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High Performance Dynamic Typing in C++ using a Replacement for boost::any

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4.77/5 (29 votes)
9 May 2011CPOL1 min read 229.8K   77   76
A high-performance alternative to boost::any.

Introduction

The Boost library provides a very useful little class called boost::any which can contain a value of virtually any type as long as that value supports copy construction and assignment. This boost::any type allows dynamic querying of the contained type, and safe type conversions. Despite its usefulness, boost::any is not as efficient as it could be, so I have rewritten my own, which I have immodestly called cdiggins::any.

Updated in 2011: After five years, I have rewritten the class from scratch to fix some really nasty bugs and to simplify the code.

Using cdiggins::any

The cdiggins::any type can be used to hold normal value types, and provides a mechanism to safely and explicitly cast back to the appropriate type.

C++
any a = 42;
cout << a.cast<int>() << endl;
a = 13;
cout << a.cast<int>() << endl;
a = "hello";
cout << a.cast<const char*>() << endl;
a = std::string("1234567890");
cout << a.cast<std::string>() << endl;
int n = 42;
a = &n;
cout << *a.cast<int*>() << endl;
any b = true;
cout << b.cast<bool>() << endl;
swap(a, b);        
cout << a.cast<bool>() << endl;
a.cast<bool>() = false;
cout << a.cast<bool>() << endl;

Design of cdiggins::any

The cdiggins::any class contains two pointers: one to a policy class (any::policy) and one is a pointer to the data (any::data) which may in certain cases be used to contain the data itself (for example, if storing a primitive data type smaller than or equal to the size of a pointer).

The policy class is used for performing allocations, deallocation, copies, etc., and determines whether to use any::data to hold the data or point to the data.

Finally, without further ado, here is the implementation of the cdiggins::any class for your enjoyment:

C++
#pragma once
/*
 * (C) Copyright Christopher Diggins 2005-2011
 * (C) Copyright Pablo Aguilar 2005
 * (C) Copyright Kevlin Henney 2001
 *
 * Distributed under the Boost Software License, Version 1.0. (See
 * accompanying file LICENSE_1_0.txt or copy at
 * http://www.boost.org/LICENSE_1_0.txt
 */

#include <stdexcept>

namespace cdiggins
{
namespace anyimpl
{
    struct bad_any_cast 
    {
    };

    struct empty_any 
    {
    };

    struct base_any_policy 
    {
        virtual void static_delete(void** x) = 0;
        virtual void copy_from_value(void const* src, void** dest) = 0;
        virtual void clone(void* const* src, void** dest) = 0;
        virtual void move(void* const* src, void** dest) = 0;
        virtual void* get_value(void** src) = 0;
        virtual size_t get_size() = 0;
    };

    template<typename T>
    struct typed_base_any_policy : base_any_policy
    {
        virtual size_t get_size() { return sizeof(T); } 
    };

    template<typename T>
    struct small_any_policy : typed_base_any_policy<T>
    {
        virtual void static_delete(void** x) { }
        virtual void copy_from_value(void const* src, void** dest)
            { new(dest) T(*reinterpret_cast<T const*>(src)); }
        virtual void clone(void* const* src, void** dest) { *dest = *src; }
        virtual void move(void* const* src, void** dest) { *dest = *src; }
        virtual void* get_value(void** src) { return reinterpret_cast<void*>(src); }
    };

    template<typename T>
    struct big_any_policy : typed_base_any_policy<T>
    {
        virtual void static_delete(void** x) { if (*x) 
            delete(*reinterpret_cast<T**>(x)); *x = NULL; }
        virtual void copy_from_value(void const* src, void** dest) { 
           *dest = new T(*reinterpret_cast<T const*>(src)); }
        virtual void clone(void* const* src, void** dest) { 
           *dest = new T(**reinterpret_cast<T* const*>(src)); }
        virtual void move(void* const* src, void** dest) { 
          (*reinterpret_cast<T**>(dest))->~T(); 
          **reinterpret_cast<T**>(dest) = **reinterpret_cast<T* const*>(src); }
        virtual void* get_value(void** src) { return *src; }
    };

    template<typename T>
    struct choose_policy 
    {
        typedef big_any_policy<T> type;
    };

    template<typename T> 
    struct choose_policy<T*> 
    { 
        typedef small_any_policy<T*> type; 
    };
    
    struct any;

