C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  static data members access          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Alexander Mahr Guest Posted: Wed Sep 24, 2003 10:31 pm    Post subject: static data members access Dear Newsgroup, As far as I now static data members can be used to share data between objects of a class but they are for a reason I don't know why only accesable within the file they are declared. In my opinionthis access limitation makes no real sence for it is now impossible to use a decleration file like cclass.h within you declare the class and a file cclass.cpp in which you write the member functions of the class which of couse shall acces and change the static data member? Can anyone tell me why static duration comes with a access limitation? Regards Alexander [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ulrich Eckhardt Guest Posted: Thu Sep 25, 2003 1:38 pm    Post subject: Re: static data members access Alexander Mahr wrote: Quote: As far as I now static data members can be used to share data between objects of a class but they are for a reason I don't know why only accesable within the file they are declared. You messed up the syntax and possibly the difference between declaraion and definition. Here's an example how to get it right: /////////// A.h struct A { // declaration static float hoover; void foo() const; }; void bar(); ////////// A.cpp // definition float A::hoover = 3.14f; void A::foo() { print(hoover); } void bar() { print(A::hoover+1.0f); } // note: doing this is something different static long lag; Ulrich Eckhardt -- Questions ? see C++-FAQ Lite: http://parashift.com/c++-faq-lite/ first ! [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gerhard Menzl Guest Posted: Thu Sep 25, 2003 1:45 pm    Post subject: Re: static data members access Alexander Mahr wrote: Quote: As far as I now static data members can be used to share data between objects of a class but they are for a reason I don't know why only accesable within the file they are declared. In my opinionthis access limitation makes no real sence for it is now impossible to use a decleration file like cclass.h within you declare the class and a file cclass.cpp in which you write the member functions of the class which of couse shall acces and change the static data member? Can anyone tell me why static duration comes with a access limitation? There is no such limitation. Could it be that you have declared the static member in the class definition but forgotten to define it in the implementation file? It is hard to tell without seeing the code that gives you troubles, so please post a stripped-down example that exhibits the problem. Gerhard Menzl [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Thu Sep 25, 2003 3:42 pm    Post subject: Re: static data members access In article <bksbbk$upf$07$1 (AT) news (DOT) t-online.com>, Alexander Mahr <mahralex (AT) gmx (DOT) de> writes Quote: Dear Newsgroup, As far as I now static data members can be used to share data between objects of a class but they are for a reason I don't know why only accesable within the file they are declared. In my opinionthis access limitation makes no real sence for it is now impossible to use a decleration file like cclass.h within you declare the class and a file cclass.cpp in which you write the member functions of the class which of couse shall acces and change the static data member? Can anyone tell me why static duration comes with a access limitation? You are confusing different uses of static. If I write: static int i; At file scope the result will be that i has what is called internal linkage and will not be exposed outside the file in which it is declared. That means that if another file contains a declaration of i (static or not) at file scope it will be a different i. The combination of: int i; in one file and extern int i; in other files is the way to inform the compiler that all these files are sharing the definition of i provided by the first declaration. i is said to have external linkage and will be visible to the linker. class mt { static int i; // rest of class }; is an entirely different beast. It is a pure declaration of i as a member of mt that belongs to the class rather than to individual instances of mt. As a pure declaration there must be a single definition available to the linker. This should be provided by something such as: int mt::i = 0; usually placed in the implementation file for class mt. It would almost certainly be an error to write: static int mt::i = 0; I say almost certainly because I want to protect myself from someone coming up with some bizarre instance where that was the right thing to do (my creative imagination fails to produce such a case). -- Francis Glassborow ACCU 64 Southfield Rd Oxford OX4 1PA +44(0)1865 246490 All opinions are mine and do not represent those of any organisation [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Tonci Tomic Guest Posted: Thu Sep 25, 2003 8:41 pm    Post subject: Re: static data members access "Alexander Mahr" <mahralex (AT) gmx (DOT) de> wrote Quote: Dear Newsgroup, As far as I now static data