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## Improved numeric_cast<>

### 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`). `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.

### 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 bad_numeric_cast, negative_overflow and positive_overflow ).

### 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 User Defined Types for details. )

### Examples

The following example performs some typical conversions between numeric types:

1. include <boost/numeric/conversion/cast.hpp>
2. 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;
    }