an lvalue? Gabriel Dos Reis <gdr (AT) integrable-solutions (DOT) net> wrote: + [email]thp (AT) cs (DOT) ucr.edu[/email] writes: + + | IIRC, the C++ standard never actually defines "lvalue", + + C++ standard defines lvalue by extension (as opposed to comprehension). IMHO, it would have been simpler and clearer to define lvalue syntactically. Tom Payne [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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