Partly initialized array default values in C

[jirik]

Hi, I have a question concerning partly initialized array. I'll give an example of code:

int array[20] = {[10] = 55};

Does compiler grants me any assurance, that other values (except with index 10) will have some particular value? I've read, that when using C99, other values will be 0, but what if I don't specify any standard?

[AdrianH]

Hi, I have a question concerning partly initialized array. I'll give an example of code:

int array[20] = {[10] = 55};

Does compiler grants me any assurance, that other values (except with index 10) will have some particular value? I've read, that when using C99, other values will be 0, but what if I don't specify any standard?

Huh? What do you mean you don't specify any standard?

If you specify an initialiser, even if it is empty (i.e. {}) it will default the uninitialised values to 0 or NULL (NULL doen't necessarly mean 0 on all CPUs).


Adrian

[weaknessforcats]

What kind of code is this??

int array[20] = {[10] = 55};
 

It's not C or C++.

[jirik]

Huh? What do you mean you don't specify any standard?
Adrian

I meant if compile it without -std switch

[jirik]

What kind of code is this??

int array[20] = {[10] = 55};
 

It's not C or C++.

Sorry, I thought it'd be much clearer, if I didn't specify anything else, just this declaration of an array.. I guess, this should be a correct snippet of C code

[jirik]

If you specify an initialiser, even if it is empty (i.e. {}) it will default the uninitialised values to 0 or NULL (NULL doen't necessarly mean 0 on all CPUs).
Adrian

Thanks, this is just what I wanted to hear

[AdrianH]

What kind of code is this??

int array[20] = {[10] = 55};
 

It's not C or C++.

It is C from the C99 standard. You can also initialialise parts of structs in a similar way:

struct A {
  int a;
  char b[30];
  double c;
  char d[30];
};
 
struct A obj = { .b = "Hello",  .d = { [29] = '\0' } };
 

Anything not initialised is initialised to 0 or NULL. So initialising obj.d[29] to '\0' doesn't save you from not initialising obj.d[0-28].

However, if you do not specifiy an initialiser list, the the elements are not initialised UNLESS it is not automatic storage, in which case it is set to 0 or NULL.

I found this to be an interesting read.


Adrian

[weaknessforcats]

It is C from the C99 standard. You can also initialialise parts of structs in a similar way:

Obviously I know only K&R C. Further research reveals Visual Studio.NET 2005 doesn't know about it either although there are various C99 doc pages all marked "unsupported".

I based my comment on Visual Studio failing to compile the snippet as either C or C++.

[AdrianH]

Obviously I know only K&R C. Further research reveals Visual Studio.NET 2005 doesn't know about it either although there are various C99 doc pages all marked "unsupported".

I based my comment on Visual Studio failing to compile the snippet as either C or C++.

Hey, a standard is a standard. If the compiler you are using doesn't conform to it, what can you do but not use the standard or use a compiler that does.


Adrian

[Savage]

Hey, a standard is a standard. If the compiler you are using doesn't conform to it, what can you do but not use the standard or use a compiler that does.


Adrian

I'm starting to hate all that mumbo jumbo about standards

Savage

[AdrianH]

I'm starting to hate all that mumbo jumbo about standards

Savage

Well, it is not so bad. At least the standards are incremental and not just willy nilly. ;)


Adrian