convert

© 2007,2009,2013 John Abbott
GNU Free Documentation License, Version 1.2



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The header file convert.H supplies several conversion functions. They are for converting a numerical value of one type into another numerical type (at least one of the types must be a CoCoALib type). There is also a way of safely converting machine integer values into other integral types.

There are two families of conversion functions:

  1. ConvertTo<DestType>(src) the result is the converted value; if src cannot be converted then an error is thrown (with code ERR::BadConvert)
  2. ConvertTo<DestType>(src, ErrMesg) the result is the converted value; if src cannot be converted then an error is thrown (with code ErrMesg typically created by calling ``ErrorInfo(ERR::code, "fn name"))
  3. IsConvertible(dest,src) the result is a boolean: true means the conversion was successful (and the result was placed in dest, the 1st arg)

Here is a summary of the conversions currently offered:

"to" type "from" type notes
(unsigned) long BigInt
(unsigned) int BigInt
(unsigned) long BigRat
(unsigned) int BigRat
long RingElem equiv to IsInteger & range check
BigInt RingElem same as IsInteger
BigRat RingElem same as IsRational
long double value must be integral & in range
BigInt double
BigRat double
double BigInt may have rounding error!!
double BigRat may have rounding error!!

NOTE 1: Conversion to a string can be achieved via ostringstream:

    ostringstream buffer;
    buffer << value;
    const string& ConvertedValue = buffer.str();

NOTE 2: Conversion fails if overflow occurs. Currently converting a non-zero BigRat to a double does not fail if the closest double is 0.

NumericCast

There is a templated class called NumericCast; it is roughly analogous to BOOST::numeric_cast, and will eventually be replaced by direct use of this BOOST feature. It is to be used for converting safely from one machine integer type to another: the conversion succeeds only if the value supplied can be represented by the destination type. In case of failure an ERR::BadConvert exception is thrown.

Maintenance notes for convert

The ConvertTo fns simply call the corresponding IsConvertible function -- indeed a template implementation is appropriate here.

Only some combinations of IsConvertible functions are present so far.

The class NumericCast has a single template argument, and the constructor has a separate template argument to allow the "natural syntax" like that of static_cast (or BOOST's numeric_cast). I used a class rather than a templated function because a function would have required the user to specify two template arguments (i.e. unnatural syntax). I don't know if this is the best way to achieve what I want, but it is simple enough that there are obviously no deficiencies.

Bugs, Shortcomings, etc

Conversion to C++ integral types other than (unsigned) int/long is not yet supported. Indeed the IsConvertible functions are a hotch potch, but how can it be done better?

BOOST has numeric_cast which is like NumericCast for built-in numerical types. Sooner or later we should use that.

Should conversion of BigRat to double ignore underflow, or should it fail?