    /// Choosing the policy for an any type is illegal, but should never happen.
    /// This is designed to throw a compiler error.
    template<>
    struct choose_policy<any>
    {
        typedef void type;
    };

    /// Specializations for small types.
    #define SMALL_POLICY(TYPE) template<> struct 
       choose_policy<TYPE> { typedef small_any_policy<TYPE> type; };

    SMALL_POLICY(signed char);
    SMALL_POLICY(unsigned char);
    SMALL_POLICY(signed short);
    SMALL_POLICY(unsigned short);
    SMALL_POLICY(signed int);
    SMALL_POLICY(unsigned int);
    SMALL_POLICY(signed long);
    SMALL_POLICY(unsigned long);
    SMALL_POLICY(float);
    SMALL_POLICY(bool);

    #undef SMALL_POLICY

    /// This function will return a different policy for each type. 
    template<typename T>
    base_any_policy* get_policy()
    {
        static typename choose_policy<T>::type policy;
        return &policy;
    };
}

struct any
{
private:
    // fields
    anyimpl::base_any_policy* policy;
    void* object;

public:
    /// Initializing constructor.
    template <typename T>
    any(const T& x) 
        : policy(anyimpl::get_policy<anyimpl::empty_any>()), object(NULL) 
    {
        assign(x);
    }       

    /// Empty constructor. 
    any() 
        : policy(anyimpl::get_policy<anyimpl::empty_any>()), object(NULL) 
    { }

    /// Special initializing constructor for string literals. 
    any(const char* x) 
        : policy(anyimpl::get_policy<anyimpl::empty_any>()), object(NULL) 
    { 
        assign(x);
    }

    /// Copy constructor. 
    any(const any& x) 
        : policy(anyimpl::get_policy<anyimpl::empty_any>()), object(NULL)         
    {            
        assign(x);
    }

    /// Destructor. 
    ~any() {
        policy->static_delete(&object);
    }

    /// Assignment function from another any. 
    any& assign(const any& x) {
        reset();
        policy = x.policy;
        policy->clone(&x.object, &object);
        return *this;
    }

    /// Assignment function. 
    template <typename T>
    any& assign(const T& x) {
        reset();
        policy = anyimpl::get_policy<T>();
        policy->copy_from_value(&x, &object);
        return *this;
    }

    /// Assignment operator.
    template<typename T>
    any& operator=(const T& x) {
        return assign(x);
    }

    /// Assignment operator, specialed for literal strings. 
    /// They have types like const char [6] which don't work as expected. 
    any& operator=(const char* x) {
        return assign(x);
    }

    /// Utility functions
    any& swap(any& x) {
        std::swap(policy, x.policy);
        std::swap(object, x.object);
        return *this;
    }

    /// Cast operator. You can only cast to the original type.
    template<typename T>
    T& cast() {
        if (policy != anyimpl::get_policy<T>()) 
            throw anyimpl::bad_any_cast();
        T* r = reinterpret_cast<T*>(policy->get_value(&object)); 
        return *r;
    }

    /// Returns true if the any contains no value. 
    bool empty() const {
        return policy == anyimpl::get_policy<anyimpl::empty_any>();
    }

    /// Frees any allocated memory, and sets the value to NULL.
    void reset() {
        policy->static_delete(&object);
        policy = anyimpl::get_policy<anyimpl::empty_any>();
    }

    /// Returns true if the two types are the same. 
    bool compatible(const any& x) const {
        return policy == x.policy;
    }
};
}

Final Words

Thanks to Pablo Aguilar who helped with an early version of the cdiggins::any class. Thanks to Raute who helped me identify the critical problems in the original version of cdiggins::any.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


Written By
Christopher Diggins
Software Developer Ara 3D
Canada Canada
I am the designer of the Plato programming language and I am the founder of Ara 3D. I can be reached via email at cdiggins@gmail.com

Comments and Discussions

 
First PrevNext
QuestionCode dump of current, for copy & pasters: Pin
David O'Neil30-May-17 10:07
professionalDavid O'Neil30-May-17 10:07 
Had to look at this this week, and figured I'd code dump it here for anyone who comes afterwards. The following is the code as it currently stands. I believe I have incorporated everything from the below comments.

The code posted in my 'Last version still buggy' no longer blows up in Visual Studio 2015 CE with these changes in place.

