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Merge branch 'feature/move-semantics' into develop

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	include/boost/optional/optional.hpp
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This commit is contained in:
Andrzej Krzemienski
2014-04-29 01:24:10 +02:00
parent 655a9e3035 c9f1422560
commit d59f47156f
16 changed files with 1656 additions and 71 deletions
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398
include/boost/optional/optional.hpp
View File

@ -21,22 +21,34 @@
#include <boost/config.hpp>
#include <boost/assert.hpp>
#include <boost/static_assert.hpp>
#include <boost/type.hpp>
#include <boost/type_traits/alignment_of.hpp>
#include <boost/type_traits/has_nothrow_constructor.hpp>
#include <boost/type_traits/type_with_alignment.hpp>
#include <boost/type_traits/remove_reference.hpp>
#include <boost/type_traits/decay.hpp>
#include <boost/type_traits/is_base_of.hpp>
#include <boost/type_traits/is_lvalue_reference.hpp>
#include <boost/type_traits/is_nothrow_move_assignable.hpp>
#include <boost/type_traits/is_nothrow_move_constructible.hpp>
#include <boost/type_traits/is_reference.hpp>
#include <boost/type_traits/is_rvalue_reference.hpp>
#include <boost/type_traits/is_same.hpp>
#include <boost/mpl/if.hpp>
#include <boost/mpl/bool.hpp>
#include <boost/mpl/not.hpp>
#include <boost/detail/reference_content.hpp>
#include <boost/move/utility.hpp>
#include <boost/none.hpp>
#include <boost/utility/swap.hpp>
#include <boost/utility/addressof.hpp>
#include <boost/utility/compare_pointees.hpp>
#include <boost/utility/in_place_factory.hpp>
#include <boost/utility/enable_if.hpp>
#include <boost/utility/explicit_operator_bool.hpp>
#include <boost/utility/in_place_factory.hpp>
#include <boost/utility/swap.hpp>
#include <boost/optional/optional_fwd.hpp>
@ -132,22 +144,39 @@ struct types_when_isnt_ref
{
typedef T const& reference_const_type ;
typedef T & reference_type ;
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
typedef T && rval_reference_type ;
static rval_reference_type move(reference_type r) { return boost::move(r); }
#endif
typedef T const* pointer_const_type ;
typedef T * pointer_type ;
typedef T const& argument_type ;
} ;
template<class T>
struct types_when_is_ref
{
typedef BOOST_DEDUCED_TYPENAME remove_reference<T>::type raw_type ;
typedef raw_type& reference_const_type ;
typedef raw_type& reference_type ;
typedef raw_type* pointer_const_type ;
typedef raw_type* pointer_type ;
typedef raw_type& argument_type ;
typedef raw_type& reference_const_type ;
typedef raw_type& reference_type ;
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
typedef raw_type&& rval_reference_type ;
static reference_type move(reference_type r) { return r; }
#endif
typedef raw_type* pointer_const_type ;
typedef raw_type* pointer_type ;
typedef raw_type& argument_type ;
} ;
template <class To, class From>
void prevent_binding_rvalue_ref_to_optional_lvalue_ref()
{
BOOST_STATIC_ASSERT_MSG(
!boost::is_lvalue_reference<To>::value || !boost::is_rvalue_reference<From>::value,
"binding rvalue references to optional lvalue references is disallowed");
}
struct optional_tag {} ;
template<class T>
@ -183,6 +212,9 @@ class optional_base : public optional_tag
protected:
typedef BOOST_DEDUCED_TYPENAME types::reference_type reference_type ;
typedef BOOST_DEDUCED_TYPENAME types::reference_const_type reference_const_type ;
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
typedef BOOST_DEDUCED_TYPENAME types::rval_reference_type rval_reference_type ;
#endif
typedef BOOST_DEDUCED_TYPENAME types::pointer_type pointer_type ;
typedef BOOST_DEDUCED_TYPENAME types::pointer_const_type pointer_const_type ;
typedef BOOST_DEDUCED_TYPENAME types::argument_type argument_type ;
@ -208,6 +240,17 @@ class optional_base : public optional_tag
construct(val);
}
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
// move-construct an optional<T> initialized from an rvalue-ref to 'val'.
