website2022/community/implementation_variations.html

---
title: Boost Implementation Variations
copyright: Beman Dawes 2001.
revised: 
---


Boost Implementation Variations



Boost Implementation Variations
===============================

Separation of interface and implementation
------------------------------------------


The interface specifications for boost.org library
 components (as well as for quality software in general) are
 conceptually separate from implementations of those interfaces.
 This may not be obvious, particularly when a component is
 implemented entirely within a header, but this separation of
 interface and implementation is always assumed. From the
 perspective of those concerned with software design,
 portability, and standardization, the interface is what is
 important, while the implementation is just a detail.


Dietmar Kühl, one of the original boost.org
 contributors, comments "The main contribution is the interface,
 which is augmented with an implementation, proving that it is
 possible to implement the corresponding class and providing a
 free implementation."


Implementation variations
-------------------------


There may be a need for multiple implementations of an
 interface, to accommodate either platform dependencies or
 performance tradeoffs. Examples of platform dependencies
 include compiler shortcomings, file systems, thread mechanisms,
 and graphical user interfaces. The classic example of a
 performance tradeoff is a fast implementation that uses a lot
 of memory versus a slower implementation which uses less
 memory.


Boost libraries generally use a [configuration
 header](/doc/libs/release/libs/config/config.htm), boost/config.hpp, to capture compiler and platform
 dependencies. Although the use of boost/config.hpp is not
 required, it is the preferred approach for simple configuration
 problems.


Boost policy
------------


The Boost policy is to avoid platform dependent variations
 in interface specifications, but supply implementations which
 are usable over a wide range of platforms and applications.
 That means boost libraries will use the techniques below
 described as appropriate for dealing with platform
 dependencies.


The Boost policy toward implementation variations designed
 to enhance performance is to avoid them unless the benefits
 greatly exceed the full costs. The term "full costs" is
 intended to include both tangible costs like extra maintenance,
 and intangible cost like increased difficulty in user
 understanding.


Techniques for providing implementation variations
--------------------------------------------------


Several techniques may be used to provide implementation
 variations. Each is appropriate in some situations, and not
 appropriate in other situations.


### Single general purpose implementation


The first technique is to simply not provide implementation
 variation at all. Instead, provide a single general-purpose
 implementation, and forgo the increased complexity implied by
 all other techniques.


**Appropriate:** When it is possible to write a
 single portable implementation which has reasonable performance
 across a wide range of platforms. Particularly appropriate when
 alternative implementations differ only in esoteric ways.


**Not appropriate:** When implementation
 requires platform specific features, or when there are multiple
 implementation possible with widely differing performance
 characteristics.


Beman Dawes comments "In design discussions, some
 implementation is often alleged to be much faster than another,
 yet a timing test discovers no significant difference. The
 lesson is that while algorithmic differences may affect speed
 dramatically, coding differences such as changing a class from
 virtual to non-virtual members or removing a level of
 indirection are unlikely to make any measurable difference
 unless deep in an inner loop. And even in an inner loop, modern
 CPUs often execute such competing code sequences in the same
 number of clock cycles! A single general purpose implementation
 is often just fine."


Or as Donald Knuth said, "Premature optimization is the root
 of all evil." (Computing Surveys, vol 6, #4, p 268).


### Macros


While the evils of macros are well known, there remain a few
 cases where macros are the preferred solution:


* Preventing multiple inclusion of headers via #include
 guards.
* Passing minor configuration information from a
 configuration header to other files.


**Appropriate:** For small compile-time
 variations that would otherwise be costly or confusing to
 install, use, or maintain. More appropriate to communicate
 within and between library components than to communicate with
 library users.


**Not appropriate:** If other techniques will
 do.


To minimize the negative aspects of macros:


* Only use macros when they are clearly superior to other
 techniques. They should be viewed as a last resort.
* Names should be all uppercase and begin with the
 namespace name. This will minimize the chance of name
 collisions. For example, the #include guard for a boost
 header called foobar.h might be named BOOST\_FOOBAR\_H.


### Separate files


A library component can have multiple variations, each
 contained in its own separate file or files. The files for the
 most appropriate variation are copied to the appropriate
 include or implementation directories at installation time.


The way to provide this approach in boost libraries is to
 include specialized implementations as separate files in
 separate sub-directories in the .ZIP distribution file. For
 example, the structure within the .ZIP distribution file for a
 library named foobar which has both default and specialized
 variations might look something like:
```

foobar.h                // The default header file
foobar.cpp              // The default implementation file
readme.txt              // Readme explains when to use which files
self\_contained/foobar.h // A variation with everything in the header
linux/foobar.cpp        // Implementation file to replace the default
win32/foobar.h          // Header file to replace the default
win32/foobar.cpp        // Implementation file to replace the default

```

**Appropriate:** When different platforms
 require different implementations, or when there are major
 performance differences between possible implementations.


**Not appropriate:** When it makes sense to use
 more that one of the variations in the same installation.


### Separate components


Rather than have several implementation variations of a
 single component, supply several separate components. For
 example, the Boost library currently supplies
 `scoped_ptr` and `shared_ptr` classes
 rather than a single `smart_ptr` class parameterized
 to distinguish between the two cases. There are several ways to
 make the component choice:


* Hardwired by the programmer during coding.
* Chosen by programmer written runtime logic (trading off
 some extra space, time, and program complexity for the
 ability to select the implementation at run-time.)


**Appropriate:** When the interfaces for the
 variations diverge, and when it is reasonable to use more than
 one of the variations. When run-time selection of
 implementation is called for.


**Not appropriate:** When the variations are
 data type, traits, or specialization variations which can be
 better handled by making the component a template. Also not
 appropriate when choice of variation is best done by some setup
 or installation mechanism outside of the program itself. Thus
 usually not appropriate to cope with platform differences.


**Note:** There is a related technique where
 the interface is specified as an abstract (pure virtual) base
 class (or an interface definition language), and the
 implementation choice is passed off to some third-party, such
 as a dynamic-link library or object-request broker. While that
 is a powerful technique, it is way beyond the scope of this
 discussion.


### Template-based approaches


Turning a class or function into a template is often an
 elegant way to cope with variations. Template-based approaches
 provide optimal space and time efficiency in return for
 constraining the implementation selection to compile time.


Important template techniques include:


* Data type parameterization. This allows a single
 component to operate on a variety of data types and is why
 templates were originally invented.
* Traits parameterization. If parameterization is complex,
 bundling up aspects into a single traits helper class can
 allow great variation while hiding messy details. The C++
 Standard Library provides several examples of this idiom,
 such as `iterator_traits<>` (24.3.1
 lib.iterator.traits) and char\_traits<> (21.2
 lib.char.traits).
* Specialization. A template parameter can be used purely
 for the purpose of selecting a specialization. For
 example:
```

SomeClass<fast>  my\_fast\_object;  // fast and small are empty classes
SomeClass<small> my\_small\_object; // used just to select specialization

```

**Appropriate:** When the need for variation is
 due to data type or traits or is performance-related like
 selecting among several algorithms, and when a program might
 reasonably use more than one of the variations.


**Not appropriate:** When the interfaces for
 variations are different, or when choice of variation is best
 done by some mechanism outside of the program itself. Thus
 usually not appropriate to cope with platform differences.