[Boost-users] Container iteration macro that is equivalent to handcoded iteration?

From: Erik (sigra_at_[hidden])
Date: 2007-10-08 05:44:50

I have a lot of handcoded loops that look something like this:
//////////////////////////////////////////////////////////////////////////////
#include <vector>

void f(float);
void g(std::vector<float> const & v) {
    std::vector<float>::const_iterator const v_end = v.end();
    for (std::vector<float>::const_iterator it = v.begin(); it != v_end; ++it)
        f(*it);
}
//////////////////////////////////////////////////////////////////////////////

I need to replace it with something equivalent but simpler. I tried
BOOST_FOREACH (from boost-1.34):
//////////////////////////////////////////////////////////////////////////////
#include <boost/foreach.hpp>

#include <vector>

void f(float);
void g(std::vector<float> const & v) {
   BOOST_FOREACH(float i, v)
      f(i);
}
//////////////////////////////////////////////////////////////////////////////

But when I compared the assembly output of this with that of the
handcoded version I discovered that the boost version is more
complicated, so I tried to create something better myself, with the
requirement that the generated code is not allowed to be any more
complicated than that of the handcoded loop. The following is the best I
could think of right now:
//////////////////////////////////////////////////////////////////////////////
#define iterate_vector_const(value_type, it, v)                        \
   for                                                                 \
      (struct {                                                        \
          std::vector<value_type>::const_iterator       current;       \
          std::vector<value_type>::const_iterator const end;           \
          value_type const & operator*() {return *current;}            \
       } it = {v.begin(), v.end()};                                    \
       it.current != it.end;                                           \
       ++it.current)                                                   \

#include <vector>

void f(float);
void g(std::vector<float> const & v) {
   iterate_vector_const(float, it, v)
      f(*it);
}
//////////////////////////////////////////////////////////////////////////////

I looked at the difference between my macro and the handcoded loop:
$ diff -U2 iteration-handcoded-g++-4.2.0-O3.S iteration-iterate_vector-g++-4.2.0-O3.S
--- iteration-handcoded-g++-4.2.0-O3.S  2007-10-07 19:38:56.000000000 +0200
+++ iteration-iterate_vector-g++-4.2.0-O3.S     2007-10-07
20:00:53.000000000 +0200
@@ -1,3 +1,3 @@
-       .file   "iteration-handcoded.cc"
+       .file   "iteration-iterate_vector.cc"
        .text
        .align 2
@@ -18,7 +18,7 @@
 .LCFI4:
        movl    8(%ebp), %eax
-       movl    4(%eax), %esi
        movl    (%eax), %ebx
-       cmpl    %ebx, %esi
+       movl    4(%eax), %esi
+       cmpl    %esi, %ebx
        je      .L4
        .p2align 4,,7

>From this I conclude that my macro is as good as a handcoded loop.
(Although I of course wonder why the compiler chose to move "movl   
4(%eax), %esi" further down and swap the parameters of cmpl from "%ebx,
%esi" to "%esi, %ebx".)

But my macro is not as versatile as BOOST_FOREACH. Is there any way to
improve it? In particular, is it possible get rid of the macro parameter
value_type? Is it possible to make it work with other containers than
vectors, as long as they define const_iterator/iterator, begin, end and
value_type? Something like this maybe:
//////////////////////////////////////////////////////////////////////////////
#define iterate_container_const(it, v)                                 \
   for                                                                 \
      (struct {                                                        \
          typeof(v)::const_iterator       current;                     \
          typeof(v)::const_iterator const end;                         \
          typeof(v)::value_type const & operator*() {return *current;} \
       } it = {v.begin(), v.end()};                                    \
       it.current != it.v_end;                                         \
       ++it.current)                                                   \

#include <vector>

void f(float);
void g(std::vector<float> const & v, std::list<float> const & l) {
   iterate_containter_const(it, v)
      f(*it);
   iterate_containter_const(it, l)
      f(*it);
}
//////////////////////////////////////////////////////////////////////////////

Or is it possible to configure BOOST_FOREACH to be as efficient as my macro?