Is there any situation where you wouldn't want include guards?

Question

I know why include guards exist, and that #pragma once is not standard and thus not supported by all compilers etc.

My question is of a different kind:<

Answer 1

@sbi already talked about code generation, so let me give an example.

Say that you have an enumeration of a lot of items, and that you would like to generate a bunch of functions for each of its elements...

One solution is to use this multiple inclusion trick.

// myenumeration.td
MY_ENUMERATION_META_FUNCTION(Item1)
MY_ENUMERATION_META_FUNCTION(Item2)
MY_ENUMERATION_META_FUNCTION(Item3)
MY_ENUMERATION_META_FUNCTION(Item4)
MY_ENUMERATION_META_FUNCTION(Item5)

Then people just use it like so:

#define MY_ENUMERATION_META_FUNCTION(Item_) \
  case Item_: return #Item_;

char const* print(MyEnum i)
{
  switch(i) {
    #include "myenumeration.td"
  }

  __unreachable__("print");
  return 0; // to shut up gcc
}

#undef MY_ENUMERATION_META_FUNCTION

Whether this is nice or hackish is up to you, but clearly it is useful not to have to crawl through all the utilities functions each time a new value is added to the enum.