Why do typedef and struct go together?

[dissectcode]

Hi - I just learned C about two months ago, and I don't get why typedef and struct go hand in hand ! Can someone please explain?

typedef struct t_struct{
int a;
char t;
};
 

thanks...

[donbock]

Hi - I just learned C about two months ago, and I don't get why typedef and struct go hand in hand ! Can someone please explain?

typedef struct t_struct{
int a;
char t;
};
 

You are not required to include "typedef" in structure declarations; but you're right, it is a pretty common idiom. The only incontrovertible advantage I can think of in Standard C for using a typedef name rather than a structure name is that you only have to type one word instead of two. There are some static analysis tools that will check that you used typedef names consistently, but that's beyond the scope of Standard C.

In Standard C, a typedef name is just a synonym for the type it is assigned to. You are allowed to vacillate back and forth between the two names. The only place I can think where a typedef name is preferred is when declaring a function that returns a function pointer: it is a whole lot easier to do this if you have a typedef for the returned function pointer.

Your mileage with C++ may vary. I don't know if that language tightened up typedef usage.

By the way, there is an error in your code snippet. It should be the following (assuming that you meant t_struct to be the name of the typedef). Notice the two entirely equivalent ways to declare variables of this type.

typedef struct S {
    int a;
    char t;
} t_struct;
 
t_struct v1;
struct S v2;

Cheers,
donbock

[gpraghuram]

typedef is very common when declaring function pointers also.
But as specified by the other member it is mostly used for structures to avoid the syntax of specifying struct every time you declare a variable.

raghu

[JosAH]

A bit of history may clarify the horrible typedef syntax: the earliest
C compilers (early 1970s) didn't have the typedef keyword. Repeating
the often complicating struct definitions or function prototypes all
over the place was a mess so they came up with a quick hack: the
typedef keyword acted as a storage class specifier, e. g.

static int t;
...
typedef int t;
 

Instead of defining an identifier t with a type int with a storage
class 'static' another name was defined which serves as an
alternative for the type int; (see the example). The hack caused
quite a complication for the lexical analyzer though because after
it scanned 't' it didn't know what to return to the parser: a type
or an identifier? (compare this after the lex-analyzer had seen
'int'; it would've known to tell the parse what it had just seen).

The scanner would simply return the token 'type-or-ident' and
leave the decision to the parser. The decision became a context
sensitive decision; see the next example:

typedef int t;
t t(t t) { ... }
 

Try to figure out what each 't' means in that text. Many wrong
implementations existed in those day until the Standards
attempted to straighten things out a bit. No matter what, the
typedef feature remains an ugly little hack.

kind regards,

Jos