Ran across this cryptic code for C, I got blown away

[G Patel]

Hello,

I ran across the following code, and it blew me away (I've been
programming in C for 3 months now). I've checked tutorials, books,
etc., but I can't find any idioms such as below. Can someone help me
decipher it ?

#include <stdio.h>

typedef int (* f_p)(int); /* 1: what does this do? */
static int k = 0;

int add (int x) { return k + x; } ; /* 2: what is this? ending with ;
? */

f_p addn(int n)
{
k = n;
return add ; /* 3: what is all this? Return value? */
}

int main ( int argv, char** argc ) {

printf("Calling (*addn(12))(13): %d\n",
(addn(12))(13));
printf("Calling (*addn(1))(13): %d\n",
(addn(1))(13));
printf("Calling (*addn(2))(3): %d\n",
(addn(2))(3));

return 0;
}

===

I have a feeling addn returns a pointer that was defined in typedef.
I've never seen such a thing before, but it seems like such a return
value is hard to show in the actual function header so the typedef is
used. As for the rest, I am totally lost.

Thx

 

[Artie Gold]

Hello,

I ran across the following code, and it blew me away (I've been
programming in C for 3 months now). I've checked tutorials, books,
etc., but I can't find any idioms such as below. Can someone help me
decipher it ?

#include <stdio.h>

typedef int (* f_p)(int); /* 1: what does this do? */

It says that `f_p' is a synonym for the type: "a pointer to a function
that takes an `int' as an argument and returns an `int'.

static int k = 0;

int add (int x) { return k + x; } ; /* 2: what is this? ending with ;
? */

It's a stray `;' (an empty statement).

f_p addn(int n)
{
k = n;
return add ; /* 3: what is all this? Return value? */

Yes. It is returning a pointer to the function `add', after setting the
static int `k' to the desired value.

}

The return type is `f_p', as explained above.

int main ( int argv, char** argc ) {

printf("Calling (*addn(12))(13): %d\n",
(addn(12))(13));
printf("Calling (*addn(1))(13): %d\n",
(addn(1))(13));
printf("Calling (*addn(2))(3): %d\n",
(addn(2))(3));

return 0;
}

===

I have a feeling addn returns a pointer that was defined in typedef.
I've never seen such a thing before, but it seems like such a return
value is hard to show in the actual function header so the typedef is
used. As for the rest, I am totally lost.

It's just a fragile fake closure.

HTH,
--ag

 

[Lawrence Kirby]

G Patel wrote:
....

It's a stray `;' (an empty statement).

Statements can only occur within function bodies, this is a syntax error.

Lawrence