Integer Type Selection

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## Integer Type Selection

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.

### Synopsis

    namespace boost
    {
      //  fast integers from least integers
      template<typename LeastInt>
      struct int_fast_t
      {
          typedef implementation-defined-type  type;
      };

      //  signed
      template<int Bits>
      struct int_t
      {
          /* Member exact may or may not be defined depending upon Bits */
          typedef implementation-defined-type  exact;
          typedef implementation-defined-type  least;
          typedef int_fast_t<least>::fast      fast;
      };

      //  unsigned
      template<int Bits>
      struct uint_t
      {
          /* Member exact may or may not be defined depending upon Bits */
          typedef implementation-defined-type  exact;
          typedef implementation-defined-type  least;
          typedef int_fast_t<least>::fast      fast;
      };

      //  signed
      template<long long MaxValue>
      struct int_max_value_t
      {
          typedef implementation-defined-type  least;
          typedef int_fast_t<least>::fast      fast;
      };

      template<long long MinValue>
      struct int_min_value_t
      {
          typedef implementation-defined-type  least;
          typedef int_fast_t<least>::fast      fast;
      };

      //  unsigned
      template<unsigned long long Value>
      struct uint_value_t
      {
          typedef implementation-defined-type  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. The output type is given as the nested type `fast`.

Implementation Notes: By default, the output type is identical to the input type. Eventually, this code's implementation should be customized 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 nested type `least`. The easiest-to-manipulate version of that type is given by the nested type `fast`. The following table describes each template's criteria.

Table 1. Criteria for the Sized Type Class Templates

Class Template | Template Parameter Mapping
--- | ---
`boost::int_t<N>::least` | The smallest, built-in, signed integral type with at least N 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 the largest integer type.
`boost::int_t<N>::fast` | The easiest-to-manipulate, built-in, signed integral type with at least N 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 the largest integer type.
`boost::int_t<N>::exact` | A built-in, signed integral type with exactly N bits, including the sign bit. The parameter should be a positive number. Note that the member exact is defined only if there exists a type with exactly N bits.
`boost::uint_t<N>::least` | The smallest, built-in, unsigned integral type with at least N bits. The parameter should be a positive number. A compile-time error results if the parameter is larger than the number of bits in the largest integer type.
`boost::uint_t<N>::fast` | The easiest-to-manipulate, built-in, unsigned integral type with at least N bits. The parameter should be a positive number. A compile-time error results if the parameter is larger than the number of bits in the largest integer type.
`boost::uint_t<N>::exact` | A built-in, unsigned integral type with exactly N bits. The parameter should be a positive number. A compile-time error results if the parameter is larger than the number of bits in the largest integer type. Note that the member exact is defined only if there exists a type with exactly N bits.
`boost::int_max_value_t<V>::last` | The smallest, built-in, signed integral type that can hold all the values in the inclusive range 0 - V. The parameter should be a positive number.
`boost::int_max_value_t<V>::fast` | The easiest-to-manipulate, built-in, signed integral type that can hold all the values in the inclusive range 0 - V. The parameter should be a positive number.
`boost::int_min_value_t<V>::least` | The smallest, built-in, signed integral type that can hold all the values in the inclusive range V - 0. The parameter should be a negative number.
`boost::int_min_value_t<V>::fast` | The easiest-to-manipulate, built-in, signed integral type that can hold all the values in the inclusive range V - 0. The parameter should be a negative number.
`boost::uint_value_t<V>::least` | The smallest, built-in, unsigned integral type that can hold all positive values up to and including V. The parameter should be a positive number.
`boost::uint_value_t<V>::fast` | The easiest-to-manipulate, built-in, unsigned integral type that can hold all positive values up to and including V. The parameter should be a positive number.

### Example

    #include <boost/integer.hpp>

    //...

    int main()
    {
        boost::int_t<24>::least my_var;  // my_var has at least 24-bits
        //...
        // This one is guarenteed not to be truncated:
        boost::int_max_value_t<1000>::least my1000 = 1000;
        //...
        // This one is guarenteed not to be truncated, and as fast
        // to manipulate as possible, its size may be greater than
        // that of my1000:
        boost::int_max_value_t<1000>::fast my_fast1000 = 1000;
    }

### 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:

- Avoid recursion because of concern about C++'s limited guaranteed recursion depth (17).
- Avoid macros on general principles.
- Try to keep the design as simple as possible.

### 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.