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449 lines
14 KiB
Plaintext
449 lines
14 KiB
Plaintext
[/
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Boost.Optional
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Copyright (c) 2003-2007 Fernando Luis Cacciola Carballal
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE_1_0.txt or copy at
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http://www.boost.org/LICENSE_1_0.txt)
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]
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[section Type Requirements and User-defined-types support]
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[section Type Requirements]
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Both arithmetic (built-in) and user-defined numeric types require proper
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specialization of `std::numeric_limits<>` (that is, with (in-class) integral
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constants).
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The library uses `std::numeric_limits<T>::is_specialized` to detect whether
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the type is builtin or user defined, and `std::numeric_limits<T>::is_integer`,
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`std::numeric_limits<T>::is_signed` to detect whether the type is integer
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or floating point; and whether it is signed/unsigned.
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The default `Float2IntRounder` policies uses unqualified calls to functions
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`floor()` and `ceil()`; but the standard functions are introduced in scope
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by a using directive:
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using std::floor ; return floor(s);
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Therefore, for builtin arithmetic types, the std functions will be used.
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User defined types should provide overloaded versions of these functions in
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order to use the default rounder policies. If these overloads are defined
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within a user namespace argument dependent lookup (ADL) should find them,
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but if your compiler has a weak ADL you might need to put these functions
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some place else or write your own rounder policy.
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The default `Trunc<>` rounder policy needs to determine if the source value
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is positive or not, and for this it evaluates the expression
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`s < static_cast<S>(0)`. Therefore, user defined types require a visible
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`operator<` in order to use the `Trunc<>` policy (the default).
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[endsect]
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[section UDT's special semantics]
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[heading Conversion Traits]
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If a User Defined Type is involved in a conversion, it is ['assumed] that
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the UDT has [link boost_numericconversion.definitions.range_and_precision wider range]
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than any built-in type, and consequently the values
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of some `converter_traits<>` members are hardwired regardless of the reality.
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The following table summarizes this:
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* `Target=`['UDT] and `Source=`['built-in]
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* `subranged=false`
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* `supertype=Target`
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* `subtype=Source`
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* `Target=`['built-in] and `Source=`['UDT]
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* `subranged=true`
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* `supertype=Source`
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* `subtype=Target`
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* `Target=`['UDT] and `Source=`['UDT]
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* `subranged=false`
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* `supertype=Target`
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* `subtype=Source`
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The `Traits` member `udt_mixture` can be used to detect whether a UDT is involved
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and to infer the validity of the other members as shown above.
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[heading Range Checking]
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Because User Defined Numeric Types might have peculiar ranges (such as an
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unbounded range), this library does not attempt to supply a meaningful range
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checking logic when UDTs are involved in a conversion. Therefore, if either
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Target or Source are not built-in types, the bundled range checking of the
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`converter<>` function object is automatically disabled. However, it is possible
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to supply a user-defined range-checker. See
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[link boost_numericconversion.type_requirements_and_user_defined_types_support.special_policies Special Policies]
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[endsect]
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[section Special Policies]
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There are two components of the `converter<>` class that might require special
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behavior if User Defined Numeric Types are involved: the Range Checking and the
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Raw Conversion.
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When both Target and Source are built-in types, the converter class uses an internal
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range checking logic which is optimized and customized for the combined properties
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of the types.
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However, this internal logic is disabled when either type is User Defined.
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In this case, the user can specify an ['external] range checking policy which will be
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used in place of the internal code. See
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[link boost_numericconversion.type_requirements_and_user_defined_types_support.udts_with_numeric_cast numeric_cast_traits]
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for details on using UDTs with `numeric_cast`.
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The converter class performs the actual conversion using a Raw Converter policy.
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The default raw converter simply performs a `static_cast<Target>(source)`.
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However, if the a UDT is involved, the `static_cast` might not work. In this case,
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the user can implement and pass a different raw converter policy.
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See [link boost_numericconversion.numeric_converter_policy_classes.policy_rawconverter RawConverter] policy for details.
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[endsect]
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[section UDTs with numeric_cast]
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In order to employ UDTs with `numeric_cast`, the user should define
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a `numeric_cast_traits` specialization on the UDT for each conversion.
