C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  zero initialize a large amount of class members Goto page 1, 2, 3  Next          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message marcel.lanz@gmail.com Guest Posted: Thu Sep 15, 2005 11:00 pm    Post subject: zero initialize a large amount of class members I have to use a class with a lot of members. Most of them are "int" or "bool" or just PODs. I think for one who has to maintain such a class with 100+ members, one will forget one of the members to initialize in the initializer list in a ctor. How can this be done in a better way ? what I tried: // 1st try class B { public: int a; std::string s; int c; }; B b = { 0 }; // works if members are all POD. aborts() here // 2nd try class B { public: int a; std::string s; int c; ... a lot more of them (at least 100) static B zero; }; B B::zero; B b = B::zero; // statics are zero initialized, yep // 3rd try class B { public: int a; std::string s; int c; B():a(0),c(0) {} // with 100 of them, my maintainers will miss one, I'm sure. }; // 4rd try class B { public: struct PODs { int a; int c; }; // again, +100 struct NonPODs { std::string s; }; // some of them PODs pods; NonPODS non_pods; B(){ PODs p = {0}; pods = p; }; // tried B():pods({0}){} }; well the last is somehow suboptimal since one has to know the type of a member. - How should it be done ? - Why can't members be default initialized in declaration class B { public: int a=0; std::string s; int c=0; } here no one would miss a member in the initialization list of a ctor. Marcel [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Yuri Khan Guest Posted: Fri Sep 16, 2005 8:13 am    Post subject: Re: zero initialize a large amount of class members [email]marcel.lanz (AT) gmail (DOT) com[/email] wrote: Quote: I have to use a class with a lot of members. Most of them are "int" or "bool" or just PODs. I think for one who has to maintain such a class with 100+ members, one will forget one of the members to initialize in the initializer list in a ctor. How can this be done in a better way ? Your problem is that you have way too many members. I strongly suspect that they can be factored into groups that correspond to more finely-grained entities. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Rob Guest Posted: Fri Sep 16, 2005 8:15 am    Post subject: Re: zero initialize a large amount of class members [email]marcel.lanz (AT) gmail (DOT) com[/email] wrote: Quote: I have to use a class with a lot of members. Most of them are "int" or "bool" or just PODs. I think for one who has to maintain such a class with 100+ members, one will forget one of the members to initialize in the initializer list in a ctor. I would argue that, if you have a class with 100+ members, you can probably do a bit of factoring. Group your members in some way (probably based on relationships between them) into a set of classes [or structs, and then assemble your monster class as a collection of instances of those classes. For example; struct IntegersIWant { int a,b,c,d,e,f,g; }; struct DoublesIWant { double p,q,r,s,t,u,v,w,x,y,z }; class MyClass { DoublesIWant doubles; IntegersIWant integers; std::string str; MyClass::MyClass() { // str will be initialised using it's default constructor DoublesIWant x = {0}; doubles = x; IntegersIWant i = {0}; integers = i; }; }; If you (say) have a lot of integers, you could use the scheme recursively. The key is building big things from somewhat smaller things which are built from little things that are easy to construct so the bits work correctly and can be put together. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top albrecht.fritzsche Guest Posted: Fri Sep 16, 2005 8:16 am    Post subject: Re: zero initialize a large amount of class members [email]marcel.lanz (AT) gmail (DOT) com[/email] wrote: Quote: I have to use a class with a lot of members. Most of them are "int" or "bool" or just PODs. I think for one who has to maintain such a class with 100+ members, one will forget one of the members to initialize in the initializer list in a ctor. How can this be done in a better way ? Using tools like PClint. Ali [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Matjaz Depolli Guest Posted: Fri Sep 16, 2005 8:46 am    Post subject: Re: zero initialize a large amount of class members First of all, I think having 100+ members in a struct or a class is a bad practice. I can not possibly imagine why someone would need classes that large. Leaving that aside, for PODs I believe = {0} initializer to be the simplest solution. After you add some non PODs I'm not sure anymore. The fact, that you need them all zero initialized, in my view shows that you should rethink your design. On your question of why can't members be initialized in declaration - how would that fit with the multiple ctors? [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Tony Delroy Guest Posted: Fri Sep 16, 2005 12:24 pm    Post subject: Re: zero initialize a large amount of class members Hi Marcel, Your second try isn't too bad, if people remember to assign from the static when their own object isn't also static. You could also create a simple wrapper class for int, double etc that adds the construction behaviour you want. First stab... template <typename T> class Zeroed { public: Zeroed() : x_(0) { } Zeroed(T x) : x_(x) { } operator T&() { return x_; } operator const T&() const { return x_; } private: T x_; }; then class B { public: Zeroed<int> a; std::string b; Zeroed<double> c; }; Use some typedefs if you think it's clearer/easier (I'm