poco/Foundation/include/Poco/SmallObjectAllocator.h

//
// SmallObjectAllocator.h
//
// $Id: //poco/1.4/Foundation/include/Poco/SmallObjectAllocator.h#1 $
//
// Library: Foundation
// Package: Core
// Module:  SmallObjectAllocator
//
// Definition of the SmallObjectAllocator template.
//
// Copyright (c) 2004-2006, Applied Informatics Software Engineering GmbH.
// and Contributors.
//
// Permission is hereby granted, free of charge, to any person or organization
// obtaining a copy of the software and accompanying documentation covered by
// this license (the "Software") to use, reproduce, display, distribute,
// execute, and transmit the Software, and to prepare derivative works of the
// Software, and to permit third-parties to whom the Software is furnished to
// do so, all subject to the following:
// 
// The copyright notices in the Software and this entire statement, including
// the above license grant, this restriction and the following disclaimer,
// must be included in all copies of the Software, in whole or in part, and
// all derivative works of the Software, unless such copies or derivative
// works are solely in the form of machine-executable object code generated by
// a source language processor.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
// SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
// FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
// ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
// DEALINGS IN THE SOFTWARE.
//


#ifndef Foundation_SmallObjectAllocator_INCLUDED
#define Foundation_SmallObjectAllocator_INCLUDED


#include "Poco/Foundation.h"
#include "Poco/Types.h"


namespace Poco {


template <typename T, std::size_t S = sizeof(T)>
class SmallObjectAllocator
	/// SmallObjectAllocator template is a lightweight wrapper implementing
	/// small object allocation optimization idiom. Class is statically
	/// parameterized on type created and the size threshold, which defaults
	/// to the created object size; it follows that the default behavior is
	/// use of automatically allocated memory (i.e. the value wrapped is
	/// stored in the pre-allocated buffer). To enforce heap allocation,
	/// specify S parameter to a value smaller than the size of the type 
	/// being instantiated.
	/// 
	/// The SmallObjectAllocator trade-off is the runtime check for memory 
	/// location where the value resides. The runtime check occurs at 
	/// construction/destruction as well as every value access. Value
	/// access check can be alleviated by caching the value reference.
	///
	/// Usage example:
	/// 
	///    SmallObjectAllocator<int> s; // on the stack
	///    SmallObjectAllocator<int, 1> h; // force heap alloc
	///    int& i = s.get(); // no runtime performance penalty after this point
	///    i = 5;
	/// 
{
public:
	SmallObjectAllocator()
	{
		if (isOnHeap()) _memory.ptr = new T();
		else new (_memory.buf) T;
	}

	SmallObjectAllocator(const T& val)
	{
		if (isOnHeap()) _memory.ptr = new T(val);
		else new (_memory.buf) T(val);
	}

	SmallObjectAllocator(const SmallObjectAllocator& other)
	{
		if (this != &other)
		{
			if (isOnHeap())
				_memory.ptr = new T(*other._memory.ptr);
			else
				new (_memory.buf) T(*reinterpret_cast<const T*>(other._memory.buf));
		}
	}
	
	~SmallObjectAllocator()
	{
		if (isOnHeap()) delete _memory.ptr;
		else reinterpret_cast<T*>(_memory.buf)->~T();
	}

	SmallObjectAllocator& operator =(const SmallObjectAllocator& other)
	{
		if (this != &other)
		{
			if (isOnHeap())
				_memory.ptr = new T(*other._memory.ptr);
			else
				new (_memory.buf) T(*reinterpret_cast<const T*>(other._memory.buf));
		}

		return *this;
	}

	T& get()
	{
		if (isOnHeap())
			return *_memory.ptr;
		else
			return reinterpret_cast<T&>(*(_memory.buf));
	}

	const T& get() const
	{
		if (isOnHeap())
			return *_memory.ptr;
		else
			return reinterpret_cast<const T&>(*_memory.buf);
	}

	bool isOnHeap() const
	{
		return sizeof(T) > S;
	}

private:
	union
	{
		unsigned char buf[S]; 
		T* ptr;
	} _memory;
};


template <>
class SmallObjectAllocator <char*>
	/// SmallObjectAllocator char* specialization.
	/// 
	/// The treshold between auto and heap allocation
	/// is controlled through POCO_SMALL_OBJECT_SIZE compile
	/// time constant, which on 32 or 64 systems defaults
	/// to 31 or 63 bytes respectively. This specialization
	/// adds an extra byte to indicate the allocation strategy.
	/// 
	/// To summarize, e.g. POCO_SMALL_OBJECT_SIZE value of 7 will
	/// take up at least 8 bytes of storage (for auto allocation)
	/// with 6 bytes as the effective string length:
	/// 
	///  +---+---+---+---+---+---+---+---+
	///  | c | c | c | c | c | c | 0 | f |
	///  +---+---+---+---+---+---+---+---+
	///
	/// Legend:
	///    c - character
	///    0 - terminating zero
	///    f - allocation flag
	/// 
{
public:
	static const std::size_t Size = POCO_SMALL_OBJECT_SIZE;

	SmallObjectAllocator(std::size_t sz)
	{
		_memory.buf[Size] = (sz > Size) ? 1 : 0;
		if (_memory.buf[Size]) _memory.ptr = new char[sz + 1];
	}

	SmallObjectAllocator(const char* pCh, std::size_t sz)
	{
		allocateAndAssign(pCh, sz);
	}

	SmallObjectAllocator(const char* pCh)
	{
		allocateAndAssign(pCh, strlen(pCh));
	}
	
	~SmallObjectAllocator()
	{
		if (_memory.buf[Size]) delete _memory.ptr;
	}

	SmallObjectAllocator <char*>& operator = (const char* pCh)
	{
		allocateAndAssign(pCh, strlen(pCh));
		return *this;
	}

	char* get()
	{
		if (_memory.buf[Size]) return _memory.ptr;
		else return _memory.buf;
	}

	const char* get() const
	{
		if (_memory.buf[Size]) return _memory.ptr;
		else return _memory.buf;
	}

	bool isOnHeap() const
	{
		return _memory.buf[Size] != 0;
	}

private:
	SmallObjectAllocator();

	void allocateAndAssign(const char* pCh, std::size_t sz)
	{
		_memory.buf[Size] = (sz > Size) ? 1 : 0;
		if (_memory.buf[Size])
		{
			_memory.ptr = new char[sz + 1];
			_memory.ptr[sz] = '\0';
			std::memcpy(_memory.ptr, pCh, sz);
		}
		else
		{
			_memory.buf[sz] = '\0';
			std::memcpy(_memory.buf, pCh, sz);
		}
	}

	union
	{
		char buf[Size + 1]; 
		char* ptr;
	} _memory;
};


typedef SmallObjectAllocator<char*> SmallStringAllocator;


} // namespace Poco


#endif // Foundation_SmallObjectAllocator_INCLUDED