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c++boost.gif (8819 bytes)Integer Type Selection Templates

The <boost/integer.hpp> type selection templates allow integer types to be selected based on desired characteristics such as number of bits or maximum value. This facility is particularly useful for solving generic programming problems.

Contents

Synopsis

namespace boost
{
  //  fast integers from least integers
  template< typename LeastInt >
  struct int_fast_t
  {
      typedef implementation_supplied  fast;
  };

  //  signed
  template< int Bits >
  struct int_t 
  {
      typedef implementation_supplied  least;
      typedef int_fast_t<least>::fast  fast;
  };

  //  unsigned
  template< int Bits >
  struct uint_t 
  {
      typedef implementation_supplied  least;
      typedef int_fast_t<least>::fast  fast;
  };

  //  signed
  template< long MaxValue >
  struct int_max_value_t 
  {
      typedef implementation_supplied  least;
      typedef int_fast_t<least>::fast  fast;
  };

  template< long MinValue >
  struct int_min_value_t 
  {
      typedef implementation_supplied  least;
      typedef int_fast_t<least>::fast  fast;
  };

  //  unsigned
  template< unsigned long Value >
  struct uint_value_t 
  {
      typedef implementation_supplied  least;
      typedef int_fast_t<least>::fast  fast;
  };
} // namespace boost

Easiest-to-Manipulate Types

The int_fast_t class template maps its input type to the next-largest type that the processor can manipulate the easiest, or to itself if the input type is already an easy-to-manipulate type. For instance, processing a bunch of char objects may go faster if they were converted to int objects before processing. The input type, passed as the only template parameter, must be a built-in integral type, except bool. Unsigned integral types can be used, as well as signed integral types, despite the name. The output type is given as the class member fast.

Implementation Notes
By default, the output type is identical to the input type. Eventually, this code's implementation should be conditionalized for each platform to give accurate mappings between the built-in types and the easiest-to-manipulate built-in types. Also, there is no guarantee that the output type actually is easier to manipulate than the input type.

Sized Types

The int_t, uint_t, int_max_value_t, int_min_value_t, and uint_value_t class templates find the most appropiate built-in integral type for the given template parameter. This type is given by the class member least. The easiest-to-manipulate version of that type is given by the class member fast. The following table describes each template's criteria.

Criteria for the Sized Type Class Templates
Class Template Template Parameter Mapping
boost::int_t The smallest built-in signed integral type with at least the given number of bits, including the sign bit. The parameter should be a positive number. A compile-time error results if the parameter is larger than the number of bits in a long.
boost::uint_t The smallest built-in unsigned integral type with at least the given number of bits. The parameter should be a positive number. A compile-time error results if the parameter is larger than the number of bits in an unsigned long.
boost::int_max_value_t The smallest built-in signed integral type that supports the given value as a maximum. The parameter should be a positive number.
boost::int_min_value_t The smallest built-in signed integral type that supports the given value as a minimum. The parameter should be a negative number.
boost::uint_value_t The smallest built-in unsigned integral type that supports the given value as a maximum. The parameter should be a positive number.

Example

#include <boost/integer.hpp>

//...

int main()
{
    boost::int_t<24>::least my_var;
    //...
}

Demonstration Program

The program integer_test.cpp is a simplistic demonstration of the results from instantiating various examples of the sized type class templates.

Rationale

The rationale for the design of the templates in this header includes:

Alternative

If the number of bits required is known beforehand, it may be more appropriate to use the types supplied in <boost/cstdint.hpp>.

Credits

The author of most of the Boost integer type choosing templates is Beman Dawes. He gives thanks to Valentin Bonnard and Kevlin Henney for sharing their designs for similar templates. Daryle Walker designed the value-based sized templates.


Revised May 20, 2001

© Copyright Beman Dawes 1999. Permission to copy, use, modify, sell and distribute this document is granted provided this copyright notice appears in all copies. This document is provided "as is" without express or implied warranty, and with no claim as to its suitability for any purpose.