Semantics of int (*ptr)();?

Question

int (*ptr)();

Can someone ELI5 what this code snippet does? I find it almost impossible to visually or logically make sense of it.

Also how and when is it used?

See http://stackoverflow.com/questions/1448849/how-to-understand-complicated-function-declarations — David S., May 07 '14 at 00:30
Ptr is a pointer to a function taking no parameters and returning an int, I believe. — David S., May 07 '14 at 00:32
@EJP Thats impossible. The program it belongs to works https://gist.github.com/vivekr98/5898434 — Jebathon, May 07 '14 at 00:54
cdecl.org appears to treat "ptr" as a reserved alias for "pointer". Replacing the name with "foo" causes it to work. — MooseBoys, May 07 '14 at 02:57

aknuds1 · Accepted Answer · 2014-05-07T00:43:32.877

4

It's a pointer, named ptr, to a function that returns an int and takes no arguments (i.e., int ()).

The syntax isn't beautiful, but it's like a function signature, except the second part identifies that this is a pointer to such a function, with a certain name (ptr in this case).

This tutorial might be of help in understanding the principles (excerpt follows):

int (*pt2Function)(float, char, char) = NULL;                        // C

int DoIt  (float a, char b, char c){ printf("DoIt\n");   return a+b+c; }
int DoMore(float a, char b, char c)const{ printf("DoMore\n"); return a-b+c; }

pt2Function = DoIt;      // short form
pt2Function = &DoMore;   // correct assignment using address operator

edited May 07 '14 at 00:43

answered May 07 '14 at 00:32

aknuds1

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As for how you might use it, you might implement two functions that take `void` and return `int` that represent two ways to roll dice. Depending on control flow in the program, you might want to switch to normal or cheating dice. Using a function pointer, you avoid needing an if-else statement in the implementation and can instead assign the pointer directly, e.g. `int CheatingRoll(){...} ... ptr = &CheatingRoll;` – MooseBoys May 07 '14 at 00:39
@MooseBoys lol cheatingdice. I took int (*ptr)(); from a Remote ShellCode Launcher. Close enough – Jebathon May 07 '14 at 00:59

score 1 · Answer 2 · answered May 07 '14 at 00:33

Read it from inside to outside.

(*ptr) - it's a pointer variable named ptr. Replace it with some symbol, say A. int A(); - it's a function taking no arguments and returning int.

Now recall what A is. Combine it. The answer is: this declares a variable, named ptr, which is a pointer to a function that takes no arguments and returns int.

score 0 · Answer 3 · answered May 07 '14 at 00:37

It is a function pointer, which points to a function that returns an int type.

Before calling a function through a pointer, it is necessary to initialize the pointer to actually point to a function.

Here is an example of calling 'atoi()' using a function pointer:

#include <stdio.h>
#include <stdlib.h>

int (*ptr)(const char *nptr) = atoi;

int main()
    {

    printf("%d\n", (*ptr)("42");

    return(0);
    }

Semantics of int (*ptr)();?

3 Answers3