为什么BOOST_FOREACH不完全等同于手动编码？

Question

来自 boost doc,

这导致近乎最佳的代码生成; BOOST_FOREACH的性能通常在等效手动编码循环的几个百分点内.

我想使用宏和非标准typeof运算符,我们可以生成完全等效的运算符.BOOST_FOREACH的哪些特性使其不准确？

编辑:

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？

Answer 1

推动foreach远非微不足道.与gcc 4.6:

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

生成了大量用于探测的案例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;
}

你可能想要循环的东西有很多种类.使用c ++ 11,不再需要所有这些技巧,因为你几乎可以使用所有这些技巧

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

要么

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