C++ Boost

Boost Numeric Conversion Library

Type Requirements
and
User-defined-types support


Contents

Type Requirements
UDT's special semantics
Special Policies

Type Requirements

Both arithmetic (built-in) and user-defined numeric types require proper specialization of std::numeric_limits<> (that is, with (in-class) integral constants).
The library uses std::numeric_limits<T>::is_specialized to detect whether the type is builtin or user defined, and std::numeric_limits<T>::is_integer, std::numeric_limits<T>::is_signed to detect whether the type is integer or floating point; and whether it is signed/unsigned.

The default Float2IntRounder policies uses unqualified calls to functions floor() and ceil(); but the standard functions are introduced in scope by a using directive:

using std::floor ; return floor(s); 

Therefore, for builtin arithmetic types, the std functions will be used. User defined types should provide overloaded versions of these functions in order to use the default rounder policies. If these overloads are defined within a user namespace argument dependent lookup (ADL) should find them, but if your compiler has a weak ADL you might need to put these functions some place else or write your own rounder policy.

The default Trunc<> rounder policy needs to determine if the source value is positive or not, and for this it evaluates the expression "s < static_cast<S>(0)". Therefore, user defined types require a visible operator < in order to use the Trunc<> policy (the default).


UDT's special semantics

Conversion Traits

If a User Defined Type is involved in a conversion, it is assumed that the UDT has wider range than any built-in type, and consequently the values of some converter_traits<> members are hardwired regardless of the reality. The following table summarizes this:

  • Target=UDT and Source=built-in
    subranged=false
    supertype=Target
    subtype=Source
  • Target=built-in and Source=UDT
    subranged=true
    supertype=Source
    subtype=Target
  • Target=UDT and Source=UDT
    subranged=false
    supertype=Target
    subtype=Source
  • The Traits member udt_mixture can be used to detect whether a UDT is involved and to infer the validity of the other members as shown above.

    Range Checking

    Because User Defined Numeric Types might have peculiar ranges (such as an unbounded range), this library does not attempt to supply a meaningful range checking logic when UDTs are involved in a conversion. Therefore, if either Target or Source are not built-in types, the bundled range checking of the converter<> function object is automatically disabled. However, it is possible to supply a user-defined range-checker. See Special Policies


    Special Policies

    There are two components of the converter<> class that might require special behavior if User Defined Numeric Types are involved: the Range Checking and the Raw Conversion.

    When both Target and Source are built-in types, the converter class uses an internal range checking logic which is optimized and customized for the combined properties of the types.
    However, this internal logic is disabled when either type is User Defined. In this case, the user can specify an external range checking policy which will be used in place of the internal code. See UserRangeChecker policy for details.

    The converter class performs the actual conversion using a Raw Converter policy. The default raw converter simply performs a "static_cast<Target>(source)".
    However, if the a UDT is involved, the static_cast might not work. In this case, the user can implement and pass a different raw converter policy. See RawConverter policy for details


    Back to Numeric Conversion library index


    Revised 23 June 2004

    Copyright Fernando Luis Cacciola Carballal, 2004

    Use, modification, and distribution are subject to the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at www.boost.org/LICENSE_1_0.txt)