// Can throw if T::T(T&&) does
optional_base ( rval_reference_type val )
:
m_initialized(false)
{
construct( boost::move(val) );
}
#endif
// Creates an optional<T> initialized with 'val' IFF cond is true, otherwise creates an uninitialzed optional<T>.
// Can throw if T::T(T const&) does
optional_base ( bool cond, argument_type val )
@ -228,7 +271,29 @@ class optional_base : public optional_tag
construct(rhs.get_impl());
}
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
// Creates a deep move of another optional<T>
// Can throw if T::T(T&&) does
optional_base ( optional_base&& rhs )
:
m_initialized(false)
{
if ( rhs.is_initialized() )
construct( boost::move(rhs.get_impl()) );
}
#endif
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
template<class Expr, class PtrExpr>
explicit optional_base ( Expr&& expr, PtrExpr const* tag )
:
m_initialized(false)
{
construct(boost::forward<Expr>(expr),tag);
}
#else
// This is used for both converting and in-place constructions.
// Derived classes use the 'tag' to select the appropriate
// implementation (the correct 'construct()' overload)
@ -240,6 +305,7 @@ class optional_base : public optional_tag
construct(expr,tag);
}
#endif
// No-throw (assuming T::~T() doesn't)
@ -260,6 +326,24 @@ class optional_base : public optional_tag
construct(rhs.get_impl());
}
}
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
// Assigns from another optional<T> (deep-moves the rhs value)
void assign ( optional_base&& rhs )
{
if (is_initialized())
{
if ( rhs.is_initialized() )
assign_value(boost::move(rhs.get_impl()), is_reference_predicate() );
else destroy();
}
else
{
if ( rhs.is_initialized() )
construct(boost::move(rhs.get_impl()));
}
}
#endif
// Assigns from another _convertible_ optional<U> (deep-copies the rhs value)
template<class U>
@ -278,6 +362,26 @@ class optional_base : public optional_tag
}
}
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
// move-assigns from another _convertible_ optional<U> (deep-moves from the rhs value)
template<class U>
void assign ( optional<U>&& rhs )
{
typedef BOOST_DEDUCED_TYPENAME optional<U>::rval_reference_type ref_type;
if (is_initialized())
{
if ( rhs.is_initialized() )
assign_value(static_cast<ref_type>(rhs.get()), is_reference_predicate() );
else destroy();
}
else
{
if ( rhs.is_initialized() )
construct(static_cast<ref_type>(rhs.get()));
}
}
#endif
// Assigns from a T (deep-copies the rhs value)
void assign ( argument_type val )
{
@ -285,19 +389,41 @@ class optional_base : public optional_tag
assign_value(val, is_reference_predicate() );
else construct(val);
}
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
// Assigns from a T (deep-moves the rhs value)
void assign ( rval_reference_type val )
{
if (is_initialized())
assign_value( boost::move(val), is_reference_predicate() );
else construct( boost::move(val) );
}
#endif
// Assigns from "none", destroying the current value, if any, leaving this UNINITIALIZED
// No-throw (assuming T::~T() doesn't)
void assign ( none_t ) { destroy(); }
#ifndef BOOST_OPTIONAL_NO_INPLACE_FACTORY_SUPPORT
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
template<class Expr, class ExprPtr>
void assign_expr ( Expr&& expr, ExprPtr const* tag )
{
if (is_initialized())
assign_expr_to_initialized(boost::forward<Expr>(expr),tag);
else construct(boost::forward<Expr>(expr),tag);
}
#else
template<class Expr>
void assign_expr ( Expr const& expr, Expr const* tag )
{
if (is_initialized())
assign_expr_to_initialized(expr,tag);
else construct(expr,tag);
}
{
if (is_initialized())
assign_expr_to_initialized(expr,tag);
else construct(expr,tag);
}
#endif
#endif
public :
@ -324,8 +450,51 @@ class optional_base : public optional_tag
new (m_storage.address()) internal_type(val) ;
m_initialized = true ;
}
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
void construct ( rval_reference_type val )
{
