ceph/src/boost/libs/proto/doc/rationale.qbk

   1 [/
   2  / Copyright (c) 2008 Eric Niebler
   3  /
   4  / Distributed under the Boost Software License, Version 1.0. (See accompanying
   5  / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
   6  /]
   7
   8 [section:rationale Appendix C: Rationale]
   9
  10 [/==================================================]
  11 [section:static_initialization Static Initialization]
  12 [/==================================================]
  13
  14 Proto expression types are PODs (Plain Old Data), and do not have constructors. They are brace-initialized, as follows:
  15
  16     terminal<int>::type const _i = {1};
  17
  18 The reason is so that expression objects like `_i` above can be ['statically
  19 initialized]. Why is static initialization important? The terminals of many embedded
  20 domain-specific languages are likely to be global const objects, like `_1` and
  21 `_2` from the Boost Lambda Library. Were these object to require run-time
  22 initialization, it might be possible to use these objects before they are
  23 initialized. That would be bad. Statically initialized objects cannot be misused
  24 that way.
  25
  26 [endsect]
  27
  28 [/=========================================================]
  29 [section:preprocessor Why Not Reuse MPL, Fusion, et cetera?]
  30 [/=========================================================]
  31
  32 Anyone who has peeked at Proto's source code has probably wondered, "Why all the
  33 dirty preprocessor gunk? Couldn't this have been all implemented cleanly on top of
  34 libraries like MPL and Fusion?" The answer is that Proto could have been
  35 implemented this way, and in fact was at one point. The problem is that template
  36 metaprogramming (TMP) makes for longer compile times. As a foundation upon which
  37 other TMP-heavy libraries will be built, Proto itself should be as lightweight as
  38 possible. That is achieved by prefering preprocessor metaprogramming to template
  39 metaprogramming. Expanding a macro is far more efficient than instantiating a
  40 template. In some cases, the "clean" version takes 10x longer to compile than the
  41 "dirty" version.
  42
  43 The "clean and slow" version of Proto can still be found at
  44 http://svn.boost.org/svn/boost/branches/proto/v3. Anyone who is interested can
  45 download it and verify that it is, in fact, unusably slow to compile. Note that
  46 this branch's development was abandoned, and it does not conform exactly with
  47 Proto's current interface.
  48
  49 [endsect]
  50
  51 [endsect]