Is there any good way to do it

Discussion in 'C Programming' started by qianz99, Oct 12, 2005.

1. qianz99Guest

Hello I am writing a big program.

I have define a struction

typedef struct
{
int Name;
int Year;
}birth;

Now I'd like to define a sequence of A, such as

Birth A = {1,1990}
Birth B = {1,1991}
.....

I hope to use #define so that A,B can be used as constant.
Can I?
and is it a good way of programming?

Thanks

qianz99, Oct 12, 2005

2. Walter RobersonGuest

birth A = {1,1990};
birth B = {1,1991};

:I hope to use #define so that A,B can be used as constant.
:Can I?

No.

What you can do is

#define B90 {1,1990}
#define B91 {1,1991}

birth A = B90;
birth B = B91;

and use B90 or B91 wherever else in the code that you happen to need
to initialize new variables to those values.

But A and B will be writable. You cannot create an unwritable structure
with any particular contents: the closest you can get is, as in the
above, to define textual substituations that happen to expand to the
values you need.

You could also get fancier with functions that return const pointers
to structures. You could probably even have something like

const birth B90(void) { birth B90_temp = {1,1990}; return B90_temp; }

but returning whole structures tends to make old-time programmers
queasy.

Walter Roberson, Oct 12, 2005

3. Keith ThompsonGuest

Right, the {1,1990} construct can be used in an initializer, but it
can't be used as an expression. C99 has compound literals, but
they're not universally supported yet.
Of course you can:

const birth A = { 1, 1990 };
const birth B = { 2, 1991 };

A and B aren't true "constants"; they're best thought of as read-only
variables. But they're probably suitable for the OP's purposes.

Keith Thompson, Oct 12, 2005
4. tanmoy87544Guest

The answer is correct for C89
Actually, you can define const objects. Except for odd uses of strchr
etc. or by use of casts, attempts to write to them always involves a
constraint violation; in the exceptional cases, it invokes undefined
behaviour. So, they are kind of unwritable.
A definition of a function type which is incompatible with birth(void),
but which is functionally identical.
But using qualified values in C rather than qualified objects just to
make someone queasy!

tanmoy87544, Oct 13, 2005

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