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<div class="chapter">
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<div class="titlepage"><div>
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<div><h2 class="title">
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<a name="boost_contract"></a>Chapter 1. Boost.Contract 1.0.0</h2></div>
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<div><div class="author"><h3 class="author">
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<span class="firstname">Lorenzo</span> <span class="surname">Caminiti <code class="email"><<a class="email" href="mailto:lorcaminiti@gmail.com">lorcaminiti@gmail.com</a>></code></span>
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</h3></div></div>
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<div><p class="copyright">Copyright © 2008-2018 Lorenzo Caminiti</p></div>
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<div><div class="legalnotice">
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<a name="boost_contract.legal"></a><p>
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Distributed under the Boost Software License, Version 1.0 (see accompanying
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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>)
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</p>
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<div class="toc">
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<p><b>Table of Contents</b></p>
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<dl class="toc">
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<dt><span class="section"><a href="index.html#boost_contract.introduction">Introduction</a></span></dt>
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<dt><span class="section"><a href="boost_contract/full_table_of_contents.html">Full Table of Contents</a></span></dt>
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<dt><span class="section"><a href="boost_contract/getting_started.html">Getting Started</a></span></dt>
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<dt><span class="section"><a href="boost_contract/contract_programming_overview.html">Contract
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Programming Overview</a></span></dt>
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<dt><span class="section"><a href="boost_contract/tutorial.html">Tutorial</a></span></dt>
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<dt><span class="section"><a href="boost_contract/advanced.html">Advanced</a></span></dt>
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<dt><span class="section"><a href="boost_contract/extras.html">Extras</a></span></dt>
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<dt><span class="section"><a href="boost_contract/examples.html">Examples</a></span></dt>
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<dt><span class="section"><a href="reference.html">Reference</a></span></dt>
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<dt><span class="section"><a href="boost_contract/release_notes.html">Release Notes</a></span></dt>
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<dt><span class="section"><a href="boost_contract/bibliography.html">Bibliography</a></span></dt>
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<dt><span class="section"><a href="boost_contract/acknowledgments.html">Acknowledgments</a></span></dt>
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</dl>
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</div>
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<div class="blockquote"><blockquote class="blockquote"><p>
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<span class="emphasis"><em><span class="quote">“<span class="quote">Our field needs more formality, but the profession has not
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realized it yet.</span>”</span></em></span>
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</p></blockquote></div>
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<div class="blockquote"><blockquote class="blockquote"><p>
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<span class="emphasis"><em>-- Bertrand Meyer (see <a class="link" href="boost_contract/bibliography.html#Meyer97_anchor">[Meyer97]</a>
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page 400)</em></span>
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</p></blockquote></div>
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<p>
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This library implements <a href="http://en.wikipedia.org/wiki/Design_by_contract" target="_top">contract
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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
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are supported by this library: Subcontracting, class invariants (also for static
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and volatile member functions), postconditions (with old and return values),
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preconditions, customizable actions on assertion failure (e.g., terminate the
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program or throw exceptions), optional compilation of assertions, disable assertions
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while already checking other assertions (to avoid infinite recursion), and more
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(see <a class="link" href="boost_contract/contract_programming_overview.html#boost_contract.contract_programming_overview.feature_summary" title="Feature Summary">Feature
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Summary</a>).
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</p>
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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_contract.introduction"></a><a class="link" href="index.html#boost_contract.introduction" title="Introduction">Introduction</a>
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</h2></div></div></div>
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<p>
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Contract programming allows to specify preconditions, postconditions, and class
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invariants that are automatically checked when functions are executed at run-time.
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These conditions assert program specifications within the source code itself
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allowing to find bugs more quickly during testing, making the code self-documenting,
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and increasing overall software quality (see <a class="link" href="boost_contract/contract_programming_overview.html" title="Contract Programming Overview">Contract
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Programming Overview</a>).