indifferent)... typedef Zeroed<int> Int0; // hope you think of a better name... Of course it breaks the usual guidelines about explicit constructors and avoiding casting operators, with all the documented issues, which usually aren't too problematic. You can make it all explicit, have a get method etc. if you don't mind usage being affected. A "heavyweight" solution, which isn't worth the hassle unless you deal in doens of these classes, each with hundreds of members, is automated code generation. At the hacky end, use a tool like ctags to pull out the POD fields from your class definition and generate an initialise_to_zero function from that metadata: perl might be an appropriate tool. More properly, use an introspective preprocessing system, OpenC++ is freely available, and my impression is that it would be relatively easy to write a base meta-class that automatically initialises data members of particular types to zero . There's even the old memset - but then the maintainer needs to know to keep the POD data members together in the class, and where to start and end, which can change and needs maintaining. Error prone and not recommended. More robustly, but with a performance hit, you could adopt a pimpl-like mechanism using calloc to ensure 0-initialised data. Cheers, Tony [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze Guest Posted: Fri Sep 16, 2005 12:25 pm    Post subject: Re: zero initialize a large amount of class members [email]marcel.lanz (AT) gmail (DOT) com[/email] wrote: Quote: I have to use a class with a lot of members. Most of them are "int" or "bool" or just PODs. I think for one who has to maintain such a class with 100+ members, one will forget one of the members to initialize in the initializer list in a ctor. How can this be done in a better way ? what I tried: // 1st try class B { public: int a; std::string s; int c; }; B b = { 0 }; // works if members are all POD. aborts() here Shouldn't. This is the correct way to do it. Of course, you can use any values: B b = { 5, "abc", 10 } ; -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top marcel.lanz@gmail.com Guest Posted: Fri Sep 16, 2005 12:37 pm    Post subject: Re: zero initialize a large amount of class members Thanks for far for the answers. Some of you say that +100 members is "bad design". I agree on that. The struct I use is part of a legacy old system, at the moment I have no choice. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Andrew Koenig Guest Posted: Fri Sep 16, 2005 3:54 pm    Post subject: Re: zero initialize a large amount of class members <marcel.lanz (AT) gmail (DOT) com> wrote Quote: I have to use a class with a lot of members. Most of them are "int" or "bool" or just PODs. I think for one who has to maintain such a class with 100+ members, one will forget one of the members to initialize in the initializer list in a ctor. How can this be done in a better way ? How about putting all the members you want to initialize in an inner structure and value-initialize it? That is: class Large { struct Initialized { int a; int b; // and so on } m; Large(): m(Initialized()) { /* ... */ } }; Now if you add a new member to struct Initialized, it will automatically start out as zero. Of course, you will have to refer to this->m.a instead of this->a, but that's not a big hassle. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Walter Bright Guest Posted: Sat Sep 17, 2005 1:59 pm    Post subject: Re: zero initialize a large amount of class members <marcel.lanz (AT) gmail (DOT) com> wrote Quote: - How should it be done ? A couple ways come to mind: 1) Overload operator new() for the class, and have that implementation of new() zero out the memory. Of course, this will only have an effect for new'd allocations, not stack allocations. 2) Create a global static instance of your class, as in: B bzero; this will default initialize to all 0 all members. Then, as the first line in your constructor: *this = bzero; This won't work if there are virtual functions and if class C derives from B and overrides any virtual functions. It won't work if any members need to be member-initialized by the default constructor to anything but 0. If you use memset(this, 0, sizeof(B)), that won't work if there are any virtual functions. There isn't any easy answer to this for C++. I know I've been bitten by this particular problem several times, and wasted lots of time tracking the problem down. That's why, in the D programming language, all members get default initialized. -Walter www.digitalmars.com C, C++, D compilers "code of the nerds" [ 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: Sat Sep 17, 2005 2:00 pm    Post subject: Re: zero initialize a large amount of class members "Andrew Koenig" <ark (AT) acm (DOT) org> writes: Quote: marcel.lanz (AT) gmail (DOT) com> wrote in message news:1126795481.692658.75960 (AT) z14g2000cwz (DOT) googlegroups.com... I have to use a class with a lot of members. Most of them are "int" or "bool" or just PODs. I think for one who has to maintain such a class with 100+ members, one will forget one of the members to initialize in the initializer list in a ctor. How can this be done in a better way ? How about putting all the members you want to initialize in an inner structure and value-initialize it? That is: class Large { struct Initialized { int a; int b; // and so on } m; Large(): m(Initialized()) { /* ... */ } }; Now if you add a new member to struct Initialized, it will automatically start out as zero. Of course, you will have to refer to this->m.a instead of this->a, but that's not