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

, 9 May 2011 CPOL
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A high-performance alternative to boost::any.


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.

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:

#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

#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) = **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.
    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);

    #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
    // fields
    anyimpl::base_any_policy* policy;
    void* object;

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

    /// Empty constructor. 
        : 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) 

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

    /// Destructor. 
    ~any() {

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

    /// Assignment function. 
    template <typename T>
    any& assign(const T& x) {
        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 = 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.


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


About the Author

Christopher Diggins
Software Developer Autodesk
Canada Canada
This article was written by Christopher Diggins, a computer science nerd who currently works at Autodesk as an SDK specialist.
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Comments and Discussions

QuestionLast version still buggy (but I don't have a solution) PinmemberDavid O'Neil4-Oct-14 12:45 
QuestionPlease someone add Move constructor Pinmemberjeefo124-May-14 17:54 
Questionnot working with "unsigned int" Pinmembercristi.arghiroiu19-Feb-14 2:00 
Questionfix the const problem [modified] Pinmemberpj220_20068-Aug-13 23:12 
AnswerRe: fix the const problem Pinmemberskyformat99@gmail.com31-Jan-14 22:47 
AnswerRe: fix the const problem PinmemberDavid O'Neil28-Feb-14 21:23 
QuestionBugfix [modified] PinmemberDavid O'Neil9-Jul-13 21:45 
QuestionAn overlooked usage case in new code PinmemberDavid O'Neil26-Jan-13 20:01 
QuestionA small helper function for you [modified] PinmemberDavid O'Neil24-Jan-13 18:53 
I just updated my projects to use your new code. First, congratulations on the changes - this updated version is SO much cleaner than the original I can almost wrap my head around it! But I was missing the 'type' function of your old version, as my main project using this tested against that. So I came up with the following small addition which makes replacing that functionality much easier.
    template<typename T>
    bool compatible() {
      T temp;
      return compatible(temp);
Just add this under the definition of the other 'compatible' function. For those readers who try to use it and are unfamiliar with the template terminology, an example invocation would be something like
if (!buttonC->user.compatible<int>()) doSomething();
Pretty simple, and not much of an addition, but hopefully helpful to some.
[Edit] Having used this for an hour, I'm changing 'compatible' to 'isA', as it better describes the usage context. For example,
if (!buttonC->user.isA<int>()) doSomething();
is a richer and more meaningful syntax than the original.

modified 25-Jan-13 1:32am.

Questionerror: 'TYPE' was not declared in this scope Pinmemberyurenjimi198520-Nov-12 16:44 
AnswerRe: error: 'TYPE' was not declared in this scope PinmemberMember 968039912-Dec-12 13:10 
GeneralIt looks good, but is that dynamic typing? PinmemberJamming114-May-11 13:20 
GeneralRe: It looks good, but is that dynamic typing? PinmemberMetaVoid29-Sep-11 10:46 
GeneralMemory Leaks and Crashes when storing "non-primitiv" data! PinmemberRaute11-Aug-10 4:57 
GeneralRe: Memory Leaks and Crashes when storing "non-primitiv" data! PinmemberChristopher Diggins9-May-11 16:31 
GeneralQuestion on checking empty PinmemberMember 421168429-Mar-10 10:26 
GeneralPreventing Reallocations PinmemberAnand Krishnamoorthi18-Apr-07 7:15 
QuestionSmall Types Assignment Problem? PinmemberAnand Krishnamoorthi18-Apr-07 7:10 
AnswerRe: Small Types Assignment Problem? Pinmemberxryl6695-Sep-07 3:28 
GeneralCompile problem with cast PinmemberNeville Franks17-Jul-06 14:15 
GeneralRe: Compile problem with cast Pinmemberpablo_mag17-Jul-06 15:00 
GeneralRe: Compile problem with cast [modified] PinmemberNeville Franks17-Jul-06 15:31 
GeneralRe: Compile problem with cast [modified] Pinmemberpablo_mag17-Jul-06 16:55 
GeneralRe: Compile problem with cast PinmemberNeville Franks17-Jul-06 19:53 
GeneralCompile error with const char* Pinmemberfarshizzo26-Apr-06 8:23 

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