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<h1>C++ use in Chromium</h1>
<p><i>This document lives at src/styleguide/c++/c++11.html in a Chromium
checkout and is part of the more general
<a href="https://chromium.googlesource.com/chromium/src/+/master/styleguide/c++/c++.md">
Chromium C++ style guide</a>. It summarizes the supported state of new and
updated language and library features in recent C++ standards. This guide
applies to both Chromium and its subprojects, though subprojects can choose to
be more restrictive if necessary for toolchain support.</i></p>
<p>The C++ language has in recent years received an updated standard every three
years (C++11, C++14, C++17). For various reasons, Chromium does not immediately
allow new features on the publication of such a standard. Instead, once
toolchain support is sufficient, a standard is declared "initially supported",
with new language/library features banned pending discussion.</p>
<p>You can propose changing the status of a feature by sending an email to
<a href="https://groups.google.com/a/chromium.org/forum/#!forum/cxx">
cxx@chromium.org</a>. Include a short blurb on what the feature is and why you
think it should or should not be allowed, along with links to any relevant
previous discussion. If the list arrives at some consensus, send a codereview to
change this file accordingly, linking to your discussion thread.</p>
<p>Two years after a standard is initially supported in Chromium, style arbiters
should make a final decision on any remaining TBD features to be banned, then
default-allow all non-banned portions of the standard. The current status of
existing standards is:
<ul><li><b>C++11:</b> <i>Default allowed; see banned features below</i></li>
<li><b>C++14:</b> <i>Initially supported August 15, 2017; see allowed/banned/TBD features below</i></li>
<li><b>C++17:</b> <i>Not yet supported in Chromium</i></li>
<li><b>C++20:</b> <i>Not yet standardized</i></li></ul></p>
<h2>Table of Contents</h2>
<ol class="toc">
<li>Allowed Features<ol>
<li>Language
<a href="#core-whitelist-14">C++14</a>
</li>
<li>Library
<a href="#library-whitelist-14">C++14</a>
</li>
</ol></li>
<li>Banned Features<ol>
<li>Language
<a href="#core-blocklist">C++11</a>
</li>
<li>Library
<a href="#library-blocklist">C++11</a>
<a href="#library-blocklist-14">C++14</a>
</li>
</ol></li>
<li>To Be Discussed<ol>
<li>Language
<a href="#core-review-14">C++14</a>
</li>
<li>Library
<a href="#library-review-14">C++14</a>
</li>
</ol></li>
</ol>
<h2 id="whitelist"><a name="core-whitelist-14"></a>C++14 Allowed Language Features</h2>
<p>The following C++14 language features are allowed in the Chromium codebase.</p>
<table id="whitelist_lang_list_14" class="unlined striped">
<tbody>
<tr>
<th style='width:220px;'>Feature</th>
<th style='width:260px;'>Snippet</th>
<th style='width:240px;'>Description</th>
<th style='width:240px;'>Documentation Link</th>
<th style='width:240px;'>Notes and Discussion Thread</th>
</tr>
<tr>
<td>Aggregate member initialization</td>
<td><code>struct Point { int x, y, z = 0; };<br>Point p = {2, 3};</code></td>
<td>Allows classes with default member initializers to be initialized with aggregate initialization, optionally omitting data members with such initializers.</td>
<td><a href="http://en.cppreference.com/w/cpp/language/aggregate_initialization">aggregate initialization</a></td>
<td><a href="https://groups.google.com/a/chromium.org/d/topic/cxx/WMBs3K9OQ_E/discussion">Discussion thread</a></td>
</tr>
<tr>
<td>Binary literals</td>
<td><code>int i = 0b1001;</code></td>
<td>Allows defining literals in base two.</td>
<td><a href="http://en.cppreference.com/w/cpp/language/integer_literal">Integer literals</a></td>
<td><a href="https://groups.google.com/a/chromium.org/d/topic/cxx/zsGhgaKLmIk/discussion">Discussion thread</a></td>
</tr>
<tr>
<td>Function return type deduction</td>
