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<html>
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<div class="section" title="Appendices">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="proto.appendices"></a><a class="link" href="appendices.html" title="Appendices">Appendices</a>
</h2></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="appendices.html#boost_proto.appendices.history"> Appendix A: History</a></span></dt>
<dt><span class="section"><a href="appendices.html#boost_proto.appendices.rationale"> Appendix B: Rationale</a></span></dt>
<dt><span class="section"><a href="appendices.html#boost_proto.appendices.implementation"> Appendix C: Implementation
Notes</a></span></dt>
<dt><span class="section"><a href="appendices.html#boost_proto.appendices.acknowledgements"> Appendix D:
Acknowledgements</a></span></dt>
</dl></div>
<div class="section" title="Appendix A: History">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_proto.appendices.history"></a><a class="link" href="appendices.html#boost_proto.appendices.history" title="Appendix A: History"> Appendix A: History</a>
</h3></div></div></div>
<div class="variablelist">
<p class="title"><b></b></p>
<dl>
<dt><span class="term">August 11, 2008</span></dt>
<dd><p>
Proto v4 is merged to Boost trunk with more powerful transform protocol.
</p></dd>
<dt><span class="term">April 7, 2008</span></dt>
<dd><p>
Proto is accepted into Boost.
</p></dd>
<dt><span class="term">March 1, 2008</span></dt>
<dd><p>
Proto's Boost review begins.
</p></dd>
<dt><span class="term">January 11, 2008</span></dt>
<dd><p>
Boost.Proto v3 brings separation of grammars and transforms and a "round"
lambda syntax for defining transforms in-place.
</p></dd>
<dt><span class="term">April 15, 2007</span></dt>
<dd><p>
Boost.Xpressive is ported from Proto compilers to Proto transforms. Support
for old Proto compilers is dropped.
</p></dd>
<dt><span class="term">April 4, 2007</span></dt>
<dd><p>
Preliminary submission of Proto to Boost.
</p></dd>
<dt><span class="term">December 11, 2006</span></dt>
<dd><p>
The idea for transforms that decorate grammar rules is born in a private
email discussion with Joel de Guzman and Hartmut Kaiser. The first transforms
are committed to CVS 5 days later on December 16.
</p></dd>
<dt><span class="term">November 1, 2006</span></dt>
<dd><p>
The idea for <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><></span></code>
and the whole grammar facility is hatched during a discussion with Hartmut
Kaiser on the spirit-devel list. The first version of <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">matches</span><span class="special"><></span></code> is checked into CVS 3 days later.
Message is <a href="http://osdir.com/ml/parsers.spirit.devel/2006-11/msg00003.html" target="_top">here</a>.
</p></dd>
<dt><span class="term">October 28, 2006</span></dt>
<dd><p>
Proto is reborn, this time with a uniform expression types that are POD.
Announcement is <a href="http://lists.boost.org/Archives/boost/2006/10/112453.php" target="_top">here</a>.
</p></dd>
<dt><span class="term">April 20, 2005</span></dt>
<dd><p>
Proto is born as a major refactorization of Boost.Xpressive's meta-programming.
Proto offers expression types, operator overloads and "compilers",
an early formulation of what later became transforms. Announcement is
<a href="http://lists.boost.org/Archives/boost/2005/04/85256.php" target="_top">here</a>.
</p></dd>
</dl>
</div>
</div>
<div class="section" title="Appendix B: Rationale">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_proto.appendices.rationale"></a><a class="link" href="appendices.html#boost_proto.appendices.rationale" title="Appendix B: Rationale"> Appendix B: Rationale</a>
</h3></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="appendices.html#boost_proto.appendices.rationale.static_initialization">
Static Initialization</a></span></dt>
<dt><span class="section"><a href="appendices.html#boost_proto.appendices.rationale.preprocessor"> Why
Not Reuse MPL, Fusion, et cetera?</a></span></dt>
</dl></div>
<div class="section" title="Static Initialization">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.appendices.rationale.static_initialization"></a><a class="link" href="appendices.html#boost_proto.appendices.rationale.static_initialization" title="Static Initialization">
Static Initialization</a>
</h4></div></div></div>
<p>
Proto expression types are PODs (Plain Old Data), and do not have constructors.
They are brace-initialized, as follows:
</p>
<pre class="programlisting"><span class="identifier">terminal</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">type</span> <span class="keyword">const</span> <span class="identifier">_i</span> <span class="special">=</span> <span class="special">{</span><span class="number">1</span><span class="special">};</span>
</pre>
<p>
The reason is so that expression objects like <code class="computeroutput"><span class="identifier">_i</span></code>
above can be <span class="emphasis"><em>statically initialized</em></span>. Why is static
initialization important? The terminals of many domain- specific embedded
languages are likely to be global const objects, like <code class="computeroutput"><span class="identifier">_1</span></code>
and <code class="computeroutput"><span class="identifier">_2</span></code> from the Boost Lambda
Library. Were these object to require run-time initialization, it might
be possible to use these objects before they are initialized. That would
be bad. Statically initialized objects cannot be misused that way.
