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  21 <div class="chapter">
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  23 <div><h2 class="title">
  24 <a name="boost_contract"></a>Chapter&#160;1.&#160;Boost.Contract 1.0.0</h2></div>
  25 <div><div class="author"><h3 class="author">
  26 <span class="firstname">Lorenzo</span> <span class="surname">Caminiti <code class="email">&lt;<a class="email" href="mailto:lorcaminiti@gmail.com">lorcaminiti@gmail.com</a>&gt;</code></span>
  27 </h3></div></div>
  28 <div><p class="copyright">Copyright &#169; 2008-2019 Lorenzo Caminiti</p></div>
  29 <div><div class="legalnotice">
  30 <a name="boost_contract.legal"></a><p>
  31         Distributed under the Boost Software License, Version 1.0 (see accompanying
  32         file LICENSE_1_0.txt or a copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
  33       </p>
  34 </div></div>
  35 </div></div>
  36 <div class="toc">
  37 <p><b>Table of Contents</b></p>
  38 <dl class="toc">
  39 <dt><span class="section"><a href="index.html#boost_contract.introduction">Introduction</a></span></dt>
  40 <dt><span class="section"><a href="boost_contract/full_table_of_contents.html">Full Table of Contents</a></span></dt>
  41 <dt><span class="section"><a href="boost_contract/getting_started.html">Getting Started</a></span></dt>
  42 <dt><span class="section"><a href="boost_contract/contract_programming_overview.html">Contract
  43     Programming Overview</a></span></dt>
  44 <dt><span class="section"><a href="boost_contract/tutorial.html">Tutorial</a></span></dt>
  45 <dt><span class="section"><a href="boost_contract/advanced.html">Advanced</a></span></dt>
  46 <dt><span class="section"><a href="boost_contract/extras.html">Extras</a></span></dt>
  47 <dt><span class="section"><a href="boost_contract/examples.html">Examples</a></span></dt>
  48 <dt><span class="section"><a href="reference.html">Reference</a></span></dt>
  49 <dt><span class="section"><a href="boost_contract/release_notes.html">Release Notes</a></span></dt>
  50 <dt><span class="section"><a href="boost_contract/bibliography.html">Bibliography</a></span></dt>
  51 <dt><span class="section"><a href="boost_contract/acknowledgments.html">Acknowledgments</a></span></dt>
  52 </dl>
  53 </div>
  54 <div class="blockquote"><blockquote class="blockquote"><p>
  55       <span class="emphasis"><em><span class="quote">&#8220;<span class="quote">Our field needs more formality, but the profession has not
  56       realized it yet.</span>&#8221;</span></em></span>
  57     </p></blockquote></div>
  58 <div class="blockquote"><blockquote class="blockquote"><p>
  59       <span class="emphasis"><em>-- Bertrand Meyer (see <a class="link" href="boost_contract/bibliography.html#Meyer97_anchor">[Meyer97]</a>
  60       page 400)</em></span>
  61     </p></blockquote></div>
  62 <p>
  63     This library implements <a href="http://en.wikipedia.org/wiki/Design_by_contract" target="_top">contract
  64     programming</a> (a.k.a., Design by Contract or DbC) <a href="#ftn.boost_contract.f0" class="footnote" name="boost_contract.f0"><sup class="footnote">[1]</sup></a> for the C++ programming language. All contract programming features
  65     are supported by this library: Subcontracting, class invariants (also for static
  66     and volatile member functions), postconditions (with old and return values),
  67     preconditions, customizable actions on assertion failure (e.g., terminate the
  68     program or throw exceptions), optional compilation of assertions, disable assertions
  69     while already checking other assertions (to avoid infinite recursion), and more
  70     (see <a class="link" href="boost_contract/contract_programming_overview.html#boost_contract.contract_programming_overview.feature_summary" title="Feature Summary">Feature
  71     Summary</a>).
  72   </p>
  73 <div class="section">
  74 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  75 <a name="boost_contract.introduction"></a><a class="link" href="index.html#boost_contract.introduction" title="Introduction">Introduction</a>
  76 </h2></div></div></div>
  77 <p>
  78       Contract programming allows to specify preconditions, postconditions, and class
  79       invariants that are automatically checked when functions are executed at run-time.
