C array of structures keeps saying initializer element not constant

knobby67 · 04-08-2015, 05:46 AM

Hi All,
after several years working on C++ I need to do some work in C and producing an array of structures is driving me mad, can't believe I've forgotten this so quickly! Basically I have a structure, I then need an array of these, I don't want an array of pointers to the structures just a "list" of these arrays. For my simple example I have

Code:

typedef struct xtime 
{
unsigned int x;
unsigned int y;
}XTIME;

XTIME a = { 10, 10 };
XTIME b = { 5, 5 };


XTIME asstruct[ 2 ] = 
{
a,
b
};

Source/List.c:38: initializer element for `asstruct[0]' is not constant
Source/List.c:40: initializer element for `asstruct[1]' is not constant

Can anyone advise please?
Thanks in advance

edit.
Should add the reason I'm doing it this way is in real code it's a large structure which can be repeated many times. I can't use a list of pointers as a couple of elements need to change each time. So for example the struct might be ITEM A = { details, ...., number, location } all that changes is number and location. It's long winded to fill in the same details eg { "blah, .......} for each element, and a pointer will overwrite the master item for each change.
Thanks again

johnsfine · 04-08-2015, 06:35 AM

The usual C solution to this kind of problem is #define

#define is a horribly flawed construct for that purpose. All the places where #define is the only easy choice in C (despite its flaws) are indications that C++ is a better language.

I can't tell whether #define is your best choice, because I can't see your entire project. But I expect it is the only easy choice from where you are now.

Code:

#define a { 10, 10 }

etc.

Another approach would be to use run time code to initialize the array, rather than compile time. That could be an initializer list if the array is located inside a function (rather than global or static). Or the array could be global an uninitialized and have separate code to initialize it.

BTW, if you use (abuse) #define that way, you may want to have all those #define's and the global definition of that array all isolated in a separate .c file. So the #define's pollute the name space of only that .c file, rather than a wider part of your project. Then the rest of the project could see that array through an extern directive (which should be in a .h file).

knobby67 · 04-08-2015, 07:54 AM

Thanks,
I think the only to do it is some pointer jiggery pokery. But have say rather than item_a, point to item_a1, item_a2, Then most of the structure is the same at set up, but the couple of elements I need to change can be without over writing a single "master element structure.

NevemTeve · 04-08-2015, 08:09 AM

Try this:

Code:

typedef struct xtime {
    unsigned int x;
    unsigned int y;
} XTIME;

static const XTIME a = { 10, 10 };
static const XTIME b = { 5, 5 };

static const XTIME *asstruct[2] = {
    &a,
    &b
};

genss · 04-08-2015, 08:17 AM

just making a and b const should do the trick, i think

why not define those values directly in the array ?
if you plan to change them later in the program by using "a" and "b" as handles, then you are doing it wrong

SoftSprocket · 04-08-2015, 10:17 AM

This will work:

Code:

        XTIME asstruct[] = {
                { 10, 10 },
                { 5, 5 }
        };


        XTIME a = asstruct[0];
        XTIME b = asstruct[1];