<td><code>auto f() { return 42; }<br>decltype(auto) g() { return 42; }</code></td>
<td>Allows the return type of a function to be automatically deduced from its return statements, according to either template or <code>decltype</code> rules.</td>
<td><a href="http://en.cppreference.com/w/cpp/language/function#Return_type_deduction">Return type deduction</a></td>
<td>Usage should be rare, primarily for abstract template code. If you're not sure of the difference, prefer decltype(auto) for templated forwarding/wrapper functions and auto for other cases; see Effective Modern C++ item 3 for more detail. <a href="https://groups.google.com/a/chromium.org/d/topic/cxx/-Ox7YgRS_no/discussion">Discussion thread</a></td>
</tr>
<tr>
<td>Generic lambdas</td>
<td><code>[](const auto&amp; x) { <i>...</i> }</code></td>
<td>Allows lambda argument types to be deduced using <code>auto</code> (according to the rules that apply to templates).</td>
<td><a href="http://en.cppreference.com/w/cpp/language/lambda">lambda expressions</a></td>
<td><a href="https://groups.google.com/a/chromium.org/d/topic/cxx/LasGKwE3SFM/discussion">Discussion thread</a></td>
</tr>
<tr>
<td>Lambda capture expressions</td>
<td><code>auto widget = std::make_unique&lt;Widget&gt;();<br>auto lambda = [widget = std::move(widget)]() {<br>&nbsp;&nbsp;SetWidget(std::move(widget));<br>}</code></td>
<td>Allows lambda captures to be explicitly initialized with expressions.</td>
<td><a href="http://en.cppreference.com/w/cpp/language/lambda#Lambda_capture">Lambda capture</a></td>
<td> Lambda capture expressions should only be used to capture existing names in
ways that C++11's direct capture doesn't allow, and should not be used as a
way to introduce new names. That is, capture expressions should always shadow
existing variables (or data members), and not substantially change their
meaning. Usage should be rare.
<a href="https://groups.google.com/a/chromium.org/d/msg/cxx/8BHoi4T6ZhA/pWHB6IYoAgAJ">
Discussion thread</a> with
<a href="https://groups.google.com/a/chromium.org/d/msg/cxx/8BHoi4T6ZhA/WwKXh2RgAgAJ">
examples</a></td>
</tr>
<tr>
<td>Number literal separators</td>
<td><code>float f = 1'000'000.000'1;</code></td>
<td><code>'</code>s anywhere in int or float literals are ignored</td>
<td><a href="http://en.cppreference.com/w/cpp/language/integer_literal">Integer literals</a>, <a href="http://en.cppreference.com/w/cpp/language/floating_literal">Floating point literals</a></td>
<td><a href="https://groups.google.com/a/chromium.org/d/topic/cxx/exS1aGs1wes/discussion">Discussion thread</a></td>
</tr>
<tr>
<td>Relaxed constant expressions</td>
<td><code>constexpr int Factorial(int n) {<br>&nbsp;&nbsp;int result = 1;<br>&nbsp;&nbsp;while (n > 0)<br>&nbsp;&nbsp;&nbsp;&nbsp;result *= n--;<br>&nbsp;&nbsp;return result;<br>}</code></td>
<td>Allows use of more declarations, conditional statements and loops inside <code>constexpr</code> functions.</td>
<td><a href="http://en.cppreference.com/w/cpp/language/constexpr">constexpr specifier</a></td>
<td>Prefer to <code>const</code> for variables where possible. Use cautiously on functions. Don't go out of the way to convert existing code. <a href="https://google.github.io/styleguide/cppguide.html#Use_of_constexpr">Google Style Guide</a>. <a href="https://groups.google.com/a/chromium.org/forum/#!topic/cxx/Psuq_EKJBAo">Discussion thread</a></td>
</tr>
</tbody>
</table>
<h2 id="whitelist"><a name="library-whitelist-14"></a>C++14 Allowed Library Features</h2>
<p>The following C++14 library features are allowed in the Chromium codebase.</p>
<table id="whitelist_lib_list" class="unlined striped">
<tbody>
<tr>
<th style='width:240px;'>Feature or Library</th>
<th style='width:240px;'>Snippet</th>
<th style='width:240px;'>Description</th>
<th style='width:240px;'>Documentation Link</th>
<th style='width:240px;'>Notes and Discussion Thread</th>
</tr>
<tr>
<td>Constant begin/end non-member functions</td>