</p>
</div>
<div class="section" title="Why Not Reuse MPL, Fusion, et cetera?">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.appendices.rationale.preprocessor"></a><a class="link" href="appendices.html#boost_proto.appendices.rationale.preprocessor" title="Why Not Reuse MPL, Fusion, et cetera?"> Why
Not Reuse MPL, Fusion, et cetera?</a>
</h4></div></div></div>
<p>
Anyone who has peeked at Proto's source code has probably wondered, "Why
all the dirty preprocessor gunk? Couldn't this have been all implemented
cleanly on top of libraries like MPL and Fusion?" The answer is that
Proto could have been implemented this way, and in fact was at one point.
The problem is that template metaprogramming (TMP) makes for longer compile
times. As a foundation upon which other TMP-heavy libraries will be built,
Proto itself should be as lightweight as possible. That is achieved by
prefering preprocessor metaprogramming to template metaprogramming. Expanding
a macro is far more efficient than instantiating a template. In some cases,
the "clean" version takes 10x longer to compile than the "dirty"
version.
</p>
<p>
The "clean and slow" version of Proto can still be found at http://svn.boost.org/svn/boost/branches/proto/v3.
Anyone who is interested can download it and verify that it is, in fact,
unusably slow to compile. Note that this branch's development was abandoned,
and it does not conform exactly with Proto's current interface.
</p>
</div>
</div>
<div class="section" title="Appendix C: Implementation Notes">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_proto.appendices.implementation"></a><a class="link" href="appendices.html#boost_proto.appendices.implementation" title="Appendix C: Implementation Notes"> Appendix C: Implementation
Notes</a>
</h3></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="appendices.html#boost_proto.appendices.implementation.sfinae"> Quick-n-Dirty
Type Categorization</a></span></dt>
<dt><span class="section"><a href="appendices.html#boost_proto.appendices.implementation.function_arity">
Detecting the Arity of Function Objects</a></span></dt>
</dl></div>
<div class="section" title="Quick-n-Dirty Type Categorization">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.appendices.implementation.sfinae"></a><a class="link" href="appendices.html#boost_proto.appendices.implementation.sfinae" title="Quick-n-Dirty Type Categorization"> Quick-n-Dirty
Type Categorization</a>
</h4></div></div></div>
<p>
Much has already been written about dispatching on type traits using SFINAE
(Substitution Failure Is Not An Error) techniques in C++. There is a Boost
library, Boost.Enable_if, to make the technique idiomatic. Proto dispatches
on type traits extensively, but it doesn't use <code class="computeroutput"><span class="identifier">enable_if</span><span class="special"><></span></code> very often. Rather, it dispatches
based on the presence or absence of nested types, often typedefs for void.
</p>
<p>
Consider the implementation of <code class="computeroutput"><span class="identifier">is_expr</span><span class="special"><></span></code>. It could have been written as
something like this:
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">is_expr</span>
<span class="special">:</span> <span class="identifier">is_base_and_derived</span><span class="special"><</span><span class="identifier">proto</span><span class="special">::</span><span class="identifier">some_expr_base</span><span class="special">,</span> <span class="identifier">T</span><span class="special">></span>
<span class="special">{};</span>
</pre>
<p>
Rather, it is implemented as this:
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Void</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">is_expr</span>
<span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">false_</span>
<span class="special">{};</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">is_expr</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T</span><span class="special">::</span><span class="identifier">proto_is_expr_</span><span class="special">></span>
<span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span>
<span class="special">{};</span>
</pre>
<p>
This relies on the fact that the specialization will be preferred if <code class="computeroutput"><span class="identifier">T</span></code> has a nested <code class="computeroutput"><span class="identifier">proto_is_expr_</span></code>
that is a typedef for <code class="computeroutput"><span class="keyword">void</span></code>.
All Proto expression types have such a nested typedef.
</p>
<p>
Why does Proto do it this way? The reason is because, after running extensive
benchmarks while trying to improve compile times, I have found that this
approach compiles faster. It requires exactly one template instantiation.
The other approach requires at least 2: <code class="computeroutput"><span class="identifier">is_expr</span><span class="special"><></span></code> and <code class="computeroutput"><span class="identifier">is_base_and_derived</span><span class="special"><></span></code>, plus whatever templates <code class="computeroutput"><span class="identifier">is_base_and_derived</span><span class="special"><></span></code>
may instantiate.
