This project implements the following 2 complementary header-only C++ classes for C++17's std::any;

1. any_shared_ptr- a type-safe container for std::shared_ptr<T> of any type T (see include/any_shared_ptr.hpp).
2. any_ptr - a type-safe container for pointers to any type (see include/any_ptr.hpp).

that, unlike std::any, preserves pointer cv-qualifier promotion and up-cast behaviour.

any_shared_ptr

Why do we need any_shared_ptr

Consider the following trivial example:

struct Base {};

struct Derived : public Base {};

std::shared_ptr<Derived> ptr = std::make_shared<Derived>();

You can use std::any to store std::shared_ptr<Derived> but its reference semantics are lost.

using namespace std;

any any{ ptr };

// OK  - casting to the same cv-qualified type
shared_ptr<Derived> derived = any_cast< shared_ptr<Derived> >( any );  

// FAILED - throws std::bad_any_cast as there's no implicit cv-qualifier promotion
shared_ptr<const Derived> const_derived = any_cast< shared_ptr<const Derived> >( any );  

// FAILED - throws std::bad_any_cast as there's no implicit up-cast
shared_ptr<Base> base = any_cast< shared_ptr<Base> >( any );

Now consider any_shared_ptr

using namespace std;

any_share_ptr any{ ptr };

// OK  - casting to the same cv-qualified type
shared_ptr<Derived> derived = any_shared_ptr_cast< Derived >( any );  

// OK - cv-qualifier promotion is supported
shared_ptr<const Derived> const_derived = any_shared_ptr_cast< const Derived >( any );  

// OK - up-cast is supported
shared_ptr<Base> base = any_shared_ptr_cast< Base >( any );  

An occasional user of std::any may be surprised to find that std::any doesn't preserve pointer cv-qualifier promotion (e.g. shared_ptr<Derived> -> shared_ptr<const Derived>) or pointer up-cast behaviour (shared_ptr<Dervied> -> shared_ptr<Base>). This is simply a limitation of std::any. Its primary purpose is to store objects and not references. In contrast any_shared_ptr is designed to store references to an object and thus preserves normal pointer behaviour. However there's a catch if performance is critical.

any_shared_ptr interface

any_shared_ptr's interface tries to be as consistent as possible with std::any_ptr. The notable exceptions are;

1. The noexcept version of any_shared_ptr_cast returns std::optional<std::shared_ptr<T>> and not std::shared_ptr<T>* as is the case for std::any_ptr_cast. The reason is due to the temporary shared_ptr<T> returned by any_shared_ptr_cast that's necessary to handle a different pointer address that can occurr when up-casting due to multiple inheritance.
2. any_shared_ptr's interface includes the following std::shared_ptr observer member functions - use_count

Member functions

Constructors

1. // set to empty state
   constexpr any_share_ptr() noexcept;

2. template<typename T>
   any_share_ptr(std::shared_ptr ptr);

Modifers

3. // destroys contained object
   void reset();

4. // swaps two any_shared_ptr objects
   void swap(any_shared_ptr & other);

Observers

5. // checks if object holds a value
   bool has_value() const;

6. // returns typeid(std::shared_ptr) of the contained value
   const std::info_type & type() const;

7. // returns the number of shared_ptr objects referring to the same managed object
   bool use_count() const noexcept;

Non-member functions

6. // swaps two any_shared_ptr objects
   std::swap(any_shared_ptr & , any_shared_ptr & );

7. // type-safe access to the contained object
   template<typename T>
   std::optional<std::shared_ptr> any_shared_ptr_cast(any_shared_ptr const * a) noexcept;

8. // type-safe access to the contained object
   template<typename T>
   std::shared_ptr any_shared_ptr_cast(any_shared_ptr const & a);

9. // creates an any object
   template<class T, class... Args>
   any_shared_ptr make_any_shared_ptr(Args&&... args);

Helper classes

10. // exception thrown by the value-returning forms of any_shared_ptr_cast on cast failure
    bad_any_shared_ptr_cast

What's the catch

To implement any_shared_ptr we need a new function, let's call it dynamic_up_cast, that's similar to C++'s dynamic_cast except that it only performs an up-cast. We could try implementing dynamic_up_cast by accessing the compilers internal RTTI data structures in a similar manner as dynamic_cast. However many compiler/platform combinations would require its own implementation which is not very appealing. Instead a better solution is a portable implementation that needs nothing more than standard C++ that's supported by all compilers. The inspiration we need is Cassio Neri's observation [1] that throwing and catching an exception can be used to implement an up-cast as shown in the following code snippet.

struct any_ptr
{ 
  using Throw_pointer_func = void(void *);

