is it possible to convert a function pointer to an integer?

Question

I want to do something like this in a well-defined manner:

struct S
{
  static some_integral_type f() noexcept
  {
    return some_cast<...>(&f);
  }
};

The integer can be signed or unsigned. The result should be the same as the casts of object pointers to uintptr_t or intptr_t produce. AFAIK casts to those are not guaranteed to be possible for function pointers. Even casts to uintmax_t and intmax_t might not work for all I know.

I need this so I can produce a unique "handle" from a function pointer, that I can then use in a switch statement.

If you _really_ have to do this, at least use [`(u)intptr_t`](http://stackoverflow.com/q/1845482/3425536) to avoid any data loss. — Emil Laine, Nov 07 '15 at 20:39
@zenith yes, but function and member function pointers are not object pointers. — user1095108, Nov 07 '15 at 20:39
Unfortunately, no this is not possible in the current C++: http://stackoverflow.com/a/29797882/278842 — Christopher Oezbek, Nov 07 '15 at 20:47
You can put a function or even a member function pointer into the signature of a function template, declare a static variable within that function template, and return the address of that variable (potentially casted to some `intptr_t`). — dyp, Nov 07 '15 at 20:59
@dyp That's what I'm doing now yes, but I've still asked for another solution to avoid the wasted variable. — user1095108, Nov 07 '15 at 21:00

Deduplicator · Accepted Answer · 2015-11-07T21:03:02.643

4

In general, no.

First, member-function-pointers (and member-pointers) are hairy beasts the standard says very little about.
So, let's ignore them for now.

Next, it might not even be possible to round-trip plain old function-pointers through void*, and the standard only defines a type for round-tripping data-pointers through integers, if the implementation supports such:
(u)intptr_t.

Naturally, if your implementation has a large enough integer-type, you can manually do it by reading the pointer as an unsigned char[] and combining it to a single numeric value, the operation then being reversible.

Be aware that equal function-pointers can result in different numbers if you go that route though, as the same pointer can have multiple possible representations (for example padding bytes, non-value-bits, segmentation)...

edited Nov 07 '15 at 21:03

answered Nov 07 '15 at 20:51

Deduplicator

44,692
7
66
118

Be aware that equal function-pointers can result in different numbers if you go that route though... Why? – user1095108 Nov 07 '15 at 20:54
@user1095108: Well, let's go to x86, segmented memory as an example: There can be multiple ways to refer to the same function, with different segment-offset-pairs. Also, there might be padding in it (32bit-offset, 16bit-segment, 2 Byte padding). – Deduplicator Nov 07 '15 at 20:59
1

On POSIX-compliant systems you're guaranteed to be able to convert a function pointer to a void pointer; and vice versa – RamblingMad Nov 08 '15 at 15:20
@Deduplicator check my nonstandard solution please. – user1095108 Nov 08 '15 at 15:23

score 1 · Answer 2 · answered Nov 07 '15 at 20:52

1

You can attempt to select an integer of the same size as the function pointer with a metafunction- the required size being sizeof(void(*)()). However, there's no guarantee that any such integer type exists.

Realistically speaking, casting it to void* or intptr_t is gonna work on pretty much all the mainstream platforms.

answered Nov 07 '15 at 20:52

Puppy

144,682
38
256
465

I could just try with `intmax_t` or `uintmax_t` then. If those don't work, nothing will. – user1095108 Nov 07 '15 at 21:05
1

@user1095108: Actually, some compilers support integers bigger than `intmax_t`, as they couldn't change the latter due to ABI-compatibility. – Deduplicator Nov 07 '15 at 21:06
@Deduplicator How about `unsigned long long` and `long long` then? – user1095108 Nov 07 '15 at 21:21

score 0 · Answer 3 · edited May 23 '17 at 10:28

0

Taken from Can std::hash be used to hash function pointers?

In a previous attempt I attempted to cast the function pointer to void*, which isn't allowed and doesn't compile (see: https://isocpp.org/wiki/faq/pointers-to-members#cant-cvt-memfnptr-to-voidptr for details). The reason is that a void* is a data pointer, while a function pointer is a code pointer.

edited May 23 '17 at 10:28

Community

1
1

answered Nov 07 '15 at 20:49

Christopher Oezbek

23,994
6
61
85

I'm not looking for a hash, as those can collide. That is, 2 different function pointers might produce the same hash. – user1095108 Nov 07 '15 at 20:49
Since you can do equality checks with member function pointers, that should not be the problem. See http://stackoverflow.com/questions/1328238/how-to-hash-and-compare-a-pointer-to-member-function for more info. – Christopher Oezbek Nov 07 '15 at 20:54

is it possible to convert a function pointer to an integer?

3 Answers3