</p>
</div>
<div class="section" title="Detecting the Arity of Function Objects">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_proto.appendices.implementation.function_arity"></a><a class="link" href="appendices.html#boost_proto.appendices.implementation.function_arity" title="Detecting the Arity of Function Objects">
Detecting the Arity of Function Objects</a>
</h4></div></div></div>
<p>
In several places, Proto needs to know whether or not a function object
<code class="computeroutput"><span class="identifier">Fun</span></code> can be called with
certain parameters and take a fallback action if not. This happens in
<code class="computeroutput"><a class="link" href="../boost/proto/context/callable_context.html" title="Struct template callable_context">proto::callable_context<></a></code>
and in the <code class="computeroutput"><a class="link" href="../boost/proto/call.html" title="Struct template call">proto::call<></a></code> transform. How does
Proto know? It involves some tricky metaprogramming. Here's how.
</p>
<p>
Another way of framing the question is by trying to implement the following
<code class="computeroutput"><span class="identifier">can_be_called</span><span class="special"><></span></code>
Boolean metafunction, which checks to see if a function object <code class="computeroutput"><span class="identifier">Fun</span></code> can be called with parameters of
type <code class="computeroutput"><span class="identifier">A</span></code> and <code class="computeroutput"><span class="identifier">B</span></code>:
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Fun</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">B</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">can_be_called</span><span class="special">;</span>
</pre>
<p>
First, we define the following <code class="computeroutput"><span class="identifier">dont_care</span></code>
struct, which has an implicit conversion from anything. And not just any
implicit conversion; it has a ellipsis conversion, which is the worst possible
conversion for the purposes of overload resolution:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">dont_care</span>
<span class="special">{</span>
<span class="identifier">dont_care</span><span class="special">(...);</span>
<span class="special">};</span>
</pre>
<p>
We also need some private type known only to us with an overloaded comma
operator (!), and some functions that detect the presence of this type
and return types with different sizes, as follows:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">private_type</span>
<span class="special">{</span>
<span class="identifier">private_type</span> <span class="keyword">const</span> <span class="special">&</span><span class="keyword">operator</span><span class="special">,(</span><span class="keyword">int</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="special">};</span>
<span class="keyword">typedef</span> <span class="keyword">char</span> <span class="identifier">yes_type</span><span class="special">;</span> <span class="comment">// sizeof(yes_type) == 1
</span><span class="keyword">typedef</span> <span class="keyword">char</span> <span class="special">(&</span><span class="identifier">no_type</span><span class="special">)[</span><span class="number">2</span><span class="special">];</span> <span class="comment">// sizeof(no_type) == 2
</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
<span class="identifier">no_type</span> <span class="identifier">is_private_type</span><span class="special">(</span><span class="identifier">T</span> <span class="keyword">const</span> <span class="special">&);</span>
<span class="identifier">yes_type</span> <span class="identifier">is_private_type</span><span class="special">(</span><span class="identifier">private_type</span> <span class="keyword">const</span> <span class="special">&);</span>
</pre>
<p>
Next, we implement a binary function object wrapper with a very strange
conversion operator, whose meaning will become clear later.
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Fun</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">funwrap2</span> <span class="special">:</span> <span class="identifier">Fun</span>
<span class="special">{</span>
<span class="identifier">funwrap2</span><span class="special">();</span>
<span class="keyword">typedef</span> <span class="identifier">private_type</span> <span class="keyword">const</span> <span class="special">&(*</span><span class="identifier">pointer_to_function</span><span class="special">)(</span><span class="identifier">dont_care</span><span class="special">,</span> <span class="identifier">dont_care</span><span class="special">);</span>
<span class="keyword">operator</span> <span class="identifier">pointer_to_function</span><span class="special">()</span> <span class="keyword">const</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
With all of these bits and pieces, we can implement <code class="computeroutput"><span class="identifier">can_be_called</span><span class="special"><></span></code> as follows:
</p>
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Fun</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">B</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">can_be_called</span>
<span class="special">{</span>
<span class="keyword">static</span> <span class="identifier">funwrap2</span><span class="special"><</span><span class="identifier">Fun</span><span class="special">></span> <span class="special">&</span><span class="identifier">fun</span><span class="special">;</span>
<span class="keyword">static</span> <span class="identifier">A</span> <span class="special">&</span><span class="identifier">a</span><span class="special">;</span>
<span class="keyword">static</span> <span class="identifier">B</span> <span class="special">&</span><span class="identifier">b</span><span class="special">;</span>
<span class="keyword">static</span> <span class="keyword">bool</span> <span class="keyword">const</span> <span class="identifier">value</span> <span class="special">=</span> <span class="special">(</span>
<span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">no_type</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">is_private_type</span><span class="special">(</span> <span class="special">(</span><span class="identifier">fun</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span><span class="identifier">b</span><span class="special">),</span> <span class="number">0</span><span class="special">)</span> <span class="special">))</span>
<span class="special">);</span>
<span class="keyword">typedef</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">bool_</span><span class="special"><</span><span class="identifier">value</span><span class="special">></span> <span class="identifier">type</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
The idea is to make it so that <code class="computeroutput"><span class="identifier">fun</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span><span class="identifier">b</span><span class="special">)</span></code> will
always compile by adding our own binary function overload, but doing it
in such a way that we can detect whether our overload was selected or not.