a big hassle. Would not struct Initialized { // ... }; struct Large : private Initialized { Large() : Initialize() { } // ... }; also trigger value-initialization? -- Gabriel Dos Reis [email]gdr (AT) integrable-solutions (DOT) net[/email] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dave Harris Guest Posted: Sun Sep 18, 2005 1:34 pm    Post subject: Re: zero initialize a large amount of class members [email]marcel.lanz (AT) gmail (DOT) com[/email] () wrote (abridged): Quote: I think for one who has to maintain such a class with 100+ members, one will forget one of the members to initialize in the initializer list in a ctor. How can this be done in a better way ? This probably won't help you, but sometimes it's possible to organise them into arrays. class B { enum { a, c, ..., end_ints }; int ints[end_ints]; }; then instead of ++b.c; you'd use: ++b.ints[B::c]; It's now easy to write loops to initialise them, and to generally treat them in a uniform way. The occasions I've had large numbers of variables, I needed the uniform infrastructure anyway. (I used more templates, and named and exploited the enum type more.) -- Dave Harris, Nottingham, UK. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top CodeCracker Guest Posted: Sun Sep 18, 2005 5:38 pm    Post subject: Re: zero initialize a large amount of class members 1. Use a function like InitializeAll() in your class which should be private and called by all your various ctors where this will be required. In that create an array of void pointers consisting of the address of each of your member variables. Loop over them and assign zero value to them according to there type using RTTI. 2. Instead of using int, double etc. use a class for each Int, Doubles etc and in that initialize the value of each to zero depending on there type in the default constructor. One is a joke but another should work. Though the second one is clumsy to start with, once the things are in place it will be easy to add more member variables without any thought to initialize them [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Graeme Prentice Guest Posted: Sun Sep 18, 2005 5:51 pm    Post subject: Re: zero initialize a large amount of class members On 17 Sep 2005 09:59:30 -0400, Walter Bright wrote: Quote: [email]marcel.lanz (AT) gmail (DOT) com[/email]> wrote in message news:1126795481.692658.75960 (AT) z14g2000cwz (DOT) googlegroups.com... - How should it be done ? A couple ways come to mind: 1) Overload operator new() for the class, and have that implementation of new() zero out the memory. Of course, this will only have an effect for new'd allocations, not stack allocations. Writing binary zero to memory isn't the same thing as zero-initialization, but it might be a compromise you're willing to live with as it works on most platforms. Quote: 2) Create a global static instance of your class, as in: B bzero; this will default initialize to all 0 all members. Actually it will first zero initialize the object, then it will default initialize it - which means default construction for a non POD such as this. Default construction will leave POD members uninitialized if not explicitly initialized, but zero-initialization sets all scalar sub-objects to zero. Quote: Then, as the first line in your constructor: *this = bzero; This is the most dependable solution as value initialization is not supported correctly on some compilers. Quote: This won't work if there are virtual functions and if class C derives from B and overrides any virtual functions. Why not? It won't work if the class has any non-static reference members or non-static const members. Quote: It won't work if any members need to be member-initialized by the default constructor to anything but 0. Yes it will. The default constructor for bzero will be called after zero-initialization so it can assign any special values it wants. Quote: If you use memset(this, 0, sizeof(B)), that won't work if there are any virtual functions. This is a bad idea. Quote: There isn't any easy answer to this for C++. I know I've been bitten by this particular problem several times, and wasted lots of time tracking the problem down. That's why, in the D programming language, all members get default initialized. This is fine for D but for C++ the overhead is too great when the initialization isn't wanted - especially in embedded systems, which is why C++ gives the programmer the choice. Interesting how the static a1 object gets copied onto itself in the default constructor in this code. #include <iostream> #include <ostream> #include <string> struct s1 { int d1; int d2; int d3; std::string b; virtual void f1(){} s1() { static s1 a1; *this = a1; std::cout << "n* " << d1; d1 = 53; } }; struct s2 : s1 { void f1(){ std::cout << "n$ " << d1 << ' ' << b << ' ';} }; int main() { s1 a1; s2 a2; a2.f1(); (&a1)->f1(); } Graeme [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Andrei Alexandrescu (See Guest Posted: Sun Sep 18, 2005 5:53 pm    Post subject: Re: zero initialize a large amount of class members Walter Bright wrote: Quote: There isn't any easy answer to this for C++. I know I've been bitten by this particular problem several times, and wasted lots of time tracking the problem down. That's why, in the D programming language, all members get default initialized. I think a better spec is to guarantee that all members have meaningful values when read... see the difference? ) That would work great for C++, too. Andrei [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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