<td><code>std::cbegin(<i>container</i>)<br>std::cend(<i>container</i>)<br>std::crbegin(<i>container</i>)<br>std::crend(<i>container</i>)</br></code></td>
<td>Constant counterparts to <code>std::begin</code> etc.</td>
<td><a href="http://en.cppreference.com/w/cpp/iterator/begin">std::cbegin</a></td>
<td></td>
</tr>
<tr>
<td>Heterogeneous lookup in associative containers</td>
<td><code><i>// Does not construct an std::string to use as the lookup key.</i><br>std::map&lt;std::string, int, std::less&lt;&gt;&gt; map;<br>auto it = map.find("answer");</code></td>
<td>Allows searching associative containers without converting the key to exactly match the stored key type, assuming a suitable comparator exists.</td>
<td><a href="http://en.cppreference.com/w/cpp/utility/functional/less">std::less</a></td>
<td><a href="https://groups.google.com/a/chromium.org/d/msg/cxx/ow7hmdDm4yw/EDEvBRi_BQAJ">Discussion thread</a></td>
</tr>
<tr>
<td><code>std::integer_sequence</code></td>
<td><code>template &lt;size_t... I&gt;<br>void CallFooImpl(std::index_sequence&lt;I...&gt;) {<br>&nbsp;&nbsp;Foo(I...);<br>}<br><br>template &lt;size_t N&gt;<br>void CallFoo() {<br>&nbsp;&nbsp;CallFooImpl(std::make_index_sequence&lt;N&gt;());<br>}</code></td>
<td>Template metaprogramming utility for representing a sequence of integers as a type.</td>
<td><a href="http://en.cppreference.com/w/cpp/utility/integer_sequence">std::integer_sequence</a></td>
<td>This also includes the alias, <code>std::index_sequence</code>, which is the specialization for <code>size_t</code>. <a href="https://groups.google.com/a/chromium.org/d/msg/cxx/ow7hmdDm4yw/EDEvBRi_BQAJ">Discussion thread</a></td>
</tr>
<tr>
<td><code>std::make_unique</code></td>
<td><code>auto widget = std::make_unique&lt;Widget&gt;();</code></td>
<td>Allocates objects on the heap and immediately constructs an <code>std::unique_ptr</code> to assume ownership.</td>
<td><a href="http://en.cppreference.com/w/cpp/memory/unique_ptr/make_unique">std::make_unique</a></td>
<td><a href="https://groups.google.com/a/chromium.org/d/msg/cxx/ow7hmdDm4yw/EDEvBRi_BQAJ">Discussion thread</a></td>
</tr>
<tr>
<td>Transparent function objects</td>
<td>Arithmetic operations:<br><code>std::plus&lt;&gt;, std::minus&lt;&gt;</code> ...<br>Comparisons:<br><code>std::less&lt;&gt;, std::equal_to&lt;&gt;</code>...<br>Logical operations:<br><code>std::logical_and&lt;&gt;, std::logical_or&lt;&gt;</code>...<br>Bitwise operations:<br><code>std::bit_and&lt;&gt;, std::bit_or&lt;&gt;</code>...<br></td>
<td>Function objects that deduce argument types.</td>
<td><a href="http://en.cppreference.com/w/cpp/utility/functional/less_void">std::less&lt;&gt;</a></td>
<td>Should replace <code>base::less</code> and usage of these functors with explicit types where appropriate. <a href="https://groups.google.com/a/chromium.org/forum/#!topic/cxx/cPoULd2NY9k">Discussion thread</a></td>
</tr>
<tr>
<td>Tuple addressing by type</td>
<td><code>std::tuple&lt;int, char&gt; enterprise(1701, 'D');<br>int n = std::get&lt;int&gt;(enterprise);</code></td>
<td>Allows entries in a tuple to be accessed by type rather than entry, if it is not ambiguous.</td>
<td><a href="http://en.cppreference.com/w/cpp/utility/tuple/get">std::get(std::tuple)</a></td>
<td></td>
</tr>
<tr>
<td>Reverse Iterator Adaptor</td>
<td><code>std::make_reverse_iterator()</code></td>
<td>For a given iterator, deduces the type of a corresponding reverse iterator and constructs it.</td>
<td><a href="http://en.cppreference.com/w/cpp/iterator/make_reverse_iterator">std::make_reverse_iterator</a></td>
<td>Useful to reduce boilerplate when constructing reverse iterators. The alternative is using <code>std::reverse_iterator<T>(i)</code> where <code>T</code> is the, usually long, type of the iterator <code>i</code>. <a href="https://groups.google.com/a/chromium.org/forum/#!topic/cxx/qOE1XA0b6Dk">Discussion thread</a></td>
</tr>
</tbody>
</table>
