how to change at runtime number precision with boost::multiprecision

Question

I\'ve read from the boost::multiprecision documentation:

Depending upon the number type, precision may be arbitrarily large (limited only by available mem

Answer 1

Most of the number backends have a fixed capacity only optionally.

Also note some backends (notable the cpp ones) take an allocator, so they /implicitly/ grow as required (until the given limit):

Normally cpp_bin_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 a template parameter causes cpp_bin_float to dynamically allocate the memory it needs

Let me check the documentation for the most popular backends:

• gmp's mpf_float adheres to this:

  typedef number >     mpf_float;
  

  Type gmp_float can be used at fixed precision by specifying a non-zero Digits10 template parameter, or at variable precision by setting the template argument to zero.

  The typedef mpf_float provides a variable precision type whose precision can be controlled via the numbers member functions.

• Similar for MPFR backends:

  Type mpfr_float_backend can be used at fixed precision by specifying a non-zero Digits10 template parameter, or at variable precision by setting the template argument to zero.

  The typedef mpfr_float provides a variable precision type whose precision can be controlled via the numbers member functions.

A sample of the dynamic precision usage:

// Operations at variable precision and no numeric_limits support:
mpfr_float a = 2;
mpfr_float::default_precision(1000);
std::cout << mpfr_float::default_precision() << std::endl;
std::cout << sqrt(a) << std::endl; // print root-2

It will leave numeric_limits undefined