typemanip.h

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////////////////////////////////////////////////////////////////////////////////
// The Loki Library
// Copyright (c) 2001 by Andrei Alexandrescu
// This code accompanies the book:
// Alexandrescu, Andrei. "Modern C++ Design: Generic Programming and Design
//     Patterns Applied". Copyright (c) 2001. Addison-Wesley.
// Permission to use, copy, modify, distribute and sell this software for any
//     purpose is hereby granted without fee, provided that the above copyright
//     notice appear in all copies and that both that copyright notice and this
//     permission notice appear in supporting documentation.
// The author or Addison-Welsey Longman make no representations about the
//     suitability of this software for any purpose. It is provided "as is"
//     without express or implied warranty.
////////////////////////////////////////////////////////////////////////////////

// Last update: August 9, 2002

#ifndef TYPEMANIP_INC_
#define TYPEMANIP_INC_

namespace Loki
{
////////////////////////////////////////////////////////////////////////////////
// class template Int2Type
// Converts each integral constant into a unique type
// Invocation: Int2Type<v> where v is a compile-time constant integral
// Defines 'value', an enum that evaluates to v
////////////////////////////////////////////////////////////////////////////////

    template <int v>
    struct Int2Type
    {
        static const int value = v;
    };

////////////////////////////////////////////////////////////////////////////////
// class template Type2Type
// Converts each type into a unique, insipid type
// Invocation Type2Type<T> where T is a type
// Defines the type OriginalType which maps back to T
////////////////////////////////////////////////////////////////////////////////

    template <typename T>
    struct Type2Type
    {
        typedef T OriginalType;
    };

////////////////////////////////////////////////////////////////////////////////
// class template Select
// Selects one of two types based upon a boolean constant
// Invocation: Select<flag, T, U>::Result
// where:
// flag is a compile-time boolean constant
// T and U are types
// Result evaluates to T if flag is true, and to U otherwise.
////////////////////////////////////////////////////////////////////////////////

    template <bool flag, typename T, typename U>
    struct Select
    {
        typedef T Result;
    };
    template <typename T, typename U>
    struct Select<false, T, U>
    {
        typedef U Result;
    };

////////////////////////////////////////////////////////////////////////////////
// class template IsSameType
// Return true iff two given types are the same
// Invocation: SameType<T, U>::value
// where:
// T and U are types
// Result evaluates to true iff U == T (types equal)
////////////////////////////////////////////////////////////////////////////////

    template <typename T, typename U>
    struct IsSameType
    {
        enum { value = false };
    };
    
    template <typename T>
    struct IsSameType<T,T>
    {
        enum { value = true };
    };

////////////////////////////////////////////////////////////////////////////////
// Helper types Small and Big - guarantee that sizeof(Small) < sizeof(Big)
////////////////////////////////////////////////////////////////////////////////

    namespace Private
    {
        template <class T, class U>
        struct ConversionHelper
        {
            typedef char Small;
            struct Big { char dummy[2]; };
            static Big   Test(...);
            static Small Test(U);
            static T MakeT();
        };
    }

////////////////////////////////////////////////////////////////////////////////
// class template Conversion
// Figures out the conversion relationships between two types
// Invocations (T and U are types):
// a) Conversion<T, U>::exists
// returns (at compile time) true if there is an implicit conversion from T
// to U (example: Derived to Base)
// b) Conversion<T, U>::exists2Way
// returns (at compile time) true if there are both conversions from T
// to U and from U to T (example: int to char and back)
// c) Conversion<T, U>::sameType
// returns (at compile time) true if T and U represent the same type
//
// Caveat: might not work if T and U are in a private inheritance hierarchy.
////////////////////////////////////////////////////////////////////////////////

    template <class T, class U>
    struct Conversion
    {
        typedef typename Private::ConversionHelper<T, U> H;

        static const bool exists     = sizeof(typename H::Small) == sizeof((H::Test(H::MakeT())));
        static const bool exists2Way = exists && Conversion<U, T>::exists;
        static const bool sameType   = false;
    };

    template <class T>
    struct Conversion<T, T>
    {
        static const bool exists     = true;
        static const bool exists2Way = true;
        static const bool sameType   = true;
    };

    template <class T>
    struct Conversion<void, T>
    {
        static const bool exists     = false;
        static const bool exists2Way = false;
        static const bool sameType   = false;
    };

    template <class T>
    struct Conversion<T, void>
    {
        static const bool exists     = true;
        static const bool exists2Way = false;
        static const bool sameType   = false;
    };

    template <>
    struct Conversion<void, void>
    {
    public:
        static const bool exists     = true;
        static const bool exists2Way = true;
        static const bool sameType   = true;
    };

////////////////////////////////////////////////////////////////////////////////
// class template SuperSubclass
// Invocation: SuperSubclass<B, D>::value where B and D are types. 
// Returns true if B is a public base of D, or if B and D are aliases of the 
// same type.
//
// Caveat: might not work if T and U are in a private inheritance hierarchy.
////////////////////////////////////////////////////////////////////////////////

template <class T, class U>
struct SuperSubclass
{
  enum { value = (::Loki::Conversion<const volatile U*, const volatile T*>::exists &&
                  !::Loki::Conversion<const volatile T*, const volatile void*>::sameType) };
};

////////////////////////////////////////////////////////////////////////////////
// class template SuperSubclassStrict
// Invocation: SuperSubclassStrict<B, D>::value where B and D are types. 
// Returns true if B is a public base of D.
//
// Caveat: might not work if T and U are in a private inheritance hierarchy.
////////////////////////////////////////////////////////////////////////////////

template<class T,class U>
struct SuperSubclassStrict
{
  enum { value = (::Loki::Conversion<const volatile U*, const volatile T*>::exists &&
                 !::Loki::Conversion<const volatile T*, const volatile void*>::sameType &&
                 !::Loki::Conversion<const volatile T*, const volatile U*>::sameType) };
};

}   // namespace Loki

////////////////////////////////////////////////////////////////////////////////
// macro SUPERSUBCLASS
// Invocation: SUPERSUBCLASS(B, D) where B and D are types. 
// Returns true if B is a public base of D, or if B and D are aliases of the 
// same type.
//
// Caveat: might not work if T and U are in a private inheritance hierarchy.
// Deprecated: Use SuperSubclass class template instead.
////////////////////////////////////////////////////////////////////////////////

#define SUPERSUBCLASS(T, U) \
    ::Loki::SuperSubclass<T,U>::value

////////////////////////////////////////////////////////////////////////////////
// macro SUPERSUBCLASS_STRICT
// Invocation: SUPERSUBCLASS(B, D) where B and D are types. 
// Returns true if B is a public base of D.
//
// Caveat: might not work if T and U are in a private inheritance hierarchy.
// Deprecated: Use SuperSubclassStrict class template instead.
////////////////////////////////////////////////////////////////////////////////

#define SUPERSUBCLASS_STRICT(T, U) \
    ::Loki::SuperSubclassStrict<T,U>::value

////////////////////////////////////////////////////////////////////////////////
// Change log:
// June     20, 2001: ported by Nick Thurn to gcc 2.95.3. Kudos, Nick!!!
// November 22, 2001: minor change to support porting to boost
// November 22, 2001: fixed bug in Conversion<void, T>
//                    (credit due to Brad Town)
// November 23, 2001: (well it's 12:01 am) fixed bug in SUPERSUBCLASS - added
//                    the volatile qualifier to be 100% politically correct
// July     16, 2002: Ported by Terje Sletteb