如何在C中比较两个不同的并集

Question

i'm just wondering how can i compare a union with other integer, my purpose is to code a sort of printf like and i'm managing the simple case like %d/%u/%i considering the size conversion : ll/l/hh/h/j/z, so basically i've the following union :

union all_integer
{
    char                        c;
    signed int                  nb;
    short int                   snb;
    long int                    lnb;
    long long int               llnb;
    size_t                      posnb;

    unsigned char               uc;
    unsigned int                unb;
    unsigned short int          usnb;
    unsigned long int           ulnb;
    unsigned long long int      ullnb;
};

because i can't know before which type i'll need to receive, and after that when i've something like that in case i use %d :

union all_integer u_allint;
u_allint.nb = va_arg(ap, int);

i want to print my data wich's in my union u_allint so i give my union to a simple function for putmydata for exemple :

putdata(union all_integer u_allint)
{
     if (u_allint < 0)
     {
          return (ft_numlen_neg(u_allint));
     }
     if (u_allint > 9)
         return (1 + ft_numlen(u_allint / 10));
     if (u_allint > 0 && u_allint < 10)
          return (1);
     if (u_allint == 0)
        return (1);
     return (0);
}

just suppose that this function is capable to print correctly my data and the fact is i can't do that because i compare an union with an int and even if i try to do an other union in my function and give newunion.nb = 0 for have an union int compare with an union int, i can't compile with this message : invalid operands to binary expression ('union my_union' and 'union my_union'). so i'm pretty sure that i misunderstand something about union, but i didn't find a similar problem in other topic, so am i misunderstand something or maybe taking the problem by the wrong way ? thank for your rep !

Answer 1

Yes - you're confused what a union is.

The union takes up as much memory as the largest item in it; and when you attempt to compare the value of it with >; the compiler doesn't know what to do; since the representation of the int = 0 and the representation of long = 0 may be different (since the int may not have 0'ed the bytes after it).

Using your union in a printf will be interesting too; as the %d tells printf to take the next sizeof(int) bytes that's in the arguments and assume it's an int. As you've got extra data left over; it will be read for the next part of printf - leaving it very confused and printing likely some rubbish (but it won't crash, as you're reading valid memory).

Answer 2

Your request isn't clear.

However it's normal that compiler give you error in both case. For compare union you have to write some code like this one:

all_integer union1;
all_integer union2;

/*To ensure all the unused data of the unions are the same, it's
necessary to set unions, before to use it, at the same value (0 in this case).*/
memset(&union1,0,sizeof(all_integer));
memset(&union2,0,sizeof(all_integer));

...

if(!memcmp(&union1,&union2,sizeof(all_integer))
{
    //Unions are equal.
    ...
}
else
{
    //Unions aren't equal.
    ...
}

Edit: follow the suggestions in this link

Answer 3

I think what you are trying to use "union" for is wrong. At compile time, all types must be resolved and so you can't compare unions with their active member being agnostic.

For me, the easiest (yet quite clean) solution here is to parse the numbers you want to print in the largest possible integer number (unsigned long long int in your case), for instance for your "%d":

unsigned long long int mask = ~((unsigned long long int) 0);
unsigned long long int container;
bool signed = true;
...
int num1 = -548375;
container = (unsigned long long int) (((long long int) num1) & mask); //"long long int" instead of "unsigned long long int" to propagate the bit of sign

Then you can pass your container as argument as well as the sign:

void my_print(unsigned long long int container, bool signed) {
    if (signed) {
        // Print as long long int
    } else if (!signed) {
        // Print as unsigned long long int
    }
}

This way, the function my_print can be generic, you only need to manage the conversion to "unsigned long long int" and the value of signed variable depending on the type.