bdlma_localsequentialallocator.h

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/// @file bdlma_localsequentialallocator.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bdlma_localsequentialallocator.h                                   -*-C++-*-
#ifndef INCLUDED_BDLMA_LOCALSEQUENTIALALLOCATOR
#define INCLUDED_BDLMA_LOCALSEQUENTIALALLOCATOR
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bdlma_localsequentialallocator bdlma_localsequentialallocator
/// @brief  Provide an efficient managed allocator using a local buffer.
/// @addtogroup bdl
/// @{
/// @addtogroup bdlma
/// @{
/// @addtogroup bdlma_localsequentialallocator
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bdlma_localsequentialallocator-purpose"> Purpose</a>
/// * <a href="#bdlma_localsequentialallocator-classes"> Classes </a>
/// * <a href="#bdlma_localsequentialallocator-description"> Description </a>
///   * <a href="#bdlma_localsequentialallocator-usage"> Usage </a>
///     * <a href="#bdlma_localsequentialallocator-example-1-recommended-usage"> Example 1: Recommended Usage </a>
///
/// # Purpose {#bdlma_localsequentialallocator-purpose}
/// Provide an efficient managed allocator using a local buffer.
///
/// # Classes {#bdlma_localsequentialallocator-classes}
///
/// -  bdlma::LocalSequentialAllocator: allocator using a local buffer
///
/// @see  bdlma_bufferedsequentialallocator
///
/// # Description {#bdlma_localsequentialallocator-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.
/// @code
///   ,-------------------------------.
///  ( bdlma::LocalSequentialAllocator )
///   `-------------------------------'
///                   |        ctor
///                   V
///  ,----------------------------------.
/// ( bdlma::BufferedSequentialAllocator )
///  `----------------------------------'
///                   |        ctor/dtor
///                   |        allocate
///                   |        deallocate
///                   |        release
///                   |        rewind
///                   V
///       ,-----------------------.
///      ( bdlma::ManagedAllocator )
///       `-----------------------'
///                   |        release = 0
///                   V
///          ,----------------.
///         ( bslma::Allocator )
///          `----------------'
///                            allocate = 0
///                            deallocate = 0
/// @endcode
/// 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 {#bdlma_localsequentialallocator-usage}
///
///
/// This section illustrates intended use of this component.
///
/// ### Example 1: Recommended Usage {#bdlma_localsequentialallocator-example-1-recommended-usage}
///
///
/// Suppose we have a function which takes a map of items to update in some
/// database:
/// @code
/// 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()'
///     }
/// }
/// @endcode
/// 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:
/// @code
/// 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';
/// }
/// @endcode
/// Then we run our program again, and observe the following output:
/// @code
/// In use: 77
/// In use: 77
/// In use: 77
/// In use: 77
/// In use: 77
/// Max: 129
/// @endcode
/// It looks like 129 is a good choice for the size of our allocator, so we go
/// with that:
/// @code
/// 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()'
///     }
/// }
/// @endcode
/// 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.
/// @}
/** @} */
/** @} */
 
/** @addtogroup bdl
 * @{
 */
/** @addtogroup bdlma
 * @{
 */
/** @addtogroup bdlma_localsequentialallocator
 * @{
 */
 
#include <bdlscm_version.h>
 
#include <bdlma_bufferedsequentialallocator.h>
 
#include <bslma_allocator.h>
 
 
namespace bdlma {
 
                      // ==============================
                      // class LocalSequentialAllocator
                      // ==============================
 
/// 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.
///
/// See @ref bdlma_localsequentialallocator 
template <int t_SIZE>
class LocalSequentialAllocator : public BufferedSequentialAllocator {
 
    // 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
    /// 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`.
    union {
        char          d_buffer[t_SIZE];
        AlignmentType d_align;
    };
#endif
 
  private:
    // NOT IMPLEMENTED
 
     LocalSequentialAllocator(const LocalSequentialAllocator&) = delete;
     LocalSequentialAllocator& operator=(
                                  const LocalSequentialAllocator&) = delete;
 
  public:
    // CREATORS
 
    /// 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.
    explicit LocalSequentialAllocator(bslma::Allocator *basicAllocator = 0);
 
    /// Destroy this local sequential allocator.  All memory allocated from
    /// this allocator is released.
     virtual ~LocalSequentialAllocator() = default;
};
 
// ============================================================================
//                             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
 
 
#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 ----------------------------------
 
/** @} */
/** @} */
/** @} */