new (m_storage.address()) internal_type( types::move(val) ) ;
m_initialized = true ;
}
#endif
#ifndef BOOST_OPTIONAL_NO_INPLACE_FACTORY_SUPPORT
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
// Constructs in-place using the given factory
template<class Expr>
void construct ( Expr&& factory, in_place_factory_base const* )
{
BOOST_STATIC_ASSERT ( ::boost::mpl::not_<is_reference_predicate>::value ) ;
boost_optional_detail::construct<value_type>(factory, m_storage.address());
m_initialized = true ;
}
// Constructs in-place using the given typed factory
template<class Expr>
void construct ( Expr&& factory, typed_in_place_factory_base const* )
{
BOOST_STATIC_ASSERT ( ::boost::mpl::not_<is_reference_predicate>::value ) ;
factory.apply(m_storage.address()) ;
m_initialized = true ;
}
template<class Expr>
void assign_expr_to_initialized ( Expr&& factory, in_place_factory_base const* tag )
{
destroy();
construct(factory,tag);
}
// Constructs in-place using the given typed factory
template<class Expr>
void assign_expr_to_initialized ( Expr&& factory, typed_in_place_factory_base const* tag )
{
destroy();
construct(factory,tag);
}
#else
// Constructs in-place using the given factory
template<class Expr>
void construct ( Expr const& factory, in_place_factory_base const* )
@ -360,7 +529,31 @@ class optional_base : public optional_tag
}
#endif
// Constructs using any expression implicitely convertible to the single argument
#endif
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
// Constructs using any expression implicitly convertible to the single argument
// of a one-argument T constructor.
// Converting constructions of optional<T> from optional<U> uses this function with
// 'Expr' being of type 'U' and relying on a converting constructor of T from U.
template<class Expr>
void construct ( Expr&& expr, void const* )
{
new (m_storage.address()) internal_type(boost::forward<Expr>(expr)) ;
m_initialized = true ;
}
// Assigns using a form any expression implicitly convertible to the single argument
// of a T's assignment operator.
// Converting assignments of optional<T> from optional<U> uses this function with
// 'Expr' being of type 'U' and relying on a converting assignment of T from U.
template<class Expr>
void assign_expr_to_initialized ( Expr&& expr, void const* )
{
assign_value(boost::forward<Expr>(expr), is_reference_predicate());
}
#else
// Constructs using any expression implicitly convertible to the single argument
// of a one-argument T constructor.
// Converting constructions of optional<T> from optional<U> uses this function with
// 'Expr' being of type 'U' and relying on a converting constructor of T from U.
@ -371,7 +564,7 @@ class optional_base : public optional_tag
m_initialized = true ;
}
// Assigns using a form any expression implicitely convertible to the single argument
// Assigns using a form any expression implicitly convertible to the single argument
// of a T's assignment operator.
// Converting assignments of optional<T> from optional<U> uses this function with
// 'Expr' being of type 'U' and relying on a converting assignment of T from U.
@ -381,6 +574,8 @@ class optional_base : public optional_tag
assign_value(expr, is_reference_predicate());
}
#endif
#ifdef BOOST_OPTIONAL_WEAK_OVERLOAD_RESOLUTION
// BCB5.64 (and probably lower versions) workaround.
// The in-place factories are supported by means of catch-all constructors
@ -394,6 +589,20 @@ class optional_base : public optional_tag
// For VC<=70 compilers this workaround dosen't work becasue the comnpiler issues and error
// instead of choosing the wrong overload
//
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
// Notice that 'Expr' will be optional<T> or optional<U> (but not optional_base<..>)
template<class Expr>
void construct ( Expr&& expr, optional_tag const* )
{
if ( expr.is_initialized() )
{
// An exception can be thrown here.