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</p>
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<p>
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For example, consider the following function <code class="computeroutput"><span class="identifier">inc</span></code>
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that increments its argument <code class="computeroutput"><span class="identifier">x</span></code>
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by <code class="computeroutput"><span class="number">1</span></code> and let's write its contract
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using code comments (see <a href="../../example/features/introduction_comments.cpp" target="_top"><code class="literal">introduction_comments.cpp</code></a>):
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</p>
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<p>
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</p>
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<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">&</span> <span class="identifier">x</span><span class="special">)</span>
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<span class="comment">// Precondition: x < std::numeric_limit<int>::max()</span>
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<span class="comment">// Postcondition: x == oldof(x) + 1</span>
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<span class="special">{</span>
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<span class="special">++</span><span class="identifier">x</span><span class="special">;</span> <span class="comment">// Function body.</span>
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<span class="special">}</span>
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</pre>
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<p>
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</p>
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<p>
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The precondition states that at function entry the argument <code class="computeroutput"><span class="identifier">x</span></code>
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must be strictly smaller than the maximum allowable value of its type (so it
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can be incremented by <code class="computeroutput"><span class="number">1</span></code> without
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overflowing). The postcondition states that at function exit the argument
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<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>
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that <code class="computeroutput"><span class="identifier">x</span></code> had before executing
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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>).
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Note that postconditions shall be checked only when the execution of the function
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body did not throw an exception.
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</p>
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<p>
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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>
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and <a class="link" href="boost_contract/tutorial.html#boost_contract.tutorial.non_member_functions" title="Non-Member Functions">Non-Member
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Functions</a>):
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</p>
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<p>
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</p>
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<pre class="programlisting"><span class="preprocessor">#include</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">hpp</span><span class="special">></span>
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<span class="keyword">void</span> <span class="identifier">inc</span><span class="special">(</span><span class="keyword">int</span><span class="special">&</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span>
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<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"><</span><span class="keyword">int</span><span class="special">></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>
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<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>
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<span class="special">.</span><span class="identifier">precondition</span><span class="special">([&]</span> <span class="special">{</span>
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<span class="identifier">BOOST_CONTRACT_ASSERT</span><span class="special">(</span><span class="identifier">x</span> <span class="special"><</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">max</span><span class="special">());</span> <span class="comment">// Line 17.</span>
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<span class="special">})</span>
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<span class="special">.</span><span class="identifier">postcondition</span><span class="special">([&]</span> <span class="special">{</span>
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<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>
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<span class="special">})</span>
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<span class="special">;</span>
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<span class="special">++</span><span class="identifier">x</span><span class="special">;</span> <span class="comment">// Function body.</span>
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<span class="special">}</span>
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</pre>
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<p>
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</p>
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<p>
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When the above function <code class="computeroutput"><span class="identifier">inc</span></code>
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is called, this library will:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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First, execute the functor passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">precondition</span><span class="special">(...)</span></code>
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that asserts <code class="computeroutput"><span class="identifier">inc</span></code> precondition.
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</li>
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<li class="listitem">
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Then, execute <code class="computeroutput"><span class="identifier">inc</span></code> body
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(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).
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</li>
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<li class="listitem">
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Last, execute the functor passed to <code class="computeroutput"><span class="special">.</span><span class="identifier">postcondition</span><span class="special">(...)</span></code>
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that asserts <code class="computeroutput"><span class="identifier">inc</span></code> postcondition
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(unless <code class="computeroutput"><span class="identifier">inc</span></code> body threw
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an exception).
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</li>
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</ul></div>
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<p>
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For example, if there is a bug in the code calling <code class="computeroutput"><span class="identifier">inc</span></code>
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so that the function is called with <code class="computeroutput"><span class="identifier">x</span></code>
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equal to <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">max</span><span class="special">()</span></code> then the program will terminate with an error
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message similar to the following (and it will be evident that the bug is in
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the calling code):
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</p>
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<pre class="programlisting">precondition assertion "x < std::numeric_limits<int>::max()" failed: file "introduction.cpp", line 17
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</pre>
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<p>
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Instead, if there is a bug in the implementation of <code class="computeroutput"><span class="identifier">inc</span></code>
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so that <code class="computeroutput"><span class="identifier">x</span></code> is not incremented
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by <code class="computeroutput"><span class="number">1</span></code> after the execution of the
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function body then the program will terminate with an error message similar
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to the following (and it will be evident that the bug is in <code class="computeroutput"><span class="identifier">inc</span></code>
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body): <a href="#ftn.boost_contract.introduction.f0" class="footnote" name="boost_contract.introduction.f0"><sup class="footnote">[2]</sup></a>
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</p>
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<pre class="programlisting">postcondition assertion "x == *old_x + 1" failed: file "introduction.cpp", line 20
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</pre>
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<p>
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By default, when an assertion fails this library prints an error message such
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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
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terminates the program calling <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">terminate</span></code>
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(this behaviour can be customized to take any user-specified action including
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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
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on Failures</a>). Note that the error messages printed by this library contain
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all the information necessary to easily and uniquely identify the point in
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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>
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</p>
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<div class="note"><table border="0" summary="Note">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../doc/src/images/note.png"></td>
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<th align="left">Note</th>
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</tr>
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<tr><td align="left" valign="top"><p>
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C++11 lambda functions are necessary to use this library without manually
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writing a significant amount of boiler-plate code to program the functors
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that 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
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Lambda Functions</a>). That said, this library implementation does not
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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>).