  80       These conditions assert program specifications within the source code itself
  81       allowing to find bugs more quickly during testing, making the code self-documenting,
  82       and increasing overall software quality (see <a class="link" href="boost_contract/contract_programming_overview.html" title="Contract Programming Overview">Contract
  83       Programming Overview</a>).
  84     </p>
  85 <p>
  86       For example, consider the following function <code class="computeroutput"><span class="identifier">inc</span></code>
  87       that increments its argument <code class="computeroutput"><span class="identifier">x</span></code>
  88       by <code class="computeroutput"><span class="number">1</span></code> and let's write its contract
  89       using code comments (see <a href="../../example/features/introduction_comments.cpp" target="_top"><code class="literal">introduction_comments.cpp</code></a>):
  90     </p>
  91 <p>
  92 </p>
  93 <pre class="programlisting"><span class="keyword">void</span> <span class="identifier">inc</span><span class="special">(</span><span class="keyword">int</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">)</span>
  94     <span class="comment">// Precondition:    x &lt; std::numeric_limit&lt;int&gt;::max()</span>
  95     <span class="comment">// Postcondition:   x == oldof(x) + 1</span>
  96 <span class="special">{</span>
  97     <span class="special">++</span><span class="identifier">x</span><span class="special">;</span> <span class="comment">// Function body.</span>
  98 <span class="special">}</span>
  99 </pre>
 100 <p>
 101     </p>
 102 <p>
 103       The precondition states that at function entry the argument <code class="computeroutput"><span class="identifier">x</span></code>
 104       must be strictly smaller than the maximum allowable value of its type (so it
 105       can be incremented by <code class="computeroutput"><span class="number">1</span></code> without
 106       overflowing). The postcondition states that at function exit the argument
 107       <code class="computeroutput"><span class="identifier">x</span></code> must be incremented by <code class="computeroutput"><span class="number">1</span></code> with respect to the <span class="emphasis"><em>old value</em></span>
 108       that <code class="computeroutput"><span class="identifier">x</span></code> had before executing
 109       the function (indicated here by <code class="literal"><span class="emphasis"><em>oldof</em></span></code><code class="computeroutput"><span class="special">(</span><span class="identifier">x</span><span class="special">)</span></code>).
 110       Note that postconditions shall be checked only when the execution of the function
 111       body does not throw an exception.
 112     </p>
 113 <p>
 114       Now let's program this function and its contract using this library (see <a href="../../example/features/introduction.cpp" target="_top"><code class="literal">introduction.cpp</code></a>
 115       and <a class="link" href="boost_contract/tutorial.html#boost_contract.tutorial.non_member_functions" title="Non-Member Functions">Non-Member
 116       Functions</a>):
 117     </p>
 118 <p>
 119 </p>
 120 <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">contract</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
 121
 122 <span class="keyword">void</span> <span class="identifier">inc</span><span class="special">(</span><span class="keyword">int</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span>
 123     <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">old_ptr</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">old_x</span> <span class="special">=</span> <span class="identifier">BOOST_CONTRACT_OLDOF</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span> <span class="comment">// Old value.</span>
 124     <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">check</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">function</span><span class="special">()</span>
 125         <span class="special">.</span><span class="identifier">precondition</span><span class="special">([&amp;]</span> <span class="special">{</span>
 126             <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">x</span> <span class="special">&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">max</span><span class="special">());</span> <span class="comment">// Line 17.</span>
 127         <span class="special">})</span>
 128         <span class="special">.</span><span class="identifier">postcondition</span><span class="special">([&amp;]</span> <span class="special">{</span>
 129             <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">x</span> <span class="special">==</span> <span class="special">*</span><span class="identifier">old_x</span> <span class="special">+</span> <span class="number">1</span><span class="special">);</span> <span class="comment">// Line 20.</span>
 130         <span class="special">})</span>
 131     <span class="special">;</span>
 132
 133     <span class="special">++</span><span class="identifier">x</span><span class="special">;</span> <span class="comment">// Function body.</span>
 134 <span class="special">}</span>
 135 </pre>
 136 <p>
 137     </p>
 138 <p>
 139       When the above function <code class="computeroutput"><span class="identifier">inc</span></code>
 140       is called, this library will:
 141     </p>
 142 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 143 <li class="listitem">
 144           First, execute the functor passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">precondition</span><span class="special">(...)</span></code>
 145           that asserts <code class="computeroutput"><span class="identifier">inc</span></code> precondition.