For your copy/pasting pleasure:
C++
#pragma once

//CdAny.h:

//-------------------------------------------------------------------------------------------
// The following is derived from Christopher Diggins, Pablo Aguilar, and Kevlin Henney's
// 'any' work available at:
// http://www.codeproject.com/Articles/11250/High-Performance-Dynamic-Typing-in-C-using-a-Repla.
//
// Their original copyright notice is as follows:
// * (C) Copyright Christopher Diggins 2005
// * (C) Copyright Pablo Aguilar 2005
// * (C) Copyright Kevlin Henney 2001
// *
// * Distributed under the Boost Software License, Version 1.0. (See
// * accompanying file LICENSE_1_0.txt or copy at
// * http://www.boost.org/LICENSE_1_0.txt)
//
// The changes made to this code include the ones listed in the comments at the previous
// CodeProject article, and tossing it into the 'cd' namespace, as well as replacement of the
// 'bad_any_cast' with my own Exception. The last one will need to be undone for anyone
// who copy/pastes this into their code, and you may want to change the namespace as well - up
// to you.
// Thanks Christopher, Pablo, and Kevlin for making this available, and the other contributors
// for their bug fixes.
//-------------------------------------------------------------------------------------------

//namespace cdiggins
namespace cd {   //For 'Christopher Diggins', for easier typing

   namespace anyimpl {

      struct empty_any {
         };

      struct base_any_policy {

         virtual void static_delete(void** x) = 0;

         virtual void copy_from_value(void const* src, void** dest) = 0;

         virtual void clone(void* const* src, void** dest) = 0;

         virtual void move(void* const* src, void** dest) = 0;

         virtual void* get_value(void** src) = 0;

         virtual void* get_value(void* const * src) const = 0;

         virtual size_t get_size() = 0;

         };

      template<typename T>
      struct typed_base_any_policy : base_any_policy {
         virtual size_t get_size() { return sizeof(T); } 
         };

      template<typename T>
      struct small_any_policy : typed_base_any_policy<T> {

         virtual void static_delete(void** x) override { }

         virtual void copy_from_value(void const* src, void** dest) {
            //This used to be:
            //new(dest) T(*reinterpret_cast<T const*>(src)); 
            //But this is the small_any_policy, and ints and other small objects
            //shouldn't need to be pointed to by an object.  So a simple 'clone',
            //or move will suffice, so use the same code as them:
            //*dest = const_cast<void*>(src);
            //In this case, we are really receiving the address of the the 'object' variable
             *dest = *(reinterpret_cast<void**>((const_cast<void*>(src))));
            }

         virtual void clone(void* const* src, void** dest) { *dest = *src; }

         virtual void move(void* const* src, void** dest) { *dest = *src; }

         virtual void* get_value(void** src) { return reinterpret_cast<void*>(src); }

         void* get_value(void* const * src) const override {
            return reinterpret_cast<void*>(const_cast<void**>(src));
            }

         };

      template<typename T>
      struct big_any_policy : typed_base_any_policy<T> {
         virtual void static_delete(void** x) override {
            if (*x) 
               delete(*reinterpret_cast<T**>(x));
            *x = NULL;
            }

         virtual void copy_from_value(void const* src, void** dest) { 
            *dest = new T(*reinterpret_cast<T const*>(src));
            }

         virtual void clone(void* const* src, void** dest) { 
            *dest = new T(**reinterpret_cast<T* const*>(src));
            }

         virtual void move(void* const* src, void** dest) { 
           (*reinterpret_cast<T**>(dest))->~T(); 
           **reinterpret_cast<T**>(dest) = **reinterpret_cast<T* const*>(src);
           }

         virtual void* get_value(void** src) { return *src; }

         void* get_value(void* const * src) const override { return *const_cast<void**>(src); }

      };

      template<typename T>
      struct choose_policy {
         typedef big_any_policy<T> type;
         };

      template<typename T> 
      struct choose_policy<T*> { 
         typedef small_any_policy<T*> type; 
         };

      struct any; //Forward declaration

      /// Choosing the policy for an any type is illegal, but should never happen.
      /// This is designed to throw a compiler error.
      template<>
      struct choose_policy<any> {
         typedef void type;
         };

      /// Specializations for small types.
      #define SMALL_POLICY(TYPE) template<> struct choose_policy<TYPE> { \
                  typedef small_any_policy<TYPE> type; };

