What does the tilde (~) in macros mean?

On the introduction page of Boost.Preprocessor, an example is given in A.4.1.1 Horizontal Repetition

#define TINY_print(z, n, data) data

#define TINY_size(z, n, unused)                                 \
  template <BOOST_PP_ENUM_PARAMS(n, class T)>                   \
  struct tiny_size<                                             \
      BOOST_PP_ENUM_PARAMS(n,T)                                 \
      BOOST_PP_COMMA_IF(n)                                      \
      BOOST_PP_ENUM(                                            \
          BOOST_PP_SUB(TINY_MAX_SIZE,n), TINY_print, none)      \
  >                                                             \
    : mpl::int_<n> {};

BOOST_PP_REPEAT(TINY_MAX_SIZE, TINY_size, ~) // Oh! a tilde!

#undef TINY_size
#undef TINY_print

An explanation is provided below:

The code generation process is kicked off by calling BOOST_PP_REPEAT, a higher-order macro that repeatedly invokes the macro named by its second argument (TINY_size). The first argument specifies the number of repeated invocations, and the third one can be any data; it is passed on unchanged to the macro being invoked. In this case, TINY_size doesn’t use that data, so the choice to pass ~ was arbitrary. [5]

(emphasis mine)

And there is the note:

[5] ~ is not an entirely arbitrary choice. Both @ and $ might have been good choices, except that they are technically not part of the basic character set that C++ implementations are required to support. An identifier like ignored might be subject to macro expansion, leading to unexpected results.

The tilde, therefore, is simply a place holder because an argument is required, but none is necessary. Since any user-defined identifier wannabe could be expanded, you need to use something else.

It turns out that ~ is pretty much unused (binary negation is not that often called) in comparison to + or - for example, so there is little chance of confusion. Once you’ve settled on this, using it consistently gives it a new meaning to the tilde; like using operator<< and operator>> for streaming data has become a C++ idiom.