 146         </li>
 147 <li class="listitem">
 148           Then, execute <code class="computeroutput"><span class="identifier">inc</span></code> body
 149           (i.e., all the code that follows the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">check</span> <span class="identifier">c</span> <span class="special">=</span> <span class="special">...</span></code> declaration).
 150         </li>
 151 <li class="listitem">
 152           Last, execute the functor passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">postcondition</span><span class="special">(...)</span></code>
 153           that asserts <code class="computeroutput"><span class="identifier">inc</span></code> postcondition
 154           (unless <code class="computeroutput"><span class="identifier">inc</span></code> body threw
 155           an exception).
 156         </li>
 157 </ul></div>
 158 <p>
 159       For example, if there is a bug in the code calling <code class="computeroutput"><span class="identifier">inc</span></code>
 160       so that the function is called with <code class="computeroutput"><span class="identifier">x</span></code>
 161       equal to <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">max</span><span class="special">()</span></code> then the program will terminate with an error
 162       message similar to the following (and it will be evident that the bug is in
 163       the calling code):
 164     </p>
 165 <pre class="programlisting">precondition assertion "x &lt; std::numeric_limits&lt;int&gt;::max()" failed: file "introduction.cpp", line 17
 166 </pre>
 167 <p>
 168       Instead, if there is a bug in the implementation of <code class="computeroutput"><span class="identifier">inc</span></code>
 169       so that <code class="computeroutput"><span class="identifier">x</span></code> is not incremented
 170       by <code class="computeroutput"><span class="number">1</span></code> after the execution of the
 171       function body then the program will terminate with an error message similar
 172       to the following (and it will be evident that the bug is in <code class="computeroutput"><span class="identifier">inc</span></code>
 173       body): <a href="#ftn.boost_contract.introduction.f0" class="footnote" name="boost_contract.introduction.f0"><sup class="footnote">[2]</sup></a>
 174     </p>
 175 <pre class="programlisting">postcondition assertion "x == *old_x + 1" failed: file "introduction.cpp", line 20
 176 </pre>
 177 <p>
 178       By default, when an assertion fails this library prints an error message such
 179       the ones above to the standard error <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cerr</span></code> and
 180       terminates the program calling <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">terminate</span></code>
 181       (this behaviour can be customized to take any user-specified action including
 182       throwing exceptions, see <a class="link" href="boost_contract/advanced.html#boost_contract.advanced.throw_on_failures__and__noexcept__" title="Throw on Failures (and noexcept)">Throw
 183       on Failures</a>). Note that the error messages printed by this library contain
 184       all the information necessary to easily and uniquely identify the point in
 185       the code at which contract assertions fail. <a href="#ftn.boost_contract.introduction.f1" class="footnote" name="boost_contract.introduction.f1"><sup class="footnote">[3]</sup></a>
 186     </p>
 187 <div class="note"><table border="0" summary="Note">
 188 <tr>
 189 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../doc/src/images/note.png"></td>
 190 <th align="left">Note</th>
 191 </tr>
 192 <tr><td align="left" valign="top"><p>
 193         C++11 lambda functions are necessary to use this library without manually
 194         writing a significant amount of boiler-plate code to program functors that
 195         assert the contracts (see <a class="link" href="boost_contract/extras.html#boost_contract.extras.no_lambda_functions__no_c__11_" title="No Lambda Functions (No C++11)">No
 196         Lambda Functions</a>). That said, this library implementation does not
 197         use C++11 features and should work on most modern C++ compilers (see <a class="link" href="boost_contract/getting_started.html" title="Getting Started">Getting Started</a>).