      SMALL_POLICY(signed char);
      SMALL_POLICY(unsigned char);
      SMALL_POLICY(signed short);
      SMALL_POLICY(unsigned short);
      SMALL_POLICY(signed int);
      SMALL_POLICY(unsigned int);
      SMALL_POLICY(signed long);
      SMALL_POLICY(unsigned long);
      SMALL_POLICY(float);
      SMALL_POLICY(bool);

      #undef SMALL_POLICY

      /// This function will return a different policy for each type. 
      template<typename T>
      base_any_policy* get_policy() {
         static typename choose_policy<T>::type policy;
         return &policy;
         };

      }  //End of anyimpl namespace

      struct any {
         private:
            // fields
            anyimpl::base_any_policy* policy;
            void* object;

         public:
            /// Initializing constructor.
            template <typename T>
            //This used to be:
            //any(const T& x) : policy(anyimpl::get_policy<anyimpl::empty_any>()),
            //          object(NULL) {
            //I am changing it to force the initial construction to use the small_policy:
            any(const T& x) : policy(anyimpl::get_policy<unsigned int>()), object(NULL) {
               assign(x);
               }      

            /// Empty constructor. 
            //This used to be:
            //any() : policy(anyimpl::get_policy<anyimpl::empty_any>()), object(NULL) {
            //But why should an empty 'any' receive a default 'big_any_policy'???
            //This starts calling 'new' and 'delete' under the hood.
            //So let's try the following to force a small_policy:
            any() : policy(anyimpl::get_policy<unsigned int>()), object(NULL) {
               }

            /// Special initializing constructor for string literals. 
            any(const char* x) : policy(anyimpl::get_policy<anyimpl::empty_any>()), 
                        object(NULL) {

               assign(x);
               }

            /// Copy constructor. 
            any(const any& x) : policy(anyimpl::get_policy<anyimpl::empty_any>()),
                        object(NULL) {

               assign(x);
               }

            /// Destructor. 
            ~any() {
               policy->static_delete(&object);
               }

            /// Assignment function from another any. 
            any& assign(const any& x) {
               reset();
               policy = x.policy;
               policy->clone(&x.object, &object);
               return *this;
               }

            /// Assignment function. 
            template <typename T>
            any& assign(const T& x) {
               reset();
               policy = anyimpl::get_policy<T>();
               policy->copy_from_value(&x, &object);
               return *this;
               }

            /// Assignment operator.
            template<typename T>
            any& operator=(const T& x) {
               return assign(x);
               }

            any& operator=(const any & x) {
               //Was:
               //policy = x.policy;
               //policy->copy_from_value(&x.object, &object);
               //return *this;
               //reset();
               return assign(x);
               }

            any& operator()(const any & x) {
               policy = x.policy;
               policy->copy_from_value(&x.object, &object);
               return *this;
               }

            /// Assignment operator, specialed for literal strings. 
            /// They have types like const char [6] which don't work as expected. 
            any& operator=(const char* x) {
               return assign(x);
               }

            /// Utility functions
            any& swap(any& x) {
               std::swap(policy, x.policy);
               std::swap(object, x.object);
               return *this;
               }

            /// Cast operator. You can only cast to the original type.
            template<typename T>
            T& cast() {
               if (policy != anyimpl::get_policy<T>()) {
                  // the following was 'throw anyimpl::bad_any_cast();'
                  throw dwl::Exception(_T("Bad 'any' cast"));
                  }

               T* r = reinterpret_cast<T*>(policy->get_value(&object)); 
               return *r;
               }

            /// Returns true if the any contains no value. 
            bool empty() const {
               return policy == anyimpl::get_policy<anyimpl::empty_any>();
               }

            /// Frees any allocated memory, and sets the value to NULL.
            void reset() {
               policy->static_delete(&object);
               //The following used to be:
               //policy = anyimpl::get_policy<anyimpl::empty_any>();
               //But why should the policy default to the big_any_policy?  Therefore,:
               policy = anyimpl::get_policy<unsigned int>();
               }

