C++ Boost

Boost.Python

Header <boost/python/type_id.hpp>


Contents

Introduction
Classes
Class type_info
Class type_info synopsis
Class type_info constructor
Class type_info comparison functions
Class type_info observer functions
Functions
type_id
Example

Introduction

<boost/python/type_id.hpp> provides types and functions for runtime type identification like those of of <typeinfo>. It exists mostly to work around certain compiler bugs and platform-dependent interactions with shared libraries.

Classes

Class type_info

type_info instances identify a type. As std::type_info is specified to (but unlike its implementation in some compilers), boost::python::type_info never represents top-level references or cv-qualification (see section 5.2.8 in the C++ standard). Unlike std::type_info, boost::python::type_info instances are copyable, and comparisons always work reliably across shared library boundaries.

Class type_info synopsis

namespace boost { namespace python
{
  class type_info : totally_ordered<type_info>
  {
   public:
      // constructor
      type_info(std::type_info const& = typeid(void));

      // comparisons
      bool operator<(type_info const& rhs) const;
      bool operator==(type_info const& rhs) const;

      // observers
      char const* name() const;
  };
}}

Class type_info constructor

type_info(std::type_info const& = typeid(void));
Effects: constructs a type_info object which identifies the same type as its argument.
Rationale: Since it is occasionally neccessary to make an array of type_info objects a benign default argument is supplied. Note: this constructor does not correct for non-conformance of compiler typeid() implementations. See type_id, below.

Class type_info comparisons

bool operator<(type_info const& rhs) const;
Effects: yields a total order over type_info objects.
bool operator==(type_info const& rhs) const;
Returns: true iff the two values describe the same type.
Note: The use of totally_ordered<type_info> as a private base class supplies operators <=, >=, >, and !=

Class type_info observers

char const* name() const;
Returns: The result of calling name() on the argument used to construct the object.

Functions

std::ostream& operator<<(std::ostream&s, type_info const&x);
Effects: Writes a description of the type described by to x into s.
Rationale: Not every C++ implementation provides a truly human-readable type_info::name() string, but for some we may be able to decode the string and produce a reasonable representation.
template <class T> type_info type_id()
Returns: type_info(typeid(T))
Note: On some non-conforming C++ implementations, the code is not actually as simple as described above; the semantics are adjusted to work as-if the C++ implementation were conforming.

Example

The following example, though silly, illustrates how the type_id facility might be used
#include <boost/python/type_id.hpp>

// Returns true iff the user passes an int argument
template <class T>
bool is_int(T x)
{
   using boost::python::type_id;
   return type_id<T>() == type_id<int>();
}

Revised 13 November, 2002

© Copyright Dave Abrahams 2002. All Rights Reserved.