CODEWITHSUNDEEP
cstructtypedef
knoxgon
knoxgon

Reputation: 1243

How a struct being typedef-ed to multiple names?

I've recently started to study about structs and pointers but there is something I didn't fully understand about the design of a struct. I understand the declaration of the struct i.e typedef struct Alias and its content but I don't understand Get_noAllyp and *no_getOf at the end of the statement. What are they? I couldn't really find a good source either.

typedef struct  Alias {
    char    *s_a_name;
    char    **s_aliases;
    short   *s_dumr;
    int     s_get_sum;
}Get_noAllyp, *no_getOf; /*Here, I don't understand this one. 
                        Where did these two variables come from?
                        And one of them is a pointer.*/

Upvotes: 17

Views: 18033

Answers (3)

axiac
axiac

Reputation: 72425

The code:

struct Alias {
    char    *s_a_name;
    char    **s_aliases;
    short   *s_dumr;
    int     s_get_sum;
}

defines a new data type that has the name Alias and is a struct. The original design of the C language is a bit clumsy here as it requires the struct type names to be always prefixed with the struct keyword when they are used.

This means the code:

struct  Alias {
    char    *s_a_name;
    char    **s_aliases;
    short   *s_dumr;
    int     s_get_sum;
} Get_noAllyp, *no_getOf;

declares the variable Get_noAllyp of type struct Alias and the variable no_getOf of type pointer to struct Alias.

By placing the typedef keyword in front, the identifiers Get_noAllyp and no_getOf become types (and not variables).

Get_noAllyp is the same as struct Alias and no_getOf is the same as struct Alias * (i.e. a pointer to a struct Alias`).

Now you can write:

struct Alias x;
struct Alias *y;

or 

Get_noAllyp x;
no_getOf y;

to declare x as a variable of type struct Alias and y as a variable of type pointer to a struct Alias.

Upvotes: 13

Sourav Ghosh
Sourav Ghosh

Reputation: 134396

Here, there are two typedefs being crated in a short-hand manner. The above typedef can be broken down like

typedef struct  Alias {
    char    *s_a_name;
    char    **s_aliases;
    short   *s_dumr;
    int     s_get_sum;
}Get_noAllyp;                  

typedef struct  Alias * no_getOf;

So,

  • Get_noAllyp represents struct Alias
  • no_getOf represents struct Alias *

Upvotes: 15

Mark Segal
Mark Segal

Reputation: 5560

It defines multiple typedefs, i.e multilple "names" for the same thing, while the second is a pointer to it.

The first one Get_noAllyp is the name given for the struct, while no_getOf represents a pointer to it.

I.e, writing no_getOf is completely the same as writing Get_noAllyp * in function signatures or variable declarations.

Upvotes: 22

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