Add a STL Allocator using the stack (for rt safety)

This is a dumb stack allocator using LIFO allocation/free, with
a fallback to new/delete. This works well for small STL containers
in particular std::vector, but it's also suitable for std::map<>,
in particular copy constructing small POD maps (plugin pin mappings).

Eventually this could be combined with TLSF for a flexible
memory arena management. This is however not currently needed
for any the planned use-cases.

This code is ANSI C++98 compatible, and yet also works with
modern C++11, C++14

libs/pbd/pbd/stack_allocator.h

@@ -0,0 +1,159 @@
+/*
+ * Copyright (C) 2020 Robin Gareus <robin@gareus.org>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef PBD_STACK_ALLOCATOR_H
+#define PBD_STACK_ALLOCATOR_H
+
+#include <boost/type_traits/aligned_storage.hpp>
+#include <limits>
+
+#include "pbd/libpbd_visibility.h"
+
+#if 0
+# include <cstdio>
+# define DEBUG_STACK_ALLOC(...) printf (__VA_ARGS__)
+#else
+# define DEBUG_STACK_ALLOC(...)
+#endif
+
+namespace PBD {
+
+template <class T, std::size_t stack_capacity>
+class /*LIBPBD_API*/ StackAllocator
+{
+public:
+#if 0 /* may be needed for compatibility */
+	typedef typename std::allocator<T>::value_type value_type;
+	typedef typename std::allocator<T>::size_type size_type;
+	typedef typename std::allocator<T>::difference_type difference_type;
+	typedef typename std::allocator<T>::pointer pointer;
+	typedef typename std::allocator<T>::const_pointer const_pointer;
+	typedef typename std::allocator<T>::reference reference;
+	typedef typename std::allocator<T>::const_reference const_reference;
+#else
+	typedef T                 value_type;
+	typedef std::size_t       size_type;
+	typedef std::ptrdiff_t    difference_type;
+	typedef value_type*       pointer;
+	typedef const value_type* const_pointer;
+	typedef value_type&       reference;
+	typedef const value_type& const_reference;
+#endif
+
+	template <class U>
+	struct rebind {
+		typedef StackAllocator<U, stack_capacity> other;
+	};
+
+	StackAllocator ()
+		: _ptr ((pointer)&_buf)
+	{ }
+
+	StackAllocator (const StackAllocator&)
+		: _ptr ((pointer)&_buf)
+	{ }
+
+	template <typename U, size_t other_capacity>
+	StackAllocator (const StackAllocator<U, other_capacity>&)
+		: _ptr ((pointer)&_buf)
+	{ }
+
+	/* inspired by http://howardhinnant.github.io/stack_alloc.h */
+	pointer allocate (size_type n, void* hint = 0)
+	{
+		if ((pointer)&_buf + stack_capacity >= _ptr + n) {
+			DEBUG_STACK_ALLOC ("Allocate %ld item(s) of size %zu on the stack\n", n, sizeof (T));
+			pointer rv = _ptr;
+			_ptr += n;
+			return rv;
+		} else {
+			DEBUG_STACK_ALLOC ("Allocate using new (%ld * %zu)\n", n, sizeof (T));
+			return static_cast<pointer> (::operator new (n * sizeof (T)));
+		}
+	}
+
+	void deallocate (pointer p, size_type n)
+	{
+		if (pointer_in_buffer (p)) {
+			if (p + n == _ptr) {
+				DEBUG_STACK_ALLOC ("Deallocate: pop item from the top of the stack\n");
+				_ptr = p;
+			} else {
+				DEBUG_STACK_ALLOC ("Deallocate: ignored. Item is not at the top of the stack \n");
+			}
+		} else {
+			::operator delete (p);
+		}
+	}
+
+	size_type max_size () const throw ()
+	{
+		return std::numeric_limits<size_type>::max () / sizeof (T);
+	}
+
+	bool operator== (StackAllocator const& a) const
+	{
+		return &_buf == &a._buf;
+	}
+
+	bool operator!= (StackAllocator const& a) const
+	{
+		return &_buf != &a._buf;
+	}
+
+	template <class U>
+	void destroy (U* const p)
+	{
+		p->~U ();
+	}
+
+	template <class U>
+	void construct (U* const p)
+	{
+		new (p) U ();
+	}
+
+	template <class U, class A>
+	void construct (U* const p, A& a)
+	{
+		new (p) U (a);
+	}
+
+	template <class U, class A, class B>
+	void construct (U* const p, A& a, B& b)
+	{
+		new (p) U (a, b);
+	}
+
+private:
+	StackAllocator& operator= (const StackAllocator&);
+
+	bool pointer_in_buffer (pointer const p)
+	{
+		return ((pointer const)&_buf <= p && p < (pointer const)&_buf + stack_capacity);
+	}
+
+	typedef typename boost::aligned_storage<sizeof (T) * stack_capacity, 16>::type align_t;
+
+	align_t _buf;
+	pointer _ptr;
+};
+
+} // namespace PBD
+
+#endif