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Here is an example of specializations for converting between the UDT
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and any other type:
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namespace boost { namespace numeric {
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template <typename Source>
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struct numeric_cast_traits<UDT, Source>
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{
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typedef conversion_traits<UDT, Source> conv_traits;
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//! The following are required:
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typedef YourOverflowHandlerPolicy overflow_policy;
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typedef YourRangeCheckerPolicy<conv_traits> range_checking_policy;
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typedef YourFloat2IntRounderPolicy<Source> rounding_policy;
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};
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template <typename Target>
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struct numeric_cast_traits<Target, UDT>
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{
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typedef conversion_traits<Target, UDT> conv_traits;
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//! The following are required:
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typedef YourOverflowHandlerPolicy overflow_policy;
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typedef YourRangeCheckerPolicy<conv_traits> range_checking_policy;
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typedef YourFloat2IntRounderPolicy<UDT> rounding_policy;
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};
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}}//namespace boost::numeric;
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These specializations are already defined with default values for the built-in
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numeric types. It is possible to disable the generation of specializations for
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built-in types by defining `BOOST_NUMERIC_CONVERSION_RELAX_BUILT_IN_CAST_TRAITS`.
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For details on defining custom policies see [link boost_numericconversion.numeric_converter_policy_classes Converter Policies].
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Here is a full example of how to define a custom UDT for use with `numeric_cast`:
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//! Define a simple custom number
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struct Double
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: boost::ordered_field_operators
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<
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Double
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, boost::ordered_field_operators2< Double, long double
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, boost::ordered_field_operators2< Double, double
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, boost::ordered_field_operators2< Double, float
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, boost::ordered_field_operators2< Double, int
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, boost::ordered_field_operators2< Double, unsigned int
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, boost::ordered_field_operators2< Double, long
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, boost::ordered_field_operators2< Double, unsigned long
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, boost::ordered_field_operators2< Double, long long
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, boost::ordered_field_operators2< Double, unsigned long long
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, boost::ordered_field_operators2< Double, char
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, boost::ordered_field_operators2< Double, unsigned char
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, boost::ordered_field_operators2< Double, short
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, boost::ordered_field_operators2< Double, unsigned short
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> > > > > > > > > > > > > >
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{
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Double()
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: v(0)
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{}
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template <typename T>
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explicit Double( T v )
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: v(static_cast<double>(v))
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{}
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template <typename T>
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Double& operator= ( T t )
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{
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v = static_cast<double>(t);
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return *this;
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}
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bool operator < ( const Double& rhs ) const
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{
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return v < rhs.v;
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}
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template <typename T>
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bool operator < ( T rhs ) const
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{
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return v < static_cast<double>(rhs);
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}
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bool operator > ( const Double& rhs ) const
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{
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return v > rhs.v;
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}
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template <typename T>
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bool operator > ( T rhs ) const
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{
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return v > static_cast<double>(rhs);
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}
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bool operator ==( const Double& rhs ) const
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{
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return v == rhs.v;
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}
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template <typename T>
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bool operator == ( T rhs ) const
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{
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return v == static_cast<double>(rhs);
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}
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bool operator !() const
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{
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return v == 0;
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}
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Double operator -() const
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{
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return Double(-v);
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}
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Double& operator +=( const Double& t )
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{
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v += t.v;
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return *this;
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}
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template <typename T>
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Double& operator +=( T t )
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{
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v += static_cast<double>(t);
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return *this;
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}
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Double& operator -=( const Double& t )
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{
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v -= t.v;
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return *this;
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}
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template <typename T>
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Double& operator -=( T t )
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{
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v -= static_cast<double>(t);
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return *this;
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}
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Double& operator *= ( const Double& factor )
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{
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v *= factor.v;
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return *this;
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}
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template <typename T>
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Double& operator *=( T t )
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{
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v *= static_cast<double>(t);
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return *this;
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}
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Double& operator /= (const Double& divisor)
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{
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v /= divisor.v;
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return *this;
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}
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template <typename T>
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Double& operator /=( T t )
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{
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v /= static_cast<double>(t);
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return (*this);
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}
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double v;
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};
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//! Define numeric_limits for the custom type.
