Consider:
#define MAXROW 20
#define MAXCOL 60
typedef State Grid[MAXROW+2] [MAXCOL+2]
typedef enum state {DEAD,ALIVE} State
How do I use typedef
and typedef enum
in C? What does this part of the code do?
typedef enum state {DEAD,ALIVE} State;
| | | | | |^ terminating semicolon, required!
| | | type specifier | | |
| | | | ^^^^^ declarator (simple name)
| | | |
| | ^^^^^^^^^^^^^^^^^^^^^^^
| |
^^^^^^^-- storage class specifier (in this case typedef)
The typedef
keyword is a pseudo-storage-class specifier. Syntactically, it is used in the same place where a storage class specifier like extern
or static
is used. It doesn't have anything to do with storage. It means that the declaration doesn't introduce the existence of named objects , but rather, it introduces names which are type aliases .
After the above declaration, the State
identifier becomes an alias for the type enum state {DEAD,ALIVE}
. The declaration also provides that type itself. However that isn't typedef
doing it. Any declaration in which enum state {DEAD,ALIVE}
appears as a type specifier introduces that type into the scope:
enum state {DEAD, ALIVE} stateVariable;
If enum state
has previously been introduced the typedef
has to be written like this:
typedef enum state State;
otherwise the enum
is being redefined, which is an error.
Like other declarations (except function parameter declarations), the typedef
declaration can have multiple declarators, separated by a comma. Moreover, they can be derived declarators, not only simple names:
typedef unsigned long ulong, *ulongptr;
| | | | | 1 | | 2 |
| | | | | | ^^^^^^^^^--- "pointer to" declarator
| | | | ^^^^^^------------- simple declarator
| | ^^^^^^^^^^^^^-------------------- specifier-qualifier list
^^^^^^^---------------------------------- storage class specifier
This typedef
introduces two type names ulong
and ulongptr
, based on the unsigned long
type given in the specifier-qualifier list. ulong
is just a straight alias for that type. ulongptr
is declared as a pointer to unsigned long
, thanks to the *
syntax, which in this role is a kind of type construction operator which deliberately mimics the unary *
for pointer dereferencing used in expressions. In other words ulongptr
is an alias for the "pointer to unsigned long
" type.
Alias means that ulongptr
is not a distinct type from unsigned long *
. This is valid code, requiring no diagnostic:
unsigned long *p = 0;
ulongptr q = p;
The variables q
and p
have exactly the same type.
The aliasing of typedef
isn't textual. For instance if user_id_t
is a typedef
name for the type int
, we may not simply do this:
unsigned user_id_t uid; // error! programmer hoped for "unsigned int uid".
This is an invalid type specifier list, combining unsigned
with a typedef name. The above can be done using the C preprocessor:
#define user_id_t int
unsigned user_id_t uid;
whereby user_id_t
is macro-expanded to the token int
prior to syntax analysis and translation. While this may seem like an advantage, it is a false one; avoid this in new programs.
Among the disadvantages that it doesn't work well for derived types:
#define silly_macro int *
silly_macro not, what, you, think;
This declaration doesn't declare what
, you
and think
as being of type "pointer to int" because the macro-expansion is:
int * not, what, you, think;
The type specifier is int
, and the declarators are *not
, what
, you
and think
. So not
has the expected pointer type, but the remaining identifiers do not.
And that's probably 99% of everything about typedef
and type aliasing in C.
typedef
defines a new data type. So you can have:
typedef char* my_string;
typedef struct{
int member1;
int member2;
} my_struct;
So now you can declare variables with these new data types
my_string s;
my_struct x;
s = "welcome";
x.member1 = 10;
For enum
, things are a bit different - consider the following examples:
enum Ranks {FIRST, SECOND};
int main()
{
int data = 20;
if (data == FIRST)
{
//do something
}
}
using typedef enum
creates an alias for a type:
typedef enum Ranks {FIRST, SECOND} Order;
int main()
{
Order data = (Order)20; // Must cast to defined type to prevent error
if (data == FIRST)
{
//do something
}
}
A typedef , is just an alias -
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