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</p></td></tr>
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</table></div>
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<p>
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In addition to contracts for non-member functions as shown the in the example
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above, this library allows to program contracts for constructors, destructors,
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and member functions. These can check class invariants and can also <span class="emphasis"><em>subcontract</em></span>
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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>
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and <a class="link" href="boost_contract/tutorial.html#boost_contract.tutorial.public_function_overrides__subcontracting_" title="Public Function Overrides (Subcontracting)">Public
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Function Overrides</a>): <a href="#ftn.boost_contract.introduction.f2" class="footnote" name="boost_contract.introduction.f2"><sup class="footnote">[4]</sup></a>
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</p>
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<p>
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</p>
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<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>
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<span class="keyword">class</span> <span class="identifier">vector</span>
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<span class="preprocessor">#define</span> <span class="identifier">BASES</span> <span class="keyword">public</span> <span class="identifier">pushable</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span>
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<span class="special">:</span> <span class="identifier">BASES</span>
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<span class="special">{</span>
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<span class="keyword">public</span><span class="special">:</span>
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<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>
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<span class="preprocessor">#undef</span> <span class="identifier">BASES</span>
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<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>
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<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="identifier">capacity</span><span class="special">());</span>
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<span class="special">}</span>
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<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">&</span> <span class="identifier">value</span><span class="special">,</span>
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<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>
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<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"><</span><span class="keyword">unsigned</span><span class="special">></span> <span class="identifier">old_size</span> <span class="special">=</span>
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<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>
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<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"><</span> <span class="comment">// For overrides.</span>
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<span class="identifier">override_push_back</span><span class="special">>(</span><span class="identifier">v</span><span class="special">,</span> <span class="special">&</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>
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<span class="special">.</span><span class="identifier">precondition</span><span class="special">([&]</span> <span class="special">{</span> <span class="comment">// Checked in OR with base preconditions.</span>
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<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="identifier">max_size</span><span class="special">());</span>
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<span class="special">})</span>
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<span class="special">.</span><span class="identifier">postcondition</span><span class="special">([&]</span> <span class="special">{</span> <span class="comment">// Checked in AND with base postconditions.</span>
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<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>
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<span class="special">})</span>
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<span class="special">;</span>
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<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>
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<span class="special">}</span>
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<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>
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<span class="comment">// Could program contracts for those as well.</span>
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<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>
|
|
<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>
|
|
<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>
|
|
|
|
<span class="keyword">private</span><span class="special">:</span>
|
|
<span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">vect_</span><span class="special">;</span>
|
|
<span class="special">};</span>
|
|
</pre>
|
|
<p>
|
|
</p>
|
|
<h4>
|
|
<a name="boost_contract.introduction.h0"></a>
|
|
<span class="phrase"><a name="boost_contract.introduction.language_support"></a></span><a class="link" href="index.html#boost_contract.introduction.language_support">Language
|
|
Support</a>
|
|
</h4>
|
|
<p>
|
|
The authors of this library advocate for contracts to be added to the core
|
|
language. Adding contract programming to the C++ standard has a number of advantages
|
|
over any library implementation (shorter and more concise syntax to program
|
|
contracts, specify contracts in declarations instead of definitions, enforce
|
|
contract constant-correctness, expected faster compile- and run-time, vendors
|
|
could develop static analysis tools to recognize and check contracts statically
|
|
when possible, compiler optimizations could be improved based on contract conditions,
|
|
etc.).
