在C编程中将包含结构的并集的值分配为NULL

Question

I have a union and 2 structures in the following format and I need to set one of them to NULL. For example m = NULL or t = NULL;

 typedef struct
 {
    char *population;
   char *area;

 } metropolitan;


 typedef struct
 {
    char *airport;
    char *type;

 } tourist;


 typedef union
 {
     tourist t;
     metropolitan m;
 } ex;

Answer 1

First of all, the members of your union are not pointers, so it doesn't make all that much sense setting the value to NULL .

Second, the way a union works is that if you set one of the members, you set the others as well. More precisely, all members of a union have the address, but different types. Ie the different types gives you a way to interpret the same area in memory as multiple types at once.

To differentiate from a strutct:

struct a {
    unsigned b;
    char *c;
};

In this case, the appropriate number of bytes are allocated for each of the fields a and s , one after the other.

union a {
    unsigned b;
    char *c;
};

Here, the values of b and c are stored in the same address. Ie if you're setting ab to 0 , a readout from ac would give NULL (numeric 0x0 ).

Answer 2

With unions you need to tell them apart.

So define a structure thus:

typedef enum { MET, TOURIST} Chap;

typedef struct {

   Chap human;
   union {
       tourist t;
       metropolitan m;
   }
} Plonker;

Then you know what is what an then set the appropriate values

ie

Plonker p;
p.human = MET;
p.m.population = NULL;
... etc

Answer 3

Since tourist and metropolitan are struct s, you cannot assign NULL to them.

But, if you declare like

typedef union
{
    tourist* t;
    metropolitan* m;
} ex;

you can do

ex e;
e.t=NULL; //which makes e.m=NULL as well