How to initialize static member char array with code

Question

I would like to have a static char array member initialized in terms of other static char array members - but the initialization is such that code is necessary. Is this possible?

class fred {
    static char *a;
    static char *b;
    static char c[4];
}

Now a and b will have fixed values, but I want to construct c in terms of them. EG:

fred::a = "1234"
fred::b = "ab"    
strcpy(c, b);
strncat(c, a, 1);

However I can't see anyway to initialize c, other than to make a class for the purpose which is just a char[4], with a constructor that references fred::a and fred::b, and then replace c in fred with an instance of that class - which is awkward when referencing the c char array.

Is there a better way?

Is there some really good reason why you can't use std::string here? — shuttle87
– shuttle87, Commented Jun 19, 2012 at 1:38
I don't use string, because it will be accessed a lot, and I'm assuming (perhaps erroneously) that a naked char array with have better performance. — bandjalong
– bandjalong, Commented Jun 19, 2012 at 1:45
Just stating that should make it clear that's not how you should be doing things. — David Schwartz
– David Schwartz, Commented Jun 19, 2012 at 1:58
@user1425406 Never ever base your performance-related decisions on assumptions, no matter how obvious you think they are. In general sense the compiler is better at generating high-performance code than you are, so just follow "good"/common practices whenever possible and only consider alternatives whenever you have proven*(by a *benchmark) that performance is an issue. Premature optimization is the .... You know the deal. :) — zxcdw
– zxcdw, Commented Jun 19, 2012 at 2:08
@DavidSchwartz - can you please explain? What part of what I'm doing do you think I'm not doing correctly? Using a char array rather than a string? — bandjalong
– bandjalong, Commented Jun 19, 2012 at 2:09

jxh · Accepted Answer · 2012-06-19 02:27:10Z

4

Edit: Originally, I had wilma as a friend of fred, and a static instance of wilma doing the initialization. I have changed the example to have dino declared within fred since the OP said he thought that would be cleaner.

You can create a static instance of a class inside fred whose job is to initialize c for you.

class fred {
    static char *a;
    static char *b;
    static char c[4];
    static struct dino { dino (); } dino_flintstone;
};

char *fred::a;
char *fred::b;
char fred::c[4];
fred::dino fred::dino_flintstone;

fred::dino::dino () {
    fred::a = "1234";
    fred::b = "ab";
    strcpy(fred::c, fred::b);
    strncat(fred::c, fred::a, 1);
}

edited Jun 19, 2012 at 2:27

answered Jun 19, 2012 at 1:43

jxh

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8 Comments

bandjalong Over a year ago

I tried this, and it looks like it works, thanks! I don't quite understand what you mean by calling the class something else if I want to define wilma inside fred (which would indeed be a bit neater).

jxh Over a year ago

@user1425406: Terrific, yabba dabba doo! +1 on your question from me.

zxcdw Over a year ago

@user1425406 He means it just as a joke. There's no real reason or need to rename it if it's a class member. :)

jxh Over a year ago

@user1425406: I modified the example to have dino inside of fred. Regards

Cloud Over a year ago

Hmm, Fred inside Wilma, Dino inside Fred. Double entendre if I ever saw one.

|

Andrew Tomazos · Accepted Answer · 2012-06-19 01:42:36Z

1

Consequtive string literals are concatenated at compile-time...

#define A "1234"
#define B "ab"
fred::a = A;
fred::b = B;    
fred::c = A B;

answered Jun 19, 2012 at 1:42

Andrew Tomazos

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1 Comment

jxh Over a year ago

Doesn't quite work because c becomes { 'a', 'b', '1', '2' } instead of "ab1".

Community · Accepted Answer · 2017-05-23 10:34:26Z

1

You can't in a clean way, but you can use a trick to simulate an static constructor as shown here.

In that static constructor, you can initialize c. This would be a possible implementation:

class fred {
    static char *a = "1234";
    static char *b = "ab";
    static char c[4];
    public:
    fred(){
       strcpy(fred::c, fred::b);
       strncat(fred::c, fred::a, 1);
    }    
}

class Fred_staticInitializer{  
    static fred staticInitializer;  //This does the trick (launches constructor)
}

Hope it helps. Cheers

edited May 23, 2017 at 10:34

CommunityBot

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answered Jun 19, 2012 at 1:42

Edgar Villegas Alvarado

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3 Comments

jxh Over a year ago

This is a good trick if fred is never instantiated by anyone else. It should probably have at least a check to see if it is already initialized.

Edgar Villegas Alvarado Over a year ago

I think the asker can do that with this guideline. But you're totally right

bandjalong Over a year ago

I don't quite understand, won't the fred constructor be called everytime I create a fred instance? This would not be desirable..

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