  Throw_pointer_func *    my_throw_pointer_func;
  void *                  my_ptr;

  template<typename T>
  static void throw_pointer(void * ptr)
  {
    throw static_cast<T*>(ptr);
  }

  template<typename T>
  any_ptr::any_ptr(T* ptr)
    : my_throw_pointer_func{ & any_ptr::throw_pointer<T> }
    , my_ptr{ const_cast<T*>(ptr) }
  {
    assert(ptr!=nullptr);
  }

  template<typename CastToType>
  CastToType* dynamic_up_cast()
  {
    try {
      my_throw_pointer_func(my_ptr);
    }
    catch(CastToType* to_ptr) {
      // note my_ptr != to_ptr is possible for 
      // class hierarchies using multiple inheritance 
      return to_ptr;
    }
    return nullptr;
  }
};

template<typename CastToType>
CastToType* any_ptr_cast(any_ptr const & a)
{
  CastToType* ptr = a.template dynamic_up_cast<CastToType>();
  if (ptr) {
    return ptr;
  }
  throw std::bad_cast();
}

The performance penalty

C++ exceptions are intended to used as an error reporting mechanism and thus the performance of try/catch blocks are optimised for the situation when no exception is thrown. Consequently we can expect a performance penalty if our normal non-error code path is based on throwing and catching an exception.

Using Google's microbenchmark library (see the src/benchmark folder) we observe that the implicit up-cast is ~100x slower than the basic cast to the same type held by any_shared_ptr.

any_shared_ptr	Benchmark (x64)	MSVC 2017	GCC 6.2	Clang 3.8
any_shared_ptr_cast< Derived >(any)	Cast to same type	19 ns	14 ns	14 ns
any_shared_ptr_cast< const Derived >(any)	Cv-qualifier promotion	2023 ns	2014 ns	2052 ns
any_shared_ptr_cast< Base >(any)	up-cast	2166 ns	2048 ns	2092 ns
any_shared_ptr_cast< int >(any)	Bad cast	4176 ns	3608 ns	5147 ns

The processor used for benchmark was an Intel i7-4710HQ 2.3GHz

Debug vs Release

By default, any_ptr builds as a debug library. You will see a warning in the output when this is the case. To build it as a release library, use:

cmake -DCMAKE_BUILD_TYPE=Release

To enable link-time optimisation, use

cmake -DCMAKE_BUILD_TYPE=Release -DBENCHMARK_ENABLE_LTO=true

To use Clang

cmake -DCMAKE_BUILD_TYPE=Release -DBENCHMARK_ENABLE_LTO=true -DCMAKE_CXX_COMPILER=clang++-3.9

Compiler Support

any_shared_ptr is implemented using shared_ptr's aliasing constructor that was added to the C++11 standard and thus was not in the TR1 version of shared_ptr (see Anthony Williams execellent blog std::shared_ptr's secret constructor for further details). In addition Google's benchmark and test libraries require a modern C++ toolchain, both compiler and standard library.

The following minimum versions are strongly recommended to build the library:

• GCC 5
• Clang 3.8
• Visual Studio 2015

Anything older may work.

any_ptr

any_ptr is designed to hold native pointers of any type that, like any_shared_ptr, preserves pointer cv-qualifier promotion and up-cast behaviour. Note that any_ptr doesn't attempt to manage the lifetime of the object referenced by the native pointer. Any lifetime management must be done externally.

The following is a simple usage example:

struct Base {};

struct Derived : public Base {};

std::shared_ptr<Derived> ptr = std::make_shared<Derived>();

Derived * p = ptr.get(); 

any_ptr any{ p };

// OK  - casting to the same cv-qualified type
Derived* derived = any_ptr_cast< Derived >( any );  

// OK - cv-qualifier promotion is supported
const Derived* const_derived = any_ptr_cast< const Derived >( any );  

// OK - up-cast is supported
Base* base = any_ptr_cast< Base >( any );  

Benchmarks

any_ptr	Benchmark (x64)	MSVC 2017	GCC 6.2	Clang 3.8
any_ptr_cast< Derived >(any)	Cast to same type	5 ns	2 ns	2 ns
any_ptr_cast< const Derived >(any)	Cv-qualifier promotion	2182 ns	1939 ns	2018 ns
any_ptr_cast< Base >(any)	Up-cast	2230 ns	1964 ns	2089 ns
any_ptr_cast< int >(any)	Bad cast	4441 ns	3688 ns	5102 ns

References

[1] Cassio Neri, Twisting the RTTI System for Safe Dynamic Casts of void* in C++, Dr Dobbs, (2011).