C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Constructor Initialization          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Michael McKnerney Guest Posted: Fri Jun 27, 2003 5:39 pm    Post subject: Constructor Initialization Hi, It seems I can influence how a base class is initialized beyond the 'normal' manner and I was wondering if someone can tell me why this. Here's my example. class A { public: A(int a=1, int b=1, int c=1) : _a(a), _b(b), _c(c) {} void print() { cout << _a << "," << _b << "," << _c << endl; } private: int _a, _b, _c; }; class B : public virtual A { public: B(int a=1, int b=1, int c=1) : A(a, b, c) {} }; class C : public virtual B { public: C(int a=1, int b=1, int c=1) : B(a, b, c), A(2,2,2) // I can explicitly initialize A here and override the default values {} }; main() { C c; c.print(); } thanks, Mike Back to top Ron Natalie Guest Posted: Fri Jun 27, 2003 5:49 pm    Post subject: Re: Constructor Initialization "Michael McKnerney" <michael.mcknerney (AT) baesystems (DOT) com> wrote Quote: Hi, It seems I can influence how a base class is initialized beyond the 'normal' manner and I was wondering if someone can tell me why this. Because you have virtual base classes. The most derived class must initialize all the virtual bases. The initialization of A specified in the B constructor has no bearing on the creation of C objects. Quote: main() { You must provide an int return here (even in C these days). Back to top Dhruv Guest Posted: Fri Jun 27, 2003 6:02 pm    Post subject: Re: Constructor Initialization On Fri, 27 Jun 2003 17:39:03 +0000, Michael McKnerney wrote: Quote: Hi, It seems I can influence how a base class is initialized beyond the 'normal' manner and I was wondering if someone can tell me why this. Here's my example. [snip]...... For virtual bases, the rule is different. The most derived class is responsible for constructing the virtual base class, as opposed to its immediate descendant in case of non-virtual bases. HTH, -Dhruv. Back to top Michael McKnerney Guest Posted: Fri Jun 27, 2003 6:07 pm    Post subject: Re: Constructor Initialization Dhruv wrote: Quote: On Fri, 27 Jun 2003 17:39:03 +0000, Michael McKnerney wrote: Hi, It seems I can influence how a base class is initialized beyond the 'normal' manner and I was wondering if someone can tell me why this. Here's my example. [snip]...... For virtual bases, the rule is different. The most derived class is responsible for constructing the virtual base class, as opposed to its immediate descendant in case of non-virtual bases. But wait a minute, if I remove the explicit initialization of A in C's constructor list, A will get initialized by B. Quote: HTH, -Dhruv. Back to top Ron Natalie Guest Posted: Fri Jun 27, 2003 6:40 pm    Post subject: Re: Constructor Initialization "Michael McKnerney" <michael.mcknerney (AT) baesystems (DOT) com> wrote Quote: But wait a minute, if I remove the explicit initialization of A in C's constructor list, A will get initialized by B. No it will not. If you remove the explicit initialization, it will invoke the default constructor. If A didn't have a default constructor, then you'd get an error. Back to top Jim Fischer Guest Posted: Fri Jun 27, 2003 7:30 pm    Post subject: Re: Constructor Initialization Michael McKnerney wrote: Quote: Dhruv wrote: On Fri, 27 Jun 2003 17:39:03 +0000, Michael McKnerney wrote: Hi, It seems I can influence how a base class is initialized beyond the 'normal' manner and I was wondering if someone can tell me why this. Here's my example. [snip]...... For virtual bases, the rule is different. The most derived class is responsible for constructing the virtual base class, as opposed to its immediate descendant in case of non-virtual bases. But wait a minute, if I remove the explicit initialization of A in C's constructor list, A will get initialized by B. Nope. If you remove the explicit initialization of A in C's initializer list and then invoke the class C ctor, C(int a=1, int b=1, int c=1) : B(a,b,c) { } the sequence of events (IIRC) is this: 1) Program invokes the C ctor 2) The C ctor implicitly invokes A's default ctor 3) The C ctor invokes 'B(a,b,c)' as specified in the initializer list of the class C ctor. The B ctor does not invoke the A ctor. So the class C ctor -- not the B ctor -- is still the entity that invokes the class A ctor. FWIW, if you rewrite your code a bit, this sequence of events will (IMO) be easier to observe. <example> <code> #include <iostream> using namespace std; class A { public: A(int a=1, int b=1, int c=1) : a_(a), b_(b), c_(c) { cout << "A(" << a << ',' << b << ',' << c << ")n"; } void print() { cout << a_ << ',' << b_ << ',' << c_ << 'n'; } private: int a_, b_, c_; }; class B : public virtual A { public: B(int a=2, int b=2, int c=2) : A(a, b, c) { cout << "B(" << a << ',' << b << ',' << c << ")n"; } }; class C : public virtual B { public: C(int a=3, int b=3, int c=3) // : A(4,4,4), B(a,b,c) : B(a,b,c) { } }; int main() { C c; c.print(); } <output> A(1,1,1) B(3,3,3) 1,1,1 </output> </example> Some side notes: 1) DO NOT use variable names that start with a leading underscore, e.g., '_a'. These names (and some others) are reserved for the implementation's use -- e.g., the standard library, compiler-specific language extensions, the OS's API, etc. So if you want to use underscores to denote variable names, place the underscore at the end of the name, 'a_', and not at the beginning. 2) In this code sample, class A's ctor is *always* invoked before class B's ctor. This sequence holds even if you try (futilely) to specify the base class ctor invocation sequence as B first, then A, in the class C ctor initializer list: C() : B(), A() { } IOW, the compiler ignores the B,A sequence specified above, and implicitly implements C's initializer list as A,B, i.e., C() : A(), B() { } [n.b. Most compilers are nice enough to warn you when they implicitly reorder the initializer list's arguments like this.] -- Jim To reply by email, remove "link" and change "now.here" to "yahoo" jfischer_link5809{at}now.here.com Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language (comp.lang.c++) All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.