 198       </p></td></tr>
 199 </table></div>
 200 <p>
 201       In addition to contracts for non-member functions as shown the in the example
 202       above, this library allows to program contracts for constructors, destructors,
 203       and member functions. These can check class invariants and can also <span class="emphasis"><em>subcontract</em></span>
 204       inheriting and extending contracts from base classes (see <a href="../../example/features/introduction_public.cpp" target="_top"><code class="literal">introduction_public.cpp</code></a>
 205       and <a class="link" href="boost_contract/tutorial.html#boost_contract.tutorial.public_function_overrides__subcontracting_" title="Public Function Overrides (Subcontracting)">Public
 206       Function Overrides</a>): <a href="#ftn.boost_contract.introduction.f2" class="footnote" name="boost_contract.introduction.f2"><sup class="footnote">[4]</sup></a>
 207     </p>
 208 <p>
 209 </p>
 210 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
 211 <span class="keyword">class</span> <span class="identifier">vector</span>
 212     <span class="preprocessor">#define</span> <span class="identifier">BASES</span> <span class="keyword">public</span> <span class="identifier">pushable</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span>
 213     <span class="special">:</span> <span class="identifier">BASES</span>
 214 <span class="special">{</span>
 215 <span class="keyword">public</span><span class="special">:</span>
 216     <span class="keyword">typedef</span> <span class="identifier">BOOST_CONTRACT_BASE_TYPES</span><span class="special">(</span><span class="identifier">BASES</span><span class="special">)</span> <span class="identifier">base_types</span><span class="special">;</span> <span class="comment">// For subcontracting.</span>
 217     <span class="preprocessor">#undef</span> <span class="identifier">BASES</span>
 218
 219     <span class="keyword">void</span> <span class="identifier">invariant</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span> <span class="comment">// Checked in AND with base class invariants.</span>
 220         <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">size</span><span class="special">()</span> <span class="special">&lt;=</span> <span class="identifier">capacity</span><span class="special">());</span>
 221     <span class="special">}</span>
 222
 223     <span class="keyword">virtual</span> <span class="keyword">void</span> <span class="identifier">push_back</span><span class="special">(</span><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">value</span><span class="special">,</span>
 224             <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">virtual_</span><span class="special">*</span> <span class="identifier">v</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span> <span class="comment">/* override */</span> <span class="special">{</span> <span class="comment">// For virtuals.</span>
 225         <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">old_ptr</span><span class="special">&lt;</span><span class="keyword">unsigned</span><span class="special">&gt;</span> <span class="identifier">old_size</span> <span class="special">=</span>
 226                 <span class="identifier">BOOST_CONTRACT_OLDOF</span><span class="special">(</span><span class="identifier">v</span><span class="special">,</span> <span class="identifier">size</span><span class="special">());</span> <span class="comment">// Old values for virtuals.</span>
 227         <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">check</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">contract</span><span class="special">::</span><span class="identifier">public_function</span><span class="special">&lt;</span> <span class="comment">// For overrides.</span>
 228                 <span class="identifier">override_push_back</span><span class="special">&gt;(</span><span class="identifier">v</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">vector</span><span class="special">::</span><span class="identifier">push_back</span><span class="special">,</span> <span class="keyword">this</span><span class="special">,</span> <span class="identifier">value</span><span class="special">)</span>
 229             <span class="special">.</span><span class="identifier">precondition</span><span class="special">([&amp;]</span> <span class="special">{</span> <span class="comment">// Checked in OR with base preconditions.</span>
 230                 <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">size</span><span class="special">()</span> <span class="special">&lt;</span> <span class="identifier">max_size</span><span class="special">());</span>
 231             <span class="special">})</span>
 232             <span class="special">.</span><span class="identifier">postcondition</span><span class="special">([&amp;]</span> <span class="special">{</span> <span class="comment">// Checked in AND with base postconditions.</span>
 233                 <span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">size</span><span class="special">()</span> <span class="special">==</span> <span class="special">*</span><span class="identifier">old_size</span> <span class="special">+</span> <span class="number">1</span><span class="special">);</span>
 234             <span class="special">})</span>
 235         <span class="special">;</span>
 236
 237         <span class="identifier">vect_</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">value</span><span class="special">);</span>
 238     <span class="special">}</span>
 239     <span class="identifier">BOOST_CONTRACT_OVERRIDE</span><span class="special">(</span><span class="identifier">push_back</span><span class="special">)</span> <span class="comment">// Define `override_push_back` above.</span>
 240
 241     <span class="comment">// Could program contracts for those as well.</span>
 242     <span class="keyword">unsigned</span> <span class="identifier">size</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">vect_</span><span class="special">.</span><span class="identifier">size</span><span class="special">();</span> <span class="special">}</span>
 243     <span class="keyword">unsigned</span> <span class="identifier">max_size</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">vect_</span><span class="special">.</span><span class="identifier">max_size</span><span class="special">();</span> <span class="special">}</span>
 244     <span class="keyword">unsigned</span> <span class="identifier">capacity</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">vect_</span><span class="special">.</span><span class="identifier">capacity</span><span class="special">();</span> <span class="special">}</span>
 245
 246 <span class="keyword">private</span><span class="special">:</span>
 247     <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">vect_</span><span class="special">;</span>
 248 <span class="special">};</span>
 249 </pre>
 250 <p>
 251     </p>
 252 <h4>
 253 <a name="boost_contract.introduction.h0"></a>
 254       <span class="phrase"><a name="boost_contract.introduction.language_support"></a></span><a class="link" href="index.html#boost_contract.introduction.language_support">Language
 255       Support</a>
 256     </h4>
 257 <p>
 258       The authors of this library advocate for contracts to be added to the core
 259       language. Adding contract programming to the C++ standard has a number of advantages
 260       over any library implementation (shorter and more concise syntax to program
 261       contracts, specify contracts in declarations instead of definitions, enforce
 262       contract constant-correctness, expected faster compile- and run-time, vendors
 263       could develop static analysis tools to recognize and check contracts statically
 264       when possible, compiler optimizations could be improved based on contract conditions,
 265       etc.).
