[/
  Copyright 2011 - 2020 John Maddock.
  Copyright 2013 - 2019 Paul A. Bristow.
  Copyright 2013 Christopher Kormanyos.

  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE_1_0.txt or copy at
  http://www.boost.org/LICENSE_1_0.txt).
]

[section:cpp_dec_float cpp_dec_float]

`#include <boost/multiprecision/cpp_dec_float.hpp>`

   namespace boost{ namespace multiprecision{

   template <unsigned Digits10, class ExponentType = std::int32_t, class Allocator = void>
   class cpp_dec_float;

   typedef number<cpp_dec_float<50> > cpp_dec_float_50;
   typedef number<cpp_dec_float<100> > cpp_dec_float_100;

   }} // namespaces

The `cpp_dec_float` back-end is used in conjunction with `number`: It acts as an entirely C++ (header only and dependency free)
floating-point number type that is a drop-in replacement for the native C++ floating-point types, but with
much greater precision.

Type `cpp_dec_float` can be used at fixed precision by specifying a non-zero `Digits10` template parameter.
The typedefs `cpp_dec_float_50` and `cpp_dec_float_100` provide arithmetic types at 50 and 100 decimal digits precision
respectively.  Optionally, you can specify an integer type to use for the exponent, this defaults to a 32-bit integer type
which is more than large enough for the vast majority of use cases, but larger types such as `long long` can also be specified
if you need a truly huge exponent range.  In any case the ExponentType must be a __fundamental signed integer type at least 2 bytes
and 16-bits wide.

Normally `cpp_dec_float` allocates no memory: all of the space required for its digits are allocated
directly within the class.  As a result care should be taken not to use the class with too high a digit count
as stack space requirements can grow out of control.  If that represents a problem then providing an allocator
as the final template parameter causes `cpp_dec_float` to dynamically allocate the memory it needs: this
significantly reduces the size of `cpp_dec_float` and increases the viable upper limit on the number of digits
at the expense of performance.  However, please bear in mind that arithmetic operations rapidly become ['very] expensive
as the digit count grows: the current implementation really isn't optimized or designed for large digit counts.

There is full standard library and `std::numeric_limits` support available for this type.

Things you should know when using this type:

* Default constructed `cpp_dec_float`s have a value of zero.
* The radix of this type is 10.  As a result it can behave subtly differently from base-2 types.
* The type has a number of internal guard digits over and above those specified in the template argument.
Normally these should not be visible to the user.
* The type supports both infinities and NaNs.  An infinity is generated whenever the result would overflow,
and a NaN is generated for any mathematically undefined operation.
* There is a `std::numeric_limits` specialisation for this type.
* Any `number` instantiated on this type, is convertible to any other `number` instantiated on this type -
for example you can convert from `number<cpp_dec_float<50> >` to `number<cpp_dec_float<SomeOtherValue> >`.
Narrowing conversions are truncating and `explicit`.
* Conversion from a string results in a `std::runtime_error` being thrown if the string can not be interpreted
as a valid floating-point number.
* The actual precision of a `cpp_dec_float` is always slightly higher than the number of digits specified in
the template parameter, actually how much higher is an implementation detail but is always at least 8 decimal
digits.
* Operations involving `cpp_dec_float` are always truncating.  However, note that since there are guard digits
in effect, in practice this has no real impact on accuracy for most use cases.

[h5 cpp_dec_float example:]

[cpp_dec_float_eg]

[endsect]