// It it happens, THIS will be left uninitialized.
new (m_storage.address()) internal_type(types::move(expr.get())) ;
m_initialized = true ;
}
}
#else
// Notice that 'Expr' will be optional<T> or optional<U> (but not optional_base<..>)
template<class Expr>
void construct ( Expr const& expr, optional_tag const* )
@ -407,9 +616,14 @@ class optional_base : public optional_tag
}
}
#endif
#endif // defined BOOST_OPTIONAL_WEAK_OVERLOAD_RESOLUTION
void assign_value ( argument_type val, is_not_reference_tag ) { get_impl() = val; }
void assign_value ( argument_type val, is_reference_tag ) { construct(val); }
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
void assign_value ( rval_reference_type val, is_not_reference_tag ) { get_impl() = static_cast<rval_reference_type>(val); }
void assign_value ( rval_reference_type val, is_reference_tag ) { construct( static_cast<rval_reference_type>(val) ); }
#endif
void destroy()
{
@ -483,22 +697,32 @@ class optional : public optional_detail::optional_base<T>
typedef BOOST_DEDUCED_TYPENAME base::value_type value_type ;
typedef BOOST_DEDUCED_TYPENAME base::reference_type reference_type ;
typedef BOOST_DEDUCED_TYPENAME base::reference_const_type reference_const_type ;
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
typedef BOOST_DEDUCED_TYPENAME base::rval_reference_type rval_reference_type ;
#endif
typedef BOOST_DEDUCED_TYPENAME base::pointer_type pointer_type ;
typedef BOOST_DEDUCED_TYPENAME base::pointer_const_type pointer_const_type ;
typedef BOOST_DEDUCED_TYPENAME base::argument_type argument_type ;
// Creates an optional<T> uninitialized.
// No-throw
optional() : base() {}
optional() BOOST_NOEXCEPT : base() {}
// Creates an optional<T> uninitialized.
// No-throw
optional( none_t none_ ) : base(none_) {}
optional( none_t none_ ) BOOST_NOEXCEPT : base(none_) {}
// Creates an optional<T> initialized with 'val'.
// Can throw if T::T(T const&) does
optional ( argument_type val ) : base(val) {}
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
// Creates an optional<T> initialized with 'move(val)'.
// Can throw if T::T(T &&) does
optional ( rval_reference_type val ) : base( boost::forward<T>(val) )
{optional_detail::prevent_binding_rvalue_ref_to_optional_lvalue_ref<T, rval_reference_type>();}
#endif
// Creates an optional<T> initialized with 'val' IFF cond is true, otherwise creates an uninitialized optional.
// Can throw if T::T(T const&) does
optional ( bool cond, argument_type val ) : base(cond,val) {}
@ -516,40 +740,94 @@ class optional : public optional_detail::optional_base<T>
if ( rhs.is_initialized() )
this->construct(rhs.get());
}
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
// Creates a deep move of another convertible optional<U>
// Requires a valid conversion from U to T.
// Can throw if T::T(U&&) does
template<class U>
explicit optional ( optional<U> && rhs )
:
base()
{
if ( rhs.is_initialized() )
this->construct( boost::move(rhs.get()) );
}
#endif
#ifndef BOOST_OPTIONAL_NO_INPLACE_FACTORY_SUPPORT
// Creates an optional<T> with an expression which can be either
// (a) An instance of InPlaceFactory (i.e. in_place(a,b,...,n);
// (b) An instance of TypedInPlaceFactory ( i.e. in_place<T>(a,b,...,n);
// (c) Any expression implicitely convertible to the single type
// (c) Any expression implicitly convertible to the single type
// of a one-argument T's constructor.
// (d*) Weak compilers (BCB) might also resolved Expr as optional<T> and optional<U>
// even though explicit overloads are present for these.
// Depending on the above some T ctor is called.
// Can throw is the resolved T ctor throws.
// Can throw if the resolved T ctor throws.