 266     </p>
 267 <p>
 268       The <a class="link" href="boost_contract/bibliography.html#P0380_anchor">[P0380]</a> proposal supports basic contract
 269       programming, it was accepted and it will be included in C++20. This is undoubtedly
 270       a step in the right direction, but unfortunately <a class="link" href="boost_contract/bibliography.html#P0380_anchor">[P0380]</a>
 271       only supports pre- and postconditions while missing important features such
 272       as class invariants and old values in postconditions, not to mention the lack
 273       of more advanced features like subcontracting (more complete proposals like
 274       <a class="link" href="boost_contract/bibliography.html#N1962_anchor">[N1962]</a> were rejected by the C++ standard
 275       committee). All contracting programming features are instead supported by this
 276       library (see <a class="link" href="boost_contract/contract_programming_overview.html#boost_contract.contract_programming_overview.feature_summary" title="Feature Summary">Feature
 277       Summary</a> for a detailed comparison between the features supported by
 278       this library and the ones listed in different contract programming proposals,
 279       see <a class="link" href="boost_contract/bibliography.html" title="Bibliography">Bibliography</a> for a list
 280       of references considered during the design and implementation of this library,
 281       including the vast majority of contract programming proposals submitted to
 282       the C++ standard committee).
 283     </p>
 284 </div>
 285 <div class="footnotes">
 286 <br><hr style="width:100; text-align:left;margin-left: 0">
 287 <div id="ftn.boost_contract.f0" class="footnote"><p><a href="#boost_contract.f0" class="para"><sup class="para">[1] </sup></a>
 288       Design by Contract (DbC) is a registered trademark of the Eiffel Software company
 289       and it was first introduced by the Eiffel programming language (see <a class="link" href="boost_contract/bibliography.html#Meyer97_anchor">[Meyer97]</a>).
 290     </p></div>
 291 <div id="ftn.boost_contract.introduction.f0" class="footnote"><p><a href="#boost_contract.introduction.f0" class="para"><sup class="para">[2] </sup></a>
 292         In this example the function body is composed of a single trivial instruction
 293         <code class="computeroutput"><span class="special">++</span><span class="identifier">x</span></code>
 294         so it easy to check by visual inspection that it does not contain any bug
 295         and it will always increment <code class="computeroutput"><span class="identifier">x</span></code>
 296         by <code class="computeroutput"><span class="number">1</span></code> thus the function postcondition
 297         will never fail. In real production code, function bodies are rarely this
 298         simple and can hide bugs which make checking postconditions useful.
 299       </p></div>
 300 <div id="ftn.boost_contract.introduction.f1" class="footnote"><p><a href="#boost_contract.introduction.f1" class="para"><sup class="para">[3] </sup></a>
 301         <span class="bold"><strong>Rationale:</strong></span> The assertion failure message
 302         printed by this library follows a format similar to the message printed by
 303         Clang when the C-style <code class="computeroutput"><span class="identifier">assert</span></code>
 304         macro fails.
 305       </p></div>
 306 <div id="ftn.boost_contract.introduction.f2" class="footnote"><p><a href="#boost_contract.introduction.f2" class="para"><sup class="para">[4] </sup></a>
 307         The <code class="computeroutput"><span class="identifier">pushable</span></code> base class is
 308         used in this example just to show subcontracting, it is somewhat arbitrary
 309         and it will likely not appear in real production code.
 310       </p></div>
 311 </div>
 312 </div>
 313 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 314 <td align="left"><p><small>Last revised: December 10, 2019 at 00:23:48 GMT</small></p></td>
 315 <td align="right"><div class="copyright-footer"></div></td>
 316 </tr></table>
 317 <hr>
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