为什么 BOOST_FOREACH 不完全等同于手工编码的？

From 增强文档 http://www.boost.org/doc/libs/1_48_0/doc/html/foreach.html#foreach.introduction.what_is__literal_boost_foreach__literal__,

这会导致近乎最佳的代码生成；的表现 BOOST_FOREACH 通常在等效值的百分之几内 手工编码的循环。

我想使用宏和非标准 typeof 运算符，我们可以生成完全相同的一个。 BOOST_FOREACH 的哪些特性使其不精确？

Edit:

我的版本：

    #define EACH(it,v) \
      for(typeof(v.begin()) it = v.begin();it != v.end(); ++it)

//use this if you want a const_iterator from a non-const container

    #define CONST_EACH(it,v) \
      typedef typeof(v) v_type; \
      typedef const v_type& const_type; \
      for(typeof(static_cast<const_type>(v).begin()) it = static_cast<const_type>(v).begin(); it != static_cast<const_type>(v).end(); ++it)

我正在尝试编写一个没有任何开销的版本。这使用非标准 typeof 并给出迭代器而不是 value_type。我在这里错过了什么吗？

Boost foreach 绝非小事。与海湾合作委员会4.6：

int main()
{
    std::string hello( "Hello, world!" );
    BOOST_FOREACH( char ch, hello )
    {
        std::cout << ch;
    }
    return 0;
}

产生大量调查案件A?B:C.

int main()
{
    std::string hello( "Hello, world!" );

    if (
boost::foreach_detail_::auto_any_t _foreach_col9 = 
boost::foreach_detail_::contain( (hello) , (true ? 0 : 
boost::foreach_detail_::or_( 
boost::foreach_detail_::and_( 
boost::foreach_detail_::not_(
boost::foreach_detail_::is_array_(hello)) , (true ? 0 : 
boost::foreach_detail_::is_rvalue_( (true ? 
boost::foreach_detail_::make_probe(hello) : (hello)), 0))) , 
boost::foreach_detail_::and_( 
boost::foreach_detail_::not_(boost_foreach_is_noncopyable( 
boost::foreach_detail_::to_ptr(hello) , boost_foreach_argument_dependent_lookup_hack_value)) , boost_foreach_is_lightweight_proxy( 
boost::foreach_detail_::to_ptr(hello) , boost_foreach_argument_dependent_lookup_hack_value)))))) {} else if (
boost::foreach_detail_::auto_any_t _foreach_cur9 = 
boost::foreach_detail_::begin( _foreach_col9 , (true ? 0 : 
boost::foreach_detail_::encode_type(hello, 
boost::foreach_detail_::is_const_(hello))) , (true ? 0 : 
boost::foreach_detail_::or_( 
boost::foreach_detail_::and_( 
boost::foreach_detail_::not_(
boost::foreach_detail_::is_array_(hello)) , (true ? 0 : 
boost::foreach_detail_::is_rvalue_( (true ? 
boost::foreach_detail_::make_probe(hello) : (hello)), 0))) , 
boost::foreach_detail_::and_( 
boost::foreach_detail_::not_(boost_foreach_is_noncopyable( 
boost::foreach_detail_::to_ptr(hello) , boost_foreach_argument_dependent_lookup_hack_value)) , boost_foreach_is_lightweight_proxy( 
boost::foreach_detail_::to_ptr(hello) , boost_foreach_argument_dependent_lookup_hack_value)))))) {} else if (
boost::foreach_detail_::auto_any_t _foreach_end9 = 
boost::foreach_detail_::end( _foreach_col9 , (true ? 0 : 
boost::foreach_detail_::encode_type(hello, 
boost::foreach_detail_::is_const_(hello))) , (true ? 0 : 
boost::foreach_detail_::or_( 
boost::foreach_detail_::and_( 
boost::foreach_detail_::not_(
boost::foreach_detail_::is_array_(hello)) , (true ? 0 : 
boost::foreach_detail_::is_rvalue_( (true ? 
boost::foreach_detail_::make_probe(hello) : (hello)), 0))) , 
boost::foreach_detail_::and_( 
boost::foreach_detail_::not_(boost_foreach_is_noncopyable( 
boost::foreach_detail_::to_ptr(hello) , boost_foreach_argument_dependent_lookup_hack_value)) , boost_foreach_is_lightweight_proxy( 
boost::foreach_detail_::to_ptr(hello) , boost_foreach_argument_dependent_lookup_hack_value)))))) {} else for (bool _foreach_continue9 = true; _foreach_continue9 && !
boost::foreach_detail_::done( _foreach_cur9 , _foreach_end9 , (true ? 0 : 
boost::foreach_detail_::encode_type(hello, 
boost::foreach_detail_::is_const_(hello)))); _foreach_continue9 ? 
boost::foreach_detail_::next( _foreach_cur9 , (true ? 0 : 
boost::foreach_detail_::encode_type(hello, 
boost::foreach_detail_::is_const_(hello)))) : (void)0) if (
boost::foreach_detail_::set_false(_foreach_continue9)) {} else for (char ch = 
boost::foreach_detail_::deref( _foreach_cur9 , (true ? 0 : 
boost::foreach_detail_::encode_type(hello, 
boost::foreach_detail_::is_const_(hello)))); !_foreach_continue9; _foreach_continue9 = true)
    {
        std::cout << ch;
    }

    return 0;
}

您可能想要循环的事物类型有很多。使用 c++11 不再需要所有这些技巧，因为您可以使用以下命令循环几乎所有内容

for(auto const &a: something){  .. }

or

for(auto a=begin(something);a!=end(something);++i){  .. }