Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

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Examples

An Optimized Version of std::copy
An Optimised Version of std::fill
An Example that Omits Destructor Calls For Types with Trivial Destructors
An improved Version of std::iter_swap

An Optimized Version of std::copy

Demonstrates a version of std::copy that uses has_trivial_assign to determine whether to use memcpy to optimise the copy operation (see copy_example.cpp):

//
// opt::copy
// same semantics as std::copy
// calls memcpy where appropriate.
//

namespace detail{

template<typename I1, typename I2, bool b>
I2 copy_imp(I1 first, I1 last, I2 out, const boost::integral_constant<bool, b>&)
{
   while(first != last)
   {
      *out = *first;
      ++out;
      ++first;
   }
   return out;
}

template<typename T>
T* copy_imp(const T* first, const T* last, T* out, const boost::true_type&)
{
   memcpy(out, first, (last-first)*sizeof(T));
   return out+(last-first);
}


}

template<typename I1, typename I2>
inline I2 copy(I1 first, I1 last, I2 out)
{
   //
   // We can copy with memcpy if T has a trivial assignment operator,
   // and if the iterator arguments are actually pointers (this last
   // requirement we detect with overload resolution):
   //
   typedef typename std::iterator_traits<I1>::value_type value_type;
   return detail::copy_imp(first, last, out, boost::has_trivial_assign<value_type>());
}

An Optimised Version of std::fill

Demonstrates a version of std::fill that uses has_trivial_assign to determine whether to use memset to optimise the fill operation (see fill_example.cpp):

//
// fill
// same as std::fill, but uses memset where appropriate
//
namespace detail{

template <typename I, typename T, bool b>
void do_fill(I first, I last, const T& val, const boost::integral_constant<bool, b>&)
{
   while(first != last)
   {
      *first = val;
      ++first;
   }
}

template <typename T>
void do_fill(T* first, T* last, const T& val, const boost::true_type&)
{
   std::memset(first, val, last-first);
}

}

template <class I, class T>
inline void fill(I first, I last, const T& val)
{
   //
   // We can do an optimised fill if T has a trivial assignment 
   // operator and if it's size is one:
   //
   typedef boost::integral_constant<bool, 
      ::boost::has_trivial_assign<T>::value && (sizeof(T) == 1)> truth_type;
   detail::do_fill(first, last, val, truth_type());
}

An Example that Omits Destructor Calls For Types with Trivial Destructors

Demonstrates a simple algorithm that uses __has_trivial_destruct to determine whether to destructors need to be called (see trivial_destructor_example.cpp):

//
// algorithm destroy_array:
// The reverse of std::unitialized_copy, takes a block of
// initialized memory and calls destructors on all objects therein.
//

namespace detail{

template <class T>
void do_destroy_array(T* first, T* last, const boost::false_type&)
{
   while(first != last)
   {
      first->~T();
      ++first;
   }
}

template <class T>
inline void do_destroy_array(T* first, T* last, const boost::true_type&)
{
}

} // namespace detail

template <class T>
inline void destroy_array(T* p1, T* p2)
{
   detail::do_destroy_array(p1, p2, ::boost::has_trivial_destructor<T>());
}

An improved Version of std::iter_swap

Demonstrates a version of std::iter_swap that use type traits to determine whether an it's arguments are proxying iterators or not, if they're not then it just does a std::swap of it's dereferenced arguments (the same as std::iter_swap does), however if they are proxying iterators then takes special care over the swap to ensure that the algorithm works correctly for both proxying iterators, and even iterators of different types (see iter_swap_example.cpp):

//
// iter_swap:
// tests whether iterator is a proxying iterator or not, and
// uses optimal form accordingly:
//
namespace detail{

template <typename I>
static void do_swap(I one, I two, const boost::false_type&)
{
   typedef typename std::iterator_traits<I>::value_type v_t;
   v_t v = *one;
   *one = *two;
   *two = v;
}
template <typename I>
static void do_swap(I one, I two, const boost::true_type&)
{
   using std::swap;
   swap(*one, *two);
}

}

template <typename I1, typename I2>
inline void iter_swap(I1 one, I2 two)
{
   //
   // See is both arguments are non-proxying iterators, 
   // and if both iterator the same type:
   //
   typedef typename std::iterator_traits<I1>::reference r1_t;
   typedef typename std::iterator_traits<I2>::reference r2_t;

   typedef boost::integral_constant<bool,
      ::boost::is_reference<r1_t>::value
      && ::boost::is_reference<r2_t>::value
      && ::boost::is_same<r1_t, r2_t>::value> truth_type;

   detail::do_swap(one, two, truth_type());
}
Copyright 2000, 2005 Adobe Systems Inc, David Abrahams, Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant, Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Thorsten Ottosen, Robert Ramey and Jeremy Siek

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