// bdlma_localsequentialallocator.h -*-C++-*-
#ifndef INCLUDED_BDLMA_LOCALSEQUENTIALALLOCATOR
#define INCLUDED_BDLMA_LOCALSEQUENTIALALLOCATOR
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
//@PURPOSE: Provide an efficient managed allocator using a local buffer.
//
//@CLASSES:
// bdlma::LocalSequentialAllocator: allocator using a local buffer
//
//@SEE_ALSO: bdlma_bufferedsequentialallocator
//
//@DESCRIPTION: This component provides a concrete mechanism,
// 'bdlma::LocalSequentialAllocator', that implements the
// 'bdlma::ManagedAllocator' protocol to very efficiently allocate
// heterogeneous memory blocks (of varying, user-specified sizes) from a local
// buffer. Note that it derives from 'bdlma::BufferedSequentialAllocator' so
// that the implementations of 'allocate', 'deallocate', and 'release' don't
// need to be instantiated for each user-specified size.
//..
// ,-------------------------------.
// ( bdlma::LocalSequentialAllocator )
// `-------------------------------'
// | ctor
// V
// ,----------------------------------.
// ( bdlma::BufferedSequentialAllocator )
// `----------------------------------'
// | ctor/dtor
// | allocate
// | deallocate
// | release
// | rewind
// V
// ,-----------------------.
// ( bdlma::ManagedAllocator )
// `-----------------------'
// | release = 0
// V
// ,----------------.
// ( bslma::Allocator )
// `----------------'
// allocate = 0
// deallocate = 0
//..
// If an allocation request exceeds the remaining free memory space in the
// local buffer, the allocator will fall back to a sequence of
// dynamically-allocated buffers. The 'release' method releases all memory
// allocated through the allocator, as does the destructor. Note that, even
// though a 'deallocate' method is available, it has no effect: individually
// allocated memory blocks cannot be separately deallocated.
//
// 'bdlma::LocalSequentialAllocator' is typically used when users have a
// reasonable estimation of the amount of memory needed. This amount of memory
// would then be created directly on the program stack, and used as the local
// buffer by this allocator for very fast memory allocation. Whilst the buffer
// has sufficient capacity, memory allocations will not trigger *any* dynamic
// memory allocation, will have optimal locality of reference, and will not
// require deallocation upon destruction.
//
// Once the local buffer is exhausted, subsequent allocation requests require
// dynamic memory allocation, and the performance of the allocator degrades.
//
// The main difference between a 'bdlma::LocalSequentialAllocator' and a
// 'bdlma::BufferedSequentialAllocator' is that this class internally maintains
// a buffer, rather than being given one at construction time.
//
///Usage
///-----
// This section illustrates intended use of this component.
//
///Example 1: Recommended Usage
/// - - - - - - - - - - - - - -
// Suppose we have a function which takes a map of items to update in some
// database:
//..
// typedef bsl::string DatabaseKey;
// typedef bsl::string DatabaseValue;
//
// void updateRecords_1(const bsl::map<DatabaseKey, DatabaseValue>& values)
// {
// for (bsl::map<DatabaseKey, DatabaseValue>::const_iterator
// it = values.begin(), end = values.end();
// it != end;
// ++it) {
// bsl::stringbuf stringBuf;
// bsl::ostream ostr(&stringBuf);
//
// ostr << "UPDATE myTable SET myValue = '" << it->first << "' WHERE "
// "myKey = '" << it->second << "'";
//
// // execute query using 'stringBuf.str()'
// }
// }
//..
// We call this method a lot, and after profiling, we notice that it's
// contributing a significant proportion of time, due to the allocations it is
// making. We decide to see whether a LocalSequentialAllocator would help.
//
// First, use a 'bslma::TestAllocator' to track the typical memory usage:
//..
// void updateRecords_2(const bsl::map<DatabaseKey, DatabaseValue>& values)
// {
// bslma::TestAllocator ta;
//
// for (bsl::map<DatabaseKey, DatabaseValue>::const_iterator
// it = values.begin(), end = values.end();
// it != end;
// ++it) {
// bsl::stringbuf stringBuf(&ta);
// bsl::ostream ostr(&stringBuf);
//
// ostr << "UPDATE myTable SET myValue = '" << it->first << "' WHERE "
// "myKey = '" << it->second << "'";
//
// // execute query using 'stringBuf.str()'
//
// bsl::cout << "In use: " << ta.numBytesInUse() << '\n';
// }
//
// bsl::cout << "Max: " << ta.numBytesMax() << '\n';
// }
//..