<h2 id="blocklist_banned"><a name="core-blocklist"></a>C++11 Banned Language Features</h2>
<p>The following C++11 language features are not allowed in the Chromium codebase.</p>
<table id="banned_list" class="unlined striped">
<tbody>
<tr>
<th style='width:240px;'>Feature or Library</th>
<th style='width:240px;'>Snippet</th>
<th style='width:240px;'>Description</th>
<th style='width:240px;'>Documentation Link</th>
<th style='width:240px;'>Notes and Discussion Thread</th>
</tr>
<tr>
<td>Inline Namespaces</td>
<td><code>inline namespace foo { ... }</code></td>
<td>Allows better versioning of namespaces</td>
<td><a href="http://en.cppreference.com/w/cpp/language/namespace#Inline_namespaces">Inline namespaces</a></td>
<td>Banned in the <a href="https://google.github.io/styleguide/cppguide.html#Namespaces">Google Style Guide</a>. Unclear how it will work with components.</td>
</tr>
<tr>
<td><code>long long</code> Type</td>
<td><code>long long <i>var</i> = <i>value</i>;</code></td>
<td>An integer of at least 64 bits</td>
<td><a href="http://en.cppreference.com/w/cpp/language/types">Fundamental types</a></td>
<td>Use a stdint.h type if you need a 64-bit number. <a href="https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/RxugZ-pIDxk">Discussion thread</a></td>
</tr>
<tr>
<td>User-Defined Literals</td>
<td><code><i>type</i> <i>var</i> = <i>literal_value</i>_<i>type</i></code></td>
<td>Allows user-defined literal expressions</td>
<td><a href="http://en.cppreference.com/w/cpp/language/user_literal">User-defined literals</a></td>
<td>Banned in the <a href="https://google.github.io/styleguide/cppguide.html#Operator_Overloading">Google Style Guide</a>.</td>
</tr>
<tr>
<td>thread_local storage class</td>
<td><code>thread_local int foo = 1;</code></td>
<td>Puts variables into thread local storage.</td>
<td><a href="http://en.cppreference.com/w/cpp/language/storage_duration">Storage duration</a></td>
<td>Some surprising effects on Mac (<a href="https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/2msN8k3Xzgs">discussion</a>, <a href="https://groups.google.com/a/chromium.org/forum/#!topic/cxx/h7O5BdtWCZw">fork</a>). Use <code>base::SequenceLocalStorageSlot</code> for sequence support, and <code>base::ThreadLocal</code>/<code>base::ThreadLocalStorage</code> otherwise.</td>
</tr>
</tbody>
</table>
<h2 id="blocklist_stdlib"><a name="library-blocklist"></a>C++11 Banned Library Features</h2>
<p>The following C++11 library features are not allowed in the Chromium codebase.</p>
<table id="blocklist_lib_list" class="unlined striped">
<tbody>
<tr>
<th style='width:240px;'>Feature</th>
<th style='width:240px;'>Snippet</th>
<th style='width:240px;'>Description</th>
<th style='width:240px;'>Documentation Link</th>
<th style='width:240px;'>Notes and Discussion Thread</th>
</tr>
<td>Bind Operations</td>
<td><code>std::bind(<i>function</i>, <i>args</i>, ...)</code></td>
<td>Declares a function object bound to certain arguments</td>
<td><a href="http://en.cppreference.com/w/cpp/utility/functional/bind">std::bind</a></td>
<td>Use <code>base::Bind</code> instead. Compared to <code>std::bind</code>, <code>base::Bind</code> helps prevent lifetime issues by preventing binding of capturing lambdas and by forcing callers to declare raw pointers as <code>Unretained</code>. <a href="https://groups.google.com/a/chromium.org/forum/#!topic/cxx/SoEj7oIDNuA">Discussion thread</a></td>
</tr>
<tr>
<td>C Floating-Point Environment</td>
<td><code>&lt;cfenv&gt;</code>, <code>&lt;fenv.h&gt;</code></td>
<td>Provides floating point status flags and control modes for C-compatible code</td>
<td><a href="http://en.cppreference.com/w/cpp/header/cfenv">Standard library header &lt;cfenv&gt;</a></td>
<td>Banned by the <a href="https://google.github.io/styleguide/cppguide.html#C++11">Google Style Guide</a> due to concerns about compiler support.</td>
</tr>
<tr>
<td>Date and time utilities</td>
<td><code>&lt;chrono&gt;</code></td>