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
template<class Expr>
explicit optional ( Expr&& expr,
BOOST_DEDUCED_TYPENAME boost::disable_if_c<
(boost::is_base_of<optional_detail::optional_tag, BOOST_DEDUCED_TYPENAME boost::decay<Expr>::type>::value) ||
boost::is_same<BOOST_DEDUCED_TYPENAME boost::decay<Expr>::type, none_t>::value >::type* = 0
)
: base(boost::forward<Expr>(expr),boost::addressof(expr))
{optional_detail::prevent_binding_rvalue_ref_to_optional_lvalue_ref<T, rval_reference_type>();}
#else
template<class Expr>
explicit optional ( Expr const& expr ) : base(expr,boost::addressof(expr)) {}
#endif
#endif // !defined BOOST_NO_CXX11_RVALUE_REFERENCES
#endif // !defined BOOST_OPTIONAL_NO_INPLACE_FACTORY_SUPPORT
// Creates a deep copy of another optional<T>
// Can throw if T::T(T const&) does
optional ( optional const& rhs ) : base( static_cast<base const&>(rhs) ) {}
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
// Creates a deep move of another optional<T>
// Can throw if T::T(T&&) does
optional ( optional && rhs )
BOOST_NOEXCEPT_IF(::boost::is_nothrow_move_constructible<T>::value)
: base( boost::move(rhs) )
{}
#endif
// No-throw (assuming T::~T() doesn't)
~optional() {}
#if !defined(BOOST_OPTIONAL_NO_INPLACE_FACTORY_SUPPORT) && !defined(BOOST_OPTIONAL_WEAK_OVERLOAD_RESOLUTION)
// Assigns from an expression. See corresponding constructor.
// Basic Guarantee: If the resolved T ctor throws, this is left UNINITIALIZED
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
template<class Expr>
BOOST_DEDUCED_TYPENAME boost::disable_if_c<
boost::is_base_of<optional_detail::optional_tag, BOOST_DEDUCED_TYPENAME boost::decay<Expr>::type>::value ||
boost::is_same<BOOST_DEDUCED_TYPENAME boost::decay<Expr>::type, none_t>::value,
optional&
>::type
operator= ( Expr&& expr )
{
optional_detail::prevent_binding_rvalue_ref_to_optional_lvalue_ref<T, rval_reference_type>();
this->assign_expr(boost::forward<Expr>(expr),boost::addressof(expr));
return *this ;
}
#else
template<class Expr>
optional& operator= ( Expr const& expr )
{
this->assign_expr(expr,boost::addressof(expr));
return *this ;
}
#endif
#endif // !defined BOOST_NO_CXX11_RVALUE_REFERENCES
#endif // !defined(BOOST_OPTIONAL_NO_INPLACE_FACTORY_SUPPORT) && !defined(BOOST_OPTIONAL_WEAK_OVERLOAD_RESOLUTION)
// Assigns from another convertible optional<U> (converts && deep-copies the rhs value)
// Copy-assigns from another convertible optional<U> (converts && deep-copies the rhs value)
// Requires a valid conversion from U to T.
// Basic Guarantee: If T::T( U const& ) throws, this is left UNINITIALIZED
template<class U>
@ -558,6 +836,18 @@ class optional : public optional_detail::optional_base<T>
this->assign(rhs);
return *this ;
}
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
// Move-assigns from another convertible optional<U> (converts && deep-moves the rhs value)
// Requires a valid conversion from U to T.