members can be used to share data between objects of a class but they are for a reason I don't know why only accesable within the file they are declared. Wrong. Static data members are accessible from everywhere. Quote: In my opinionthis access limitation makes no real sence for it is now impossible to use a decleration file like cclass.h within you declare the class and a file cclass.cpp in which you write the member functions of the class which of couse shall acces and change the static data member? Wrong premisse leads to wrong conclusion. Can you write a little example in which you will explain what you want to achieve and that, by your opinion, is not possible? [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Daniel Spangenberg Guest Posted: Thu Sep 25, 2003 9:24 pm    Post subject: Re: static data members access Hello, Alexander Mahr! Alexander Mahr schrieb: Quote: Dear Newsgroup, As far as I now static data members can be used to share data between objects of a class but they are for a reason I don't know why only accesable within the file they are declared. In my opinionthis access limitation makes no real sence for it is now impossible to use a decleration file like cclass.h within you declare the class and a file cclass.cpp in which you write the member functions of the class which of couse shall acces and change the static data member? Can anyone tell me why static duration comes with a access limitation? Regards Alexander It seems that you mix up the different meanings of the keyword "static" and/or the meaning of the member-access descriptors "public", "protected", and "private". Static has several meanings. The meaning of "static" for static data members can be understood as a "storage duration" descriptor. The standard defines static, automatic and dynamic storage durations where static storage duration can be loosely translated as "existing for the duration of the program". (Regrettably a static lifetime duration makes not much sense for functions. But such static member functions behave like a simple non-member function despite that its access is controlled by the class access rights.) It seems that you mix up this meaning of static with static meant as linkage descriptor, which is the (deprecated) way to say that a named entity in namespace scope shall have internal linkage (Which means that it can only be accessed via its name inside the translation unit where it is defined) One example is: static const double pipapo = 123.4556; This meaning of static here is different to that of static data members, because static (data) members always have external linkage and can thus be reached by their name across different translation units. (One exception exists: Such static class members do not have external linkage if the class name itself has not) The conclusion is: Normally static data members can be accessed by name across translation units. It depends only on the access specifier (public, private, protected) of this very member and the rights the caller has whether (s)he is **allowed** to use this name. Hope that helps, Daniel [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gareth Stockwell Guest Posted: Thu Sep 25, 2003 11:34 pm    Post subject: Re: static data members access Alexander, I'm not sure exactly what you're asking here, but I hope that the code snippet below might help to show how static members can be used. //-------------------------- // In a file x.h, declare a class X, which has a static data member, // and a function which accesses that member by returning a reference // to it class X { public: static int& i() { return i_; } private: static int i_; }; // Make sure that the member i_ is initialised somewhere - the most // obvious place would be in the implementation file x.cpp #include "x.h" int X::i_ = 50; // Now use the 'X' class - this code can be in a separate file // e.g. main.cpp, as long as the x.o object file (which contains the // initialisation of X::i_) is included at the linking stage #include "x.h" #include <iostream> int main() { X x1, x2; // Declare two instances of X std::cout << "x1.i = " << x1.i() << std::endl; std::cout << "x2.i = " << x2.i() << std::endl; x1.i() = 99; // Access the static member X::i_ via object x1 // Hey presto, it has changed in x2 also std::cout << "x2.i = " << x2.i() << std::endl; } //-------------------------- This program should print out the following x1.i = 50 x2.i = 50 x2.i = 99 I hope that helps, Gareth "Alexander Mahr" Quote: Dear Newsgroup, As far as I now static data members can be used to share data between objects of a class but they are for a reason I don't know why only accesable within the file they are declared. In my opinionthis access limitation makes no real sence for it is now impossible to use a decleration file like cclass.h within you declare the class and a file cclass.cpp in which you write the member functions of the class which of couse shall acces and change the static data member? Can anyone tell me why static duration comes with a access limitation? Regards Alexander [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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