<td>A standard date and time library</td>
<td><a href="http://en.cppreference.com/w/cpp/chrono">Date and time utilities</a></td>
<td>Overlaps with <code>Time</code> APIs in <code>base/</code>. Keep using the <code>base/</code> classes.</td>
</tr>
<tr>
<td>Exceptions</td>
<td><code>&lt;exception&gt;</code></td>
<td>Enhancements to exception throwing and handling</td>
<td><a href="http://en.cppreference.com/w/cpp/header/exception">Standard library header &lt;exception&gt;</a></td>
<td>Exceptions are banned by the <a href="https://google.github.io/styleguide/cppguide.html#Exceptions">Google Style Guide</a> and disabled in Chromium compiles. However, the <code>noexcept</code> specifier is explicitly allowed. <a href="https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/8i4tMqNpHhg">Discussion thread</a></td>
</tr>
<tr>
<td>Function Objects</td>
<td><code>std::function</code></td>
<td>Wraps a standard polymorphic function</td>
<td><a href="http://en.cppreference.com/w/cpp/utility/functional/function">std::function</a></td>
<td>Use <code>base::Callback</code> instead. Compared to <code>std::function</code>, <code>base::Callback</code> directly supports Chromium's refcounting classes and weak pointers and deals with additional thread safety concerns. <a href="https://groups.google.com/a/chromium.org/forum/#!topic/cxx/SoEj7oIDNuA">Discussion thread</a></td>
</tr>
<tr>
<td>Random Number Engines</td>
<td>The random number engines defined in <code>&lt;random&gt;</code> (see separate item for random number distributions), e.g.:<br/>
<code>linear_congruential_engine</code>, <code>mersenne_twister_engine</code><br/>
<code>minstd_rand0</code>, <code>mt19937</code>, <code>ranlinux48</code><br/>
<code>random_device</code>
</td>
<td>Random number generation algorithms and utilities.</td>
<td><a href="http://en.cppreference.com/w/cpp/numeric/random">Pseudo-random number generation</a></td>
<td>Do not use any random number engines from <code>&lt;random&gt;</code>. Instead, use <code>base::RandomBitGenerator</code>. <a href="https://groups.google.com/a/chromium.org/forum/#!topic/cxx/16Xmw05C-Y0">Discussion thread</a></td>
</tr>
<tr>
<td>Ratio Template Class</td>
<td><code>std::ratio&lt;<i>numerator</i>, <i>denominator</i>&gt;</code></td>
<td>Provides compile-time rational numbers</td>
<td><a href="http://en.cppreference.com/w/cpp/numeric/ratio/ratio">std::ratio</a></td>
<td>Banned by the <a href="https://google.github.io/styleguide/cppguide.html#C++11">Google Style Guide</a> due to concerns that this is tied to a more template-heavy interface style.</td>
</tr>
<tr>
<td>Regular Expressions</td>
<td><code>&lt;regex&gt;</code></td>
<td>A standard regular expressions library</td>
<td><a href="http://en.cppreference.com/w/cpp/regex">Regular expressions library</a></td>
<td>Overlaps with many regular expression libraries in Chromium. When in doubt, use re2.</td>
</tr>
<tr>
<td>Shared Pointers</td>
<td><code>std::shared_ptr</code></td>
<td>Allows shared ownership of a pointer through reference counts</td>
<td><a href="http://en.cppreference.com/w/cpp/memory/shared_ptr">std::shared_ptr</a></td>
<td>Needs a lot more evaluation for Chromium, and there isn't enough of a push for this feature. <a href="https://google.github.io/styleguide/cppguide.html#Ownership_and_Smart_Pointers">Google Style Guide</a>. <a href="https://groups.google.com/a/chromium.org/forum/#!topic/cxx/aT2wsBLKvzI">Discussion Thread</a></td>
</tr>
<tr>
<td>String-Number Conversion Functions</td>
<td><code>std::stoi()</code>, <code>std::stol()</code>, <code>std::stoul()</code>, <code>std::stoll</code>, <code>std::stoull()</code>, <code>std::stof()</code>, <code>std::stod()</code>, <code>std::stold()</code>, <code>std::to_string()</code></td>
<td>Converts strings to/from numbers</td>