// Basic Guarantee: If T::T( U && ) throws, this is left UNINITIALIZED
template<class U>
optional& operator= ( optional<U> && rhs )
{
this->assign(boost::move(rhs));
return *this ;
}
#endif
// Assigns from another optional<T> (deep-copies the rhs value)
// Basic Guarantee: If T::T( T const& ) throws, this is left UNINITIALIZED
@ -568,6 +858,16 @@ class optional : public optional_detail::optional_base<T>
return *this ;
}
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
// Assigns from another optional<T> (deep-moves the rhs value)
optional& operator= ( optional && rhs )
BOOST_NOEXCEPT_IF(::boost::is_nothrow_move_constructible<T>::value && ::boost::is_nothrow_move_assignable<T>::value)
{
this->assign( static_cast<base &&>(rhs) ) ;
return *this ;
}
#endif
// Assigns from a T (deep-copies the rhs value)
// Basic Guarantee: If T::( T const& ) throws, this is left UNINITIALIZED
optional& operator= ( argument_type val )
@ -576,6 +876,15 @@ class optional : public optional_detail::optional_base<T>
return *this ;
}
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
// Assigns from a T (deep-moves the rhs value)
optional& operator= ( rval_reference_type val )
{
this->assign( boost::move(val) ) ;
return *this ;
}
#endif
// Assigns from a "none"
// Which destroys the current value, if any, leaving this UNINITIALIZED
// No-throw (assuming T::~T() doesn't)
@ -586,10 +895,10 @@ class optional : public optional_detail::optional_base<T>
}
void swap( optional & arg )
BOOST_NOEXCEPT_IF(::boost::is_nothrow_move_constructible<T>::value && ::boost::is_nothrow_move_assignable<T>::value)
{
// allow for Koenig lookup
using boost::swap;
swap(*this, arg);
boost::swap(*this, arg);
}
@ -615,11 +924,19 @@ class optional : public optional_detail::optional_base<T>
reference_const_type operator *() const { return this->get() ; }
reference_type operator *() { return this->get() ; }
bool operator!() const { return !this->is_initialized() ; }
bool operator!() const BOOST_NOEXCEPT { return !this->is_initialized() ; }
BOOST_EXPLICIT_OPERATOR_BOOL()
} ;
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
template<class T>
class optional<T&&>
{
BOOST_STATIC_ASSERT_MSG(sizeof(T) == 0, "Optional rvalue references are illegal.");
} ;
#endif
// Returns optional<T>(v)
template<class T>
inline
@ -919,6 +1236,37 @@ struct swap_selector<true>
}
};
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
template<>
struct swap_selector<false>
{
template<class T>
static void optional_swap ( optional<T>& x, optional<T>& y )
//BOOST_NOEXCEPT_IF(::boost::is_nothrow_move_constructible<T>::value && BOOST_NOEXCEPT_EXPR(boost::swap(*x, *y)))
{
if(x)
{
if (y)
{
boost::swap(*x, *y);
}
else
{
y = boost::move(*x);
x = boost::none;
}
}
else
{
if (y)
{
x = boost::move(*y);
y = boost::none;
}
}
}
};
#else
template<>
struct swap_selector<false>
{
@ -945,6 +1293,7 @@ struct swap_selector<false>
}
}
};
#endif // !defined BOOST_NO_CXX11_RVALUE_REFERENCES
} // namespace optional_detail
@ -952,6 +1301,7 @@ template<class T>
struct optional_swap_should_use_default_constructor : has_nothrow_default_constructor<T> {} ;
template<class T> inline void swap ( optional<T>& x, optional<T>& y )
//BOOST_NOEXCEPT_IF(::boost::is_nothrow_move_constructible<T>::value && BOOST_NOEXCEPT_EXPR(boost::swap(*x, *y)))
{
optional_detail::swap_selector<optional_swap_should_use_default_constructor<T>::value>::optional_swap(x, y);
}

7
include/boost/optional/optional_fwd.hpp
View File

@ -11,15 +11,18 @@
//
// Revisions:
// 10 May 2008 (added swap related forward declaration) Niels Dekker
//
// 17 Apr 2014 (added noexcept) Andrzej Krzemienski
//
#ifndef BOOST_OPTIONAL_OPTIONAL_FWD_FLC_19NOV2002_HPP
#define BOOST_OPTIONAL_OPTIONAL_FWD_FLC_19NOV2002_HPP
#include <boost/config.hpp>
namespace boost {
template<class T> class optional ;
template<class T> void swap ( optional<T>& , optional<T>& ) ;
template<class T> void swap ( optional<T>& , optional<T>& );
template<class T> struct optional_swap_should_use_default_constructor ;