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</div><div><span class="editsection noprint plainlinks" title="Edit this template"><a rel="nofollow" class="external text" href="http://en.cppreference.com/mwiki/index.php?title=Template:cpp/language/navbar_content&action=edit">[edit]</a></span></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>Renders the entire program meaningless if certain rules of the language are violated.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/ub&action=edit&section=1" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>The C++ standard precisely defines the <a href="as_if.html" title="cpp/language/as if">observable behavior</a> of every C++ program that does not fall into one of the following classes:
</p>
<ul><li> <i>ill-formed</i> - the program has syntax errors or diagnosable semantic errors. A conforming C++ compiler is required to issue a diagnostic, even if it defines a language extension that assigns meaning to such code (such as with variable-length arrays). The text of the standard uses <i>shall</i>, <i>shall not</i>, and <i>ill-formed</i> to indicate these requirements.
</li><li> <i>ill-formed no diagnostic required</i> - the program has semantic errors which may not be diagnosable in general case (e.g. violations of the <a href="definition.html" title="cpp/language/definition">ODR</a> or other errors that are only detectable at link time. The behavior is undefined if such program is executed.
</li><li> <i>implementation-defined behavior</i> - the behavior of the program varies between implementations, and the conforming implementation must document the effects of each behavior. For example, the type of <span class="t-lc">std::size_t</span> or the number of bits in a byte, or the text of <span class="t-lc">std::bad_alloc::what</span>. A subset of implementation-defined behavior is <i>locale-specific behavior</i>, which depends on the implementation-supplied <a href="../locale.html" title="cpp/locale">locale</a>.
</li><li> <i>unspecified behavior</i> - the behavior of the program varies between implementations and the conforming implementation is not required to document the effects of each behavior. For example, <a href="eval_order.html" title="cpp/language/eval order">order of evaluation</a>, whether identical <a href="string_literal.html" title="cpp/language/string literal">string literals</a> are distinct, the amount of array allocation overhead, etc. Each unspecified behavior results in one of a set of valid results.
</li><li> <i>undefined behavior</i> - there are no restrictions on the behavior of the program. Examples of undefined behavior are memory accesses outside of array bounds, signed integer overflow, null pointer dereference, modification of the same scalar <a href="eval_order.html" title="cpp/language/eval order">more than once</a> in an expression without sequence points, access to an object through a pointer of a different type, etc. Compilers are not required to diagnose undefined behavior (although many simple situations are diagnosed), and the compiled program is not required to do anything meaningful.
</li></ul>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/ub&action=edit&section=2" title="Edit section: UB and optimization">edit</a>]</span> <span class="mw-headline" id="UB_and_optimization">UB and optimization</span></h3>
<p>Because correct C++ programs are free of undefined behavior, compilers may produce unexpected results when a program that actually has UB is compiled with optimization enabled:
</p><p>For example,
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> foo<span class="br0">(</span><span class="kw4">int</span> x<span class="br0">)</span> <span class="br0">{</span>
<span class="kw1">return</span> x<span class="sy2">+</span><span class="nu0">1</span> <span class="sy1">></span> x<span class="sy4">;</span> <span class="co1">// either true or UB due to signed overflow</span>
<span class="br0">}</span>
<span class="co1">// may be compiled as </span>
<span class="kw4">int</span> foo<span class="br0">(</span><span class="kw4">int</span> x<span class="br0">)</span> <span class="br0">{</span>
<span class="kw1">return</span> <span class="nu0">1</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">bool</span> p<span class="sy4">;</span> <span class="co1">// uninitialized local variable</span>
<span class="kw1">if</span><span class="br0">(</span>p<span class="br0">)</span> <span class="co1">// UB access to uninitialized scalar</span>
