CODEWITHSUNDEEP
c++c++14
user1095108
user1095108

Reputation: 14603

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

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.

Upvotes: 2

Views: 241

Answers (3)

Deduplicator
Deduplicator

Reputation: 45704

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)...

Upvotes: 4

Puppy
Puppy

Reputation: 147036

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.

Upvotes: 1

Christopher Oezbek
Christopher Oezbek

Reputation: 26483

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.

Upvotes: 0

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