structs and elements by reference?

hi, i have the following code:
BYTE is type def as unsigned char.

Code:

// header file: typedef unsigned char BYTE; struct LIGHTS { BYTE set1; // element 0 BYTE set2; // element 1 BYTE set3; // element 2 }; // inside main: LIGHTS lights_array; BYTE size; lights_array.set1 = 31; lights_array.set2 = 32; lights_array.set3 = 33; size = lights_array.set1 ; // get value of 'set1'

--------
problem is, i want to be able to access each element using a number, not the name... e.g.:

Code:

size = lights_array.set1 ; // retruns 31 size = lights_array.set3 ; // returns 33

would become (i know this is wrong) :

Code:

size = lights_array[0] ; // get value of element 0 (set1) return 31 size = lights_array[1] ; // get value of element 1 (set2) return 32 size = lights_array[2] ; // get value of element 2 (set3) return 33

how can i do this???

Joe
#

If you want to mimic an array, then why not use one?

BYTE lights_array[3];

Change the LIGHTS struct a bit

Code:

struct LIGHTS { BYTE set1; // element 0 BYTE set2; // element 1 BYTE set3; // element 2 BYTE& operator[](unsigned id) { if (id == 0) return set1; else if (id == 1) return set2; else if (id == 2) return set2; else throw "LIGHTS::[] : bad id"; } };

Although I'd probably change the struct more than that

i need to use int to access the elements because im going to use a for loop to get the data from each element:

Code:

for(x=1; x<number_of_elements; x++) // skip element 0 { size = lights_array[x]; // doesnt work DoData(size); }

element 0 must NOT be read by the for loop as it is an array (not shown here to simplify the question)

joe
#

the question is very much simplified, the actual struct contains 57 elements of all different types and identifier names, i have 7 structs.

i need code that is "identifier name independent", e.g. i dont have to use the identifier name to access the data.

This enables me to access the same element (element 3 for example) in several different structs without having to type out the element name each time. Also each struct has a different number of elements and each element has a different name.

(if that makes sence)

joe
#

Umm, perhaps you should consider restructuring your code.

so does this mean that it is not possible to index the elements?

Can i get a pointer to the 1st element, then increment the pointer by the sizeof each element until i get to the one i want?

There must be a solution! Windows uses structs all over the place!

joe
#

Quote:

Originally posted by Sang-drax
Change the LIGHTS struct a bit

Code:

struct LIGHTS { BYTE set1; // element 0 BYTE set2; // element 1 BYTE set3; // element 2 BYTE operator[](unsigned id) { if (id == 0) return set1; else if (id == 1) return set2; else if (id == 2) return set2; else throw "LIGHTS::[] : bad id"; } };

Although I'd probably change the struct more than that

Look above and see your answer. Just overload the [] operator. I'm a tad confused with the naming of the variable "size". Wouldn't it be more fitting to be "value"? You know the size, it's a BYTE. :)

Tweaked what Sang-drax posted

Code:

struct LIGHTS { BYTE set1; // element 0 BYTE set2; // element 1 BYTE set3; // element 2 BYTE operator[](unsigned id) { if (id == 0) return set1; else if (id == 1) return set2; else if (id == 2) return set3; else throw "LIGHTS::[] : bad id"; } }; void main () { LIGHTS x; x.set1 = 5; x.set2 = 10; x.set3 = 15; for (int y=0; y < 3; y++) cout << (int) x[y] << " "; }

>void main ()
Naughty naughty, look at this mess. I expect to see your code cleaned up next time I look in here young man. :p

-Prelude

void smoid

>void smoid
My thoughts exactly, int seems happier just before main than void does. And the "void main gestapo" won't go after you. ;)

-Prelude

OK, it worked, many thanks for everyones efforts. The pointer idea doesnt work because the 1st element is an array of 27 ints!! but this code did:

Code:

struct LIGHTS { BYTE set1; // element 0 BYTE set2; // element 1 BYTE set3; // element 2 BYTE operator[](unsigned id) { if (id == 0) return set1; else if (id == 1) return set2; else if (id == 2) return set3; else // throw "LIGHTS::[] : bad id"; return 255; } }; void main () { LIGHTS x; x.set1 = 5; x.set2 = 10; x.set3 = 15; for (int y=0; y < 3; y++) cout << (int) x[y] << " "; }

I have no idea how it works but it does what i need(except the 'throw' because the compiler says its "reserved for future use" (VC++ 1.53)).
-- yes, i do have VC6 and VS.net, but i prefer VC1.53 for messing with small code examples!

I cant find this overloading(?) in any of my books... can someone explain it a bit? if not, nevermiind, i'm moving on again! :D

2 more things...

1/ Is there a 'c' equlivent?

2/ What's the beef between void main() and int main()??? Does it really matter?!

joe
#

>I cant find this overloading(?) in any of my books
You need to get better books then, operator overloading is an important piece of C++ and every book I've seen at least mentions it.

>Is there a 'c' equlivent?
C doesn't support operator overloading or member functions, you would have to use another solution to get the equivalent code in C.

>What's the beef between void main() and int main()???
void main is wrong, anyone who knows this instinctively rears back in horror when they see void main.

>Does it really matter?!
Yes, void main makes the entire program undefined while int main is correct and predictable.

-Prelude

does the OS really check the return value from main() ?

I returned 4 different values:

void, -1, 0, 1

and all have the same effect - nothing!!

So what's the point??

joe
#

Quote:

Originally posted by kybert
does the OS really check the return value from main() ?

I returned 4 different values:

void, -1, 0, 1

and all have the same effect - nothing!!

So what's the point??

joe
#

Its a point of neatness....

On many compilers, the C runtime checks the return of main and then assigns that value to an int.....this int is then passed back to the OS with the method used to terminate the process...

The return is handy for scripting and such as scripts can rely on the return of a process to branch off in the script.....

Also from a more formal view, void main is undefined in std C++....

So if both practicality and the standard say dont use it.....you should avoid it IMO

Quote:

Originally posted by kybert
does the OS really check the return value from main() ?

I returned 4 different values:

void, -1, 0, 1

and all have the same effect - nothing!!

So what's the point??

joe
#

Any section of code that is deemed to be "undefined" is not technically valid, and therefore should be avoided.

>>all have the same effect - nothing!!
Lucky you. But your code might not work elsewhere. Remember, just because it appears to work, doesn't mean it's correct.

This is a popular discussion that comes up a lot. If you want to know more, try a board search.

Here's the C FAQ with some helpful info: http://www.eskimo.com/~scs/C-faq/s11.html

Quote:

Originally posted by kybert

I have no idea how it works but it does what i need(except the 'throw' because the compiler says its "reserved for future use" (VC++ 1.53)).
-- yes, i do have VC6 and VS.net, but i prefer VC1.53 for messing with small code examples!

If your compiler (VC++ 1.53 !!!???... wow :) ) cannot even handle throw, i strongly suggest using another one.

I got VS.net (VC7) but dont use it for testing simple stuff cos its just too much effort... (starting a new project... edit project... etc...)

joe
#

VC++ 1.53!!!! Wow!!! Does it take punch cards and do you need a 5 1/2 Floppy to install it?

Quote:

"reserved for future use"

Definately time to put that program in a museum!

I don't think there ever was a VC++ 1.53, wasn't Microsoft's compiler just called "Microsoft C" back then?

In either case, that thing's just old!

Code:

new char[50];

Error - overloaded operator "new" reserved for future use.

LOL

no it really does exist, and yes it does support new char[]...

lol!!

http://www.iap.uk.net/test/image1.jpg

joe
#