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namespace std
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{
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template<>
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class numeric_limits<Double> : public numeric_limits<double>
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{
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public:
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//! Limit our Double to a range of +/- 100.0
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static Double (min)()
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{
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return Double(1.e-2);
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}
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static Double (max)()
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{
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return Double(1.e2);
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}
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static Double epsilon()
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{
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return Double( std::numeric_limits<double>::epsilon() );
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}
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};
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}
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//! Define range checking and overflow policies.
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namespace custom
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{
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//! Define a custom range checker
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template<typename Traits, typename OverFlowHandler>
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struct range_checker
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{
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typedef typename Traits::argument_type argument_type ;
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typedef typename Traits::source_type S;
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typedef typename Traits::target_type T;
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//! Check range of integral types.
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static boost::numeric::range_check_result out_of_range( argument_type s )
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{
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using namespace boost::numeric;
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if( s > bounds<T>::highest() )
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return cPosOverflow;
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else if( s < bounds<T>::lowest() )
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return cNegOverflow;
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else
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return cInRange;
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}
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static void validate_range ( argument_type s )
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{
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BOOST_STATIC_ASSERT( std::numeric_limits<T>::is_bounded );
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OverFlowHandler()( out_of_range(s) );
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}
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};
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//! Overflow handler
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struct positive_overflow{};
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struct negative_overflow{};
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struct overflow_handler
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{
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void operator() ( boost::numeric::range_check_result r )
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{
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using namespace boost::numeric;
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if( r == cNegOverflow )
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throw negative_overflow() ;
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else if( r == cPosOverflow )
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throw positive_overflow() ;
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}
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};
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//! Define a rounding policy and specialize on the custom type.
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template<class S>
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struct Ceil : boost::numeric::Ceil<S>{};
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template<>
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struct Ceil<Double>
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{
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typedef Double source_type;
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typedef Double const& argument_type;
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static source_type nearbyint ( argument_type s )
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{
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#if !defined(BOOST_NO_STDC_NAMESPACE)
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using std::ceil ;
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#endif
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return Double( ceil(s.v) );
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}
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typedef boost::mpl::integral_c< std::float_round_style, std::round_toward_infinity> round_style;
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};
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//! Define a rounding policy and specialize on the custom type.
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template<class S>
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struct Trunc: boost::numeric::Trunc<S>{};
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template<>
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struct Trunc<Double>
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{
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typedef Double source_type;
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typedef Double const& argument_type;
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static source_type nearbyint ( argument_type s )
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{
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#if !defined(BOOST_NO_STDC_NAMESPACE)
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using std::floor;
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#endif
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return Double( floor(s.v) );
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}
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typedef boost::mpl::integral_c< std::float_round_style, std::round_toward_zero> round_style;
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};
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}//namespace custom;
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namespace boost { namespace numeric {
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//! Define the numeric_cast_traits specializations on the custom type.
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template <typename S>
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struct numeric_cast_traits<Double, S>
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{
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typedef custom::overflow_handler overflow_policy;
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typedef custom::range_checker
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<
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boost::numeric::conversion_traits<Double, S>
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, overflow_policy
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> range_checking_policy;
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typedef boost::numeric::Trunc<S> rounding_policy;
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};
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template <typename T>
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struct numeric_cast_traits<T, Double>
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{
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typedef custom::overflow_handler overflow_policy;
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typedef custom::range_checker
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<
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boost::numeric::conversion_traits<T, Double>
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, overflow_policy
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> range_checking_policy;
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typedef custom::Trunc<Double> rounding_policy;
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};
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//! Define the conversion from the custom type to built-in types and vice-versa.
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template<typename T>
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struct raw_converter< conversion_traits< T, Double > >
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{
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static T low_level_convert ( const Double& n )
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{
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return static_cast<T>( n.v );
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}
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};
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template<typename S>
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struct raw_converter< conversion_traits< Double, S > >
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{
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static Double low_level_convert ( const S& n )
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{
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return Double(n);
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}
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};
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}}//namespace boost::numeric;
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[endsect]
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[endsect]
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