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| Acmlm's Board - I2 Archive - Programming - Help with a tutorial? |
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Emptyeye Real American Level: 67 Posts: 206/2273 EXP: 2488421 For next: 104451 Since: 05-24-04 From: I DUNNOOOOOOOO!! Since last post: 9 hours Last activity: 4 hours |
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| Okay, yeah, I know, learning in general with a tutorial is not a good idea. But I'm pretty sure I've gotten as far as I'm going to in college, and the book isn't a great help either, so... I'm at the section on classes (I learned everything up to that in school...the tutorial in question can be found here, by the way, my question pertains to section 4.2). Specifically, static members. The following code, which I'll attempt to comment with my understanding of it, has me somewhat baffled: // static members in classes #include < class CDummy { //Creation of class public: //Declaration as public static int n; //Declaration of static member n within the class CDummy () { n++; }; //Seems to be an incrementing constructor ~CDummy () { n--; }; //Seems to be a decrementing destructor }; int CDummy::n=0; //declaration of the variable as 0 int main () { CDummy a; //Makes a a valid class parameter, increments (See above) (1) CDummy b[5]; //Makes b a valid class parameter, increments again (See above) (2) CDummy * c = new CDummy; //?? Seems to create a new instance of the class with a pointer c cout << a.n << endl; //outputs n within a, which should be 7 according to the code, how? delete c; //Deletes the above created instance; presumably decrements the n as well cout << CDummy::n << endl; //?? I read this as "outputs n within CDummy", which is probably wrong. This should be 6. return 0; } So where I need help with is getting in my head how what is output equals 7 and 6, respectively. Once I get that, I should be okay. Thanks! (edited by Emptyeye on 06-12-04 11:41 AM) |
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NightHawk Bob-Omb Level: 39  Posts: 149/621 EXP: 374743 For next: 30028 Since: 03-26-04 From: Switzerland Since last post: 432 days Last activity: 339 days |
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Originally posted by EmptyeyeAt this point, n == 1. After this, it equals 6 (it initializes 5 instances of the class, so n gets incremented 5 times). Yes, it creates a new instance, which c then points to. Also increments n. n now equals 7. Yes, it decrements n. n == 6. I think your problem might be that a static member of a class is shared between all instances of that class. Does that explain it to you? |
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Acmlm Torosu heh Level: 51  Posts: 749/1173 EXP: 981994 For next: 31944 Since: 03-15-04 From: Somewhere that isn't outside of Sherbrooke, Québec, Canada Since last post: 39 days Last activity: 3 hours |
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"CDummy b[5];" should create 5 CDummy variables, incrementing the value by 5 ... while "CDummy * c = new CDummy;" increments by 1, so you get 1 + 5 + 1 = 7, or at least that's how I see it  And then "delete c" calls the destructor and decrements the value, so you get 6 ... n is shared by all instances of the class, since it's static, and keeps track of how many instances are in memory  (I'm 40 seconds late )(edited by Acmlm on 06-12-04 12:35 PM) |
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Emptyeye Real American Level: 67 Posts: 208/2273 EXP: 2488421 For next: 104451 Since: 05-24-04 From: I DUNNOOOOOOOO!! Since last post: 9 hours Last activity: 4 hours |
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| Okay, I get it now. Thanks. I read the b[5] line as "assigns 5 to the b parameter" or "makes parameter b an array 5 characters long" as opposed to "initializes and increments 5 times". The way the two of you explained it makes sense now. |
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NightHawk Bob-Omb Level: 39 Posts: 151/621 EXP: 374743 For next: 30028 Since: 03-26-04 From: Switzerland Since last post: 432 days Last activity: 339 days |
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Originally posted by EmptyeyeI'm glad I could help  I remember what it was like to learn C++, so you have my sympathies |
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