            /// Returns true if the two types are the same. 
            template<typename T>
            bool isA() {
               return policy == anyimpl::get_policy<T>();
               }

             template <typename T> operator T&() { return cast<T>(); }
             template <typename T> operator T&() const { return cast<T>(); }

             template<typename T>
             T& cast() const {
                 if (policy != anyimpl::get_policy<T>()) {
                  throw dwl::Exception(_T("Bad 'any' cast"));
                  //was:
                  //throw anyimpl::bad_any_cast();
                    }
                 T* r = reinterpret_cast<T*>(policy->get_value(&object));
                 return *r;
                }

         }; //End of 'any' struct definition

   }
Sudden Sun Death Syndrome (SSDS) is a very real concern which we should be raising awareness of. 156 billion suns die every year before they're just 1 billion years old.
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QuestionImplicit cast fix Pin
Member 1154432525-Mar-15 11:59
Member 1154432525-Mar-15 11:59 
QuestionLast version still buggy (but I don't have a solution) Pin
David O'Neil4-Oct-14 11:45
professionalDavid O'Neil4-Oct-14 11:45 
QuestionPlease someone add Move constructor Pin
jeefo124-May-14 16:54
jeefo124-May-14 16:54 
Questionnot working with "unsigned int" Pin
cristi.arghiroiu19-Feb-14 1:00
cristi.arghiroiu19-Feb-14 1:00 
Questionfix the const problem Pin
pj220_20068-Aug-13 22:12
pj220_20068-Aug-13 22:12 
AnswerRe: fix the const problem Pin
skyformat99@gmail.com31-Jan-14 21:47
skyformat99@gmail.com31-Jan-14 21:47 
AnswerRe: fix the const problem Pin
David O'Neil28-Feb-14 20:23
professionalDavid O'Neil28-Feb-14 20:23 
BugRe: fix the const problem Pin
Martial Spirit19-Mar-16 18:26
Martial Spirit19-Mar-16 18:26 
QuestionBugfix Pin
David O'Neil9-Jul-13 20:45
professionalDavid O'Neil9-Jul-13 20:45 
QuestionAn overlooked usage case in new code Pin
David O'Neil26-Jan-13 19:01
professionalDavid O'Neil26-Jan-13 19:01 
QuestionA small helper function for you Pin
David O'Neil24-Jan-13 17:53
professionalDavid O'Neil24-Jan-13 17:53 
Questionerror: 'TYPE' was not declared in this scope Pin
yurenjimi198520-Nov-12 15:44
yurenjimi198520-Nov-12 15:44 
AnswerRe: error: 'TYPE' was not declared in this scope Pin
Member 968039912-Dec-12 12:10
Member 968039912-Dec-12 12:10 
GeneralIt looks good, but is that dynamic typing? Pin
Jamming114-May-11 12:20
Jamming114-May-11 12:20 
GeneralRe: It looks good, but is that dynamic typing? Pin
MetaVoid29-Sep-11 9:46
MetaVoid29-Sep-11 9:46 
GeneralRe: It looks good, but is that dynamic typing? Pin
jcmalek30-Jan-15 5:49
jcmalek30-Jan-15 5:49 
GeneralMemory Leaks and Crashes when storing "non-primitiv" data! Pin
Raute11-Aug-10 3:57
Raute11-Aug-10 3:57 
GeneralRe: Memory Leaks and Crashes when storing "non-primitiv" data! Pin
Christopher Diggins9-May-11 15:31
professionalChristopher Diggins9-May-11 15:31 
GeneralQuestion on checking empty Pin
Member 421168429-Mar-10 9:26
Member 421168429-Mar-10 9:26 
GeneralPreventing Reallocations Pin
Anand Krishnamoorthi18-Apr-07 6:15
Anand Krishnamoorthi18-Apr-07 6:15 
QuestionSmall Types Assignment Problem? Pin
Anand Krishnamoorthi18-Apr-07 6:10
Anand Krishnamoorthi18-Apr-07 6:10 
AnswerRe: Small Types Assignment Problem? Pin
xryl6695-Sep-07 2:28
xryl6695-Sep-07 2:28 
GeneralCompile problem with cast Pin
Neville Franks17-Jul-06 13:15
Neville Franks17-Jul-06 13:15 
GeneralRe: Compile problem with cast Pin
pablo_mag17-Jul-06 14:00
pablo_mag17-Jul-06 14:00 
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Posted 8 Aug 2005

Article Copyright 2005 by Christopher Diggins
Everything else Copyright © CodeProject, 1999-2024

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