|
|
</p>
|
|
<p>
|
|
Unfortunately, detailed and complete proposals to add contracts to the C++
|
|
standard such as <a class="link" href="boost_contract/bibliography.html#N1962_anchor">[N1962]</a> were rejected
|
|
by the C++ standard committee and it is not clear if the current proposal for
|
|
adding contracts to C++ <a class="link" href="boost_contract/bibliography.html#P0380_anchor">[P0380]</a> will actually
|
|
be accepted by the standard. <a href="#ftn.boost_contract.introduction.f3" class="footnote" name="boost_contract.introduction.f3"><sup class="footnote">[5]</sup></a> In any case, at least for now <a class="link" href="boost_contract/bibliography.html#P0380_anchor">[P0380]</a>
|
|
only supports pre- and postconditions while missing basic features such as
|
|
class invariants and old values in postconditions, not to mention the lack
|
|
of more advanced features like subcontracting. All these features are instead
|
|
supported by this library (see <a class="link" href="boost_contract/contract_programming_overview.html#boost_contract.contract_programming_overview.feature_summary" title="Feature Summary">Feature
|
|
Summary</a> for a detailed comparison between the features supported by
|
|
this library and the ones listed in different contract programming proposals,
|
|
see <a class="link" href="boost_contract/bibliography.html" title="Bibliography">Bibliography</a> for a list
|
|
of references considered during the design and implementation of this library,
|
|
including the vast majority of contract programming proposals submitted to
|
|
the C++ standard committee).
|
|
</p>
|
|
</div>
|
|
<div class="footnotes">
|
|
<br><hr style="width:100; text-align:left;margin-left: 0">
|
|
<div id="ftn.boost_contract.f0" class="footnote"><p><a href="#boost_contract.f0" class="para"><sup class="para">[1] </sup></a>
|
|
Design by Contract (DbC) is a registered trademark of the Eiffel Software company
|
|
and it was first introduced by the Eiffel programming language (see <a class="link" href="boost_contract/bibliography.html#Meyer97_anchor">[Meyer97]</a>).
|
|
</p></div>
|
|
<div id="ftn.boost_contract.introduction.f0" class="footnote"><p><a href="#boost_contract.introduction.f0" class="para"><sup class="para">[2] </sup></a>
|
|
In this example the function body is composed of a single trivial instruction
|
|
<code class="computeroutput"><span class="special">++</span><span class="identifier">x</span></code>
|
|
so it easy to check by visual inspection that it does not contain any bug
|
|
and it will always increment <code class="computeroutput"><span class="identifier">x</span></code>
|
|
by <code class="computeroutput"><span class="number">1</span></code> thus the function postcondition
|
|
will never fail. In real code, function bodies are rarely this simple and
|
|
can hide bugs which makes checking postconditions useful.
|
|
</p></div>
|
|
<div id="ftn.boost_contract.introduction.f1" class="footnote"><p><a href="#boost_contract.introduction.f1" class="para"><sup class="para">[3] </sup></a>
|
|
<span class="bold"><strong>Rationale:</strong></span> The assertion failure message
|
|
printed by this library follows a format similar to the message printed by
|
|
Clang when the C-style <code class="computeroutput"><span class="identifier">assert</span></code>
|
|
macro fails.
|
|
</p></div>
|
|
<div id="ftn.boost_contract.introduction.f2" class="footnote"><p><a href="#boost_contract.introduction.f2" class="para"><sup class="para">[4] </sup></a>
|
|
The <code class="computeroutput"><span class="identifier">pushable</span></code> base class is
|
|
used in this example just to show subcontracting, it is somewhat arbitrary
|
|
and it will likely not appear in real code.
|
|
</p></div>
|
|
<div id="ftn.boost_contract.introduction.f3" class="footnote"><p><a href="#boost_contract.introduction.f3" class="para"><sup class="para">[5] </sup></a>
|
|
The authors find attractive the syntax that uses C++11 attributes <code class="computeroutput"><span class="special">[[...]]</span></code> to specify contracts as indicated
|
|
in <a class="link" href="boost_contract/bibliography.html#P0380_anchor">[P0380]</a>.
|
|
</p></div>
|
|
</div>
|
|
</div>
|
|
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
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<td align="left"><p><small>Last revised: August 14, 2019 at 12:07:23 GMT</small></p></td>
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<td align="right"><div class="copyright-footer"></div></td>
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</tr></table>
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