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Foundation/include/Poco/SmallObjectAllocator.h

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+//
+// 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