// Then we run our program again, and observe the following output:
//..
// In use: 77
// In use: 77
// In use: 77
// In use: 77
// In use: 77
// Max: 129
//..
// It looks like 129 is a good choice for the size of our allocator, so we go
// with that:
//..
// void updateRecords_3(const bsl::map<DatabaseKey, DatabaseValue>& values)
// {
// bdlma::LocalSequentialAllocator<129> lsa;
//
// for (bsl::map<DatabaseKey, DatabaseValue>::const_iterator
// it = values.begin(), end = values.end();
// it != end;
// ++it) {
// lsa.release();
//
// bsl::stringbuf stringBuf(&lsa);
// bsl::ostream ostr(&stringBuf);
//
// ostr << "UPDATE myTable SET myValue = '" << it->first << "' WHERE "
// "myKey = '" << it->second << "'";
//
// // execute query using 'stringBuf.str()'
// }
// }
//..
// Note that we release at the end of every iteration, as the deallocate method
// is a no-op, so without this, subsequent memory would be allocated from the
// default allocator (or the allocator passed to 'bsa' at construction).
//
// Finally, we re-profile our code to determine whether the addition of a
// 'LocalSequentialAllocator' helped.
#include <bdlscm_version.h>
#include <bdlma_bufferedsequentialallocator.h>
#include <bslma_allocator.h>
namespace BloombergLP {
namespace bdlma {
// ==============================
// class LocalSequentialAllocator
// ==============================
template <int t_SIZE>
class LocalSequentialAllocator : public BufferedSequentialAllocator {
// This class implements the 'ManagedAllocator' protocol to provide a fast
// allocator that dispenses heterogeneous blocks of memory (of varying,
// user-specified sizes) from a local buffer whose capacity is the
// specified 't_SIZE' (in bytes). If an allocation request exceeds the
// remaining free memory space in the local buffer, memory will be supplied
// by an (optional) allocator supplied at construction; if no allocator is
// supplied, the currently installed default allocator is used. This class
// is *exception* *neutral*; if memory cannot be allocated, the behavior is
// defined by the (optional) allocator supplied at construction.
// PRIVATE TYPES
typedef BufferedSequentialAllocator AllocatorBase;
#if !defined(BSLS_COMPILERFEATURES_SUPPORT_ALIGNAS)
typedef bsls::AlignmentToType<
bsls::AlignmentUtil::BSLS_MAX_ALIGNMENT>::Type AlignmentType;
#endif
// DATA
#if defined(BSLS_COMPILERFEATURES_SUPPORT_ALIGNAS)
// The C++11 implementation uses the 'alignas' keyword to ensure the
// alignment of 'd_buffer'.
alignas(bsls::AlignmentUtil::BSLS_MAX_ALIGNMENT) char d_buffer[t_SIZE];
#else
union {
// This anonymous union is 'bsls::AlignedBuffer', but typed out again
// so that extra template instantiations are avoided. The C++03
// implementation uses a union data member to ensure the alignment of
// 'd_buffer'.
char d_buffer[t_SIZE];
AlignmentType d_align;
};
#endif
private:
// NOT IMPLEMENTED
//! LocalSequentialAllocator(const LocalSequentialAllocator&) = delete;
//! LocalSequentialAllocator& operator=(
// const LocalSequentialAllocator&) = delete;
public:
// CREATORS
explicit LocalSequentialAllocator(bslma::Allocator *basicAllocator = 0);
// Create a local sequential allocator for allocating memory blocks
// from a local buffer having the specified 't_SIZE' (in bytes).
// Optionally specify a 'basicAllocator' used to supply memory should
// the capacity of the local buffer be exhausted. If 'basicAllocator'
// is 0, the currently installed default allocator is used.
//! virtual ~LocalSequentialAllocator() = default;
// Destroy this local sequential allocator. All memory allocated from
// this allocator is released.
};
// ============================================================================
// INLINE DEFINITIONS
// ============================================================================
// ------------------------------
// class LocalSequentialAllocator
// ------------------------------
// CREATORS
template <int t_SIZE>
inline
LocalSequentialAllocator<t_SIZE>::LocalSequentialAllocator(
bslma::Allocator *basicAllocator)
: AllocatorBase(this->d_buffer, t_SIZE, basicAllocator)
{
}
} // close package namespace
} // close enterprise namespace
#endif
// ----------------------------------------------------------------------------
// Copyright 2015 Bloomberg Finance L.P.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// ----------------------------- END-OF-FILE ----------------------------------