And we rig it so that our overload is selected if there is really no better
option. What follows is a description of how <code class="computeroutput"><span class="identifier">can_be_called</span><span class="special"><></span></code> works.
</p>
<p>
We wrap <code class="computeroutput"><span class="identifier">Fun</span></code> in a type that
has an implicit conversion to a pointer to a binary function. An object
<code class="computeroutput"><span class="identifier">fun</span></code> of class type can be
invoked as <code class="computeroutput"><span class="identifier">fun</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">)</span></code> if it has such a conversion operator,
but since it involves a user-defined conversion operator, it is less preferred
than an overloaded <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>, which requires no such conversion.
</p>
<p>
The function pointer can accept any two arguments by virtue of the <code class="computeroutput"><span class="identifier">dont_care</span></code> type. The conversion sequence
for each argument is guaranteed to be the worst possible conversion sequence:
an implicit conversion through an ellipsis, and a user-defined conversion
to <code class="computeroutput"><span class="identifier">dont_care</span></code>. In total,
it means that <code class="computeroutput"><span class="identifier">funwrap2</span><span class="special"><</span><span class="identifier">Fun</span><span class="special">>()(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">)</span></code>
will always compile, but it will select our overload only if there really
is no better option.
</p>
<p>
If there is a better option --- for example if <code class="computeroutput"><span class="identifier">Fun</span></code>
has an overloaded function call operator such as <code class="computeroutput"><span class="keyword">void</span>
<span class="keyword">operator</span><span class="special">()(</span><span class="identifier">A</span> <span class="identifier">a</span><span class="special">,</span> <span class="identifier">B</span> <span class="identifier">b</span><span class="special">)</span></code> ---
then <code class="computeroutput"><span class="identifier">fun</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">)</span></code> will resolve to that one instead. The
question now is how to detect which function got picked by overload resolution.
</p>
<p>
Notice how <code class="computeroutput"><span class="identifier">fun</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">)</span></code> appears in <code class="computeroutput"><span class="identifier">can_be_called</span><span class="special"><></span></code>: <code class="computeroutput"><span class="special">(</span><span class="identifier">fun</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">),</span> <span class="number">0</span><span class="special">)</span></code>.
Why do we use the comma operator there? The reason is because we are using
this expression as the argument to a function. If the return type of <code class="computeroutput"><span class="identifier">fun</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span> <span class="identifier">b</span><span class="special">)</span></code> is <code class="computeroutput"><span class="keyword">void</span></code>,
it cannot legally be used as an argument to a function. The comma operator
sidesteps the issue.
</p>
<p>
This should also make plain the purpose of the overloaded comma operator
in <code class="computeroutput"><span class="identifier">private_type</span></code>. The return
type of the pointer to function is <code class="computeroutput"><span class="identifier">private_type</span></code>.
If overload resolution selects our overload, then the type of <code class="computeroutput"><span class="special">(</span><span class="identifier">fun</span><span class="special">(</span><span class="identifier">a</span><span class="special">,</span>
<span class="identifier">b</span><span class="special">),</span>
<span class="number">0</span><span class="special">)</span></code>
is <code class="computeroutput"><span class="identifier">private_type</span></code>. Otherwise,
it is <code class="computeroutput"><span class="keyword">int</span></code>. That fact is used
to dispatch to either overload of <code class="computeroutput"><span class="identifier">is_private_type</span><span class="special">()</span></code>, which encodes its answer in the size
of its return type.
</p>
<p>
That's how it works with binary functions. Now repeat the above process
for functions up to some predefined function arity, and you're done.
</p>
</div>
</div>
<div class="section" title="Appendix D: Acknowledgements">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_proto.appendices.acknowledgements"></a><a class="link" href="appendices.html#boost_proto.appendices.acknowledgements" title="Appendix D: Acknowledgements"> Appendix D:
Acknowledgements</a>
</h3></div></div></div>
<p>
I'd like to thank Joel de Guzman and Hartmut Kaiser for being willing to
take a chance on using Proto for their work on Spirit-2 and Karma when Proto
was little more than a vision. Their requirements and feedback have been
indespensable.
</p>
<p>
Thanks also to the developers of <a href="http://www.codesourcery.com/pooma/download.html" target="_top">PETE</a>.
I found many good ideas there.
</p>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2008 Eric Niebler<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
</div></td>
</tr></table>
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