<a href="../io/c/puts.html"><span class="kw1553">std::<span class="me2">puts</span></span></a><span class="br0">(</span><span class="st0">"p is true"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">if</span><span class="br0">(</span><span class="sy3">!</span>p<span class="br0">)</span> <span class="co1">// UB access to uninitialized scalar</span>
<a href="../io/c/puts.html"><span class="kw1553">std::<span class="me2">puts</span></span></a><span class="br0">(</span><span class="st0">"p is false"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// may be compiled to a program that prints both lines:</span>
<span class="co1">// p is true</span>
<span class="co1">// p is false</span>
<span class="co1">// ...or to a program that prints nothing</span></pre></div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> table<span class="br0">[</span><span class="nu0">4</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">bool</span> exists_in_table<span class="br0">(</span><span class="kw4">int</span> v<span class="br0">)</span>
<span class="br0">{</span>
<span class="co1">// return true in one of the first 4 iterations or UB due to out-of-bounds access</span>
<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> i <span class="sy1"><=</span> <span class="nu0">4</span><span class="sy4">;</span> i<span class="sy2">++</span><span class="br0">)</span> <span class="br0">{</span>
<span class="kw1">if</span> <span class="br0">(</span>table<span class="br0">[</span>i<span class="br0">]</span> <span class="sy1">==</span> v<span class="br0">)</span> <span class="kw1">return</span> <span class="kw2">true</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw1">return</span> <span class="kw2">false</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="co1">// this may be compiled as</span>
<span class="kw4">bool</span> exists_in_table<span class="br0">(</span><span class="kw4">int</span> v<span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">return</span> <span class="kw2">true</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> <span class="sy2">*</span>p <span class="sy1">=</span> <span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span><span class="br0">)</span><a href="../memory/c/malloc.html"><span class="kw640">std::<span class="me2">malloc</span></span></a><span class="br0">(</span>sizeof<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">int</span> <span class="sy2">*</span>q <span class="sy1">=</span> <span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span><span class="br0">)</span><a href="../memory/c/realloc.html"><span class="kw642">std::<span class="me2">realloc</span></span></a><span class="br0">(</span>p, sizeof<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
<span class="sy2">*</span>p <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// UB access to a pointer that was passed to realloc</span>
<span class="sy2">*</span>q <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span>
<span class="kw1">if</span> <span class="br0">(</span>p <span class="sy1">==</span> q<span class="br0">)</span> <span class="co1">// UB access to a pointer that was passed to realloc</span>
printf<span class="br0">(</span><span class="st0">"%d %d<span class="es1">\n</span>"</span>, <span class="sy2">*</span>p, <span class="sy2">*</span>q<span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// this may print 1 2</span></pre></div></div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/ub&action=edit&section=3" title="Edit section: External links">edit</a>]</span> <span class="mw-headline" id="External_links">External links</span></h3>
<ul><li> <a rel="nofollow" class="external text" href="http://blog.llvm.org/2011/05/what-every-c-programmer-should-know.html">What Every C Programmer Should Know About Undefined Behavior #1/3</a>
</li><li> <a rel="nofollow" class="external text" href="http://blog.llvm.org/2011/05/what-every-c-programmer-should-know_14.html">What Every C Programmer Should Know About Undefined Behavior #2/3</a>
</li><li> <a rel="nofollow" class="external text" href="http://blog.llvm.org/2011/05/what-every-c-programmer-should-know_21.html">What Every C Programmer Should Know About Undefined Behavior #3/3</a>
</li><li> <a rel="nofollow" class="external text" href="http://blogs.msdn.com/b/oldnewthing/archive/2014/06/27/10537746.aspx">Undefined behavior can result in time travel (among other things, but time travel is the funkiest)</a>
</li><li> <a rel="nofollow" class="external text" href="http://www.cs.utah.edu/~regehr/papers/overflow12.pdf">Understanding Integer Overflow in C/C++</a>
</li><li> <a rel="nofollow" class="external text" href="http://kukuruku.co/hub/cpp/undefined-behavior-and-fermats-last-theorem">Undefined Behavior and Fermat’s Last Theorem</a>
</li></ul>
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