boost/libs/numeric/conversion/doc/numeric_cast.qbk

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    Copyright (c) 2003-2007 Fernando Luis Cacciola Carballal

    Distributed under the Boost Software License, Version 1.0.
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[section  Improved numeric_cast<>]

[section Introduction]

The lack of preservation of range makes conversions between numeric types
error prone. This is true for both implicit conversions and explicit
conversions (through `static_cast`).
[link boost_numericconversion.improved_numeric_cast__.numeric_cast `numeric_cast`]
detects loss of range when a numeric type is converted, and throws an
exception if the range cannot be preserved.

There are several situations where conversions are unsafe:

* Conversions from an integral type with a wider range than the target integral type.
* Conversions from unsigned to signed (and vice versa) integral types.
* Conversions from floating point types to integral types.

The C++ Standard does not specify the behavior when a numeric type is
assigned a value that cannot be represented by the type, except for unsigned
integral types \[3.9.1.4\], which must obey the laws of arithmetic modulo 2n
(this implies that the result will be reduced modulo the number that is one
greater than the largest value that can be represented). The fact that the
behavior for overflow is undefined for all conversions (except the
aforementioned unsigned to unsigned) makes any code that may produce
positive or negative overflows exposed to portability issues.

By default `numeric_cast` adheres to the rules for implicit conversions mandated by
the C++ Standard, such as truncating floating point types when converting
to integral types. The implementation must guarantee that for a conversion
to a type that can hold all possible values of the source type, there will
be no runtime overhead.

[endsect]

[section numeric_cast]
    
    template <typename Target, typename Source> inline
    Target numeric_cast( Source arg )
    {
        typedef conversion_traits<Target, Source>   conv_traits;
        typedef numeric_cast_traits<Target, Source> cast_traits;
        typedef converter
            <
                Target,
                Source, 
                conv_traits,
                typename cast_traits::overflow_policy, 
                typename cast_traits::rounding_policy, 
                raw_converter<conv_traits>,
                typename cast_traits::range_checking_policy
            > converter;
        return converter::convert(arg);
    }

`numeric_cast` returns the result of converting a value of type Source
to a value of type Target. If out-of-range is detected, an overflow policy
is executed whose default behavior is to throw an an exception (see 
[link numeric_conversion_bad_numeric_cast bad_numeric_cast],
[link numeric_conversion_negative_overflow negative_overflow] and
[link numeric_conversion_possitive_overflow positive_overflow]
).

[endsect]

[section numeric_cast_traits]

    template <typename Target, typename Source, typename EnableIf = void>
    struct numeric_cast_traits
    {
        typedef def_overflow_handler    overflow_policy;
        typedef UseInternalRangeChecker range_checking_policy;
        typedef Trunc<Source>           rounding_policy;
    };

The behavior of `numeric_cast` may be tailored for custom numeric types through
the specialization of `numeric_cast_traits`. (see 
[link boost_numericconversion.type_requirements_and_user_defined_types_support User Defined Types] 
for details.
)

[endsect]

[section Examples]

The following example performs some typical conversions between numeric types:

#include <boost/numeric/conversion/cast.hpp>
#include <iostream>

    int main()
    {
        using boost::numeric_cast;

        using boost::numeric::bad_numeric_cast;
        using boost::numeric::positive_overflow;
        using boost::numeric::negative_overflow;

        try
        {
            int i=42;
            short s=numeric_cast<short>(i); // This conversion succeeds (is in range)
        }
        catch(negative_overflow& e) {
            std::cout << e.what();
        }
        catch(positive_overflow& e) {
            std::cout << e.what();
        }

        try
        {
            float f=-42.1234;

            // This will cause a boost::numeric::negative_overflow exception to be thrown
            unsigned int i=numeric_cast<unsigned int>(f);
        }
        catch(bad_numeric_cast& e) {
            std::cout << e.what();
        }

        double d= f + numeric_cast<double>(123); // int -> double

        unsigned long l=std::numeric_limits<unsigned long>::max();

        try
        {
            // This will cause a boost::numeric::positive_overflow exception to be thrown
            // NOTE: *operations* on unsigned integral types cannot cause overflow
            // but *conversions* to a signed type ARE range checked by numeric_cast.

            unsigned char c=numeric_cast<unsigned char>(l);
        }
        catch(positive_overflow& e) {
            std::cout << e.what();
        }


        return 0;
    }

[endsect]

[endsect]