<td><a href="http://en.cppreference.com/w/cpp/string/basic_string/stol">std::stoi, std::stol, std::stoll</a>, <a href="http://en.cppreference.com/w/cpp/string/basic_string/stoul">std::stoul, std::stoull</a>, <a href="http://en.cppreference.com/w/cpp/string/basic_string/stof">std::stof, std::stod, std::stold</a>, <a href="http://en.cppreference.com/w/cpp/string/basic_string/to_string">std::to_string</a></td>
<td>The string-to-number conversions rely on exceptions to communicate failure, while the number-to-string conversions have performance concerns and depend on the locale. Use the routines in <code>base/strings/string_number_conversions.h</code> instead.</td>
</tr>
<tr>
<td>Thread Library</td>
<td><code>&lt;thread&gt;</code> and related headers, including<br />
<code>&lt;future&gt;</code>, <code>&lt;mutex&gt;</code>, <code>&lt;condition_variable&gt;</code></td>
<td>Provides a standard multithreading library using <code>std::thread</code> and associates</td>
<td><a href="http://en.cppreference.com/w/cpp/thread">Thread support library</a></td>
<td>Overlaps with many classes in <code>base/</code>. Keep using the <code>base/</code> classes for now. <code>base::Thread</code> is tightly coupled to <code>MessageLoop</code> which would make it hard to replace. We should investigate using standard mutexes, or unique_lock, etc. to replace our locking/synchronization classes.</td>
</tr>
<tr>
<td>Weak Pointers</td>
<td><code>std::weak_ptr</code></td>
<td>Allows a weak reference to a <code>std::shared_ptr</code></td>
<td><a href="http://en.cppreference.com/w/cpp/memory/weak_ptr">std::weak_ptr</a></td>
<td>Banned because <code>std::shared_ptr</code> is banned. Use <code>base::WeakPtr</code> instead.</td>
</tr>
</tbody>
</table>
<h2 id="blocklist_stdlib"><a name="library-blocklist-14"></a>C++14 Banned Library Features</h2>
<p>The following C++14 library features are not allowed in the Chromium codebase.</p>
<table id="blocklist_lib_list" class="unlined striped">
<tbody>
<tr>
<th style='width:240px;'>Feature</th>
<th style='width:240px;'>Snippet</th>
<th style='width:240px;'>Description</th>
<th style='width:240px;'>Documentation Link</th>
<th style='width:240px;'>Notes and Discussion Thread</th>
</tr>
<tr>
<td><code>std::chrono</code> literals</td>
<td><code>using namespace std::chrono_literals;<br>auto timeout = 30s;</code></td>
<td>Allows <code>std::chrono</code> types to be more easily constructed.</td>
<td><a href="http://en.cppreference.com/w/cpp/chrono/operator%22%22s">std::literals::chrono_literals::operator""s</a></td>
<td>Banned because <code>&lt;chrono&gt;</code> is banned.</td>
</tr>
</tbody>
</table>
<h2 id="blocklist_review"><a name="core-review-14"></a>C++14 TBD Language Features</h2>
<p>The following C++14 language features are not allowed in the Chromium codebase. See the top of this page on how to propose moving a feature from this list into the allowed or banned sections.</p>
<table id="blocklist_review_list" class="unlined striped">
<tbody>
<tr>
<th style='width:240px;'>Feature</th>
<th style='width:240px;'>Snippet</th>
<th style='width:240px;'>Description</th>
<th style='width:240px;'>Documentation Link</th>
<th style='width:240px;'>Notes and Discussion Thread</th>
</tr>
<tr>
<td><code>[[deprecated]]</code> attribute</td>
<td><code>[[deprecated]] void f();<br>
[[deprecated("use h() instead")]] void g();</code></td>
<td>Marks a function as deprecated.</td>
<td><a href="http://en.cppreference.com/w/cpp/language/attributes">Standard attributes</a></td>
<td>We don't use deprecation warnings in Chromium; if you want to deprecate
something, remove all callers and remove the function instead.</td>
</tr>
<tr>
<td><code>decltype(auto)</code> variable declarations</td>
<td><code>decltype(auto) x = 42;</code></td>
<td>Allows deducing the type of a variable using <code>decltype</code> rules.</td>
<td><a href="http://en.cppreference.com/w/cpp/language/auto">auto specifier</a></td>
<td>Often more surprising than <code>auto</code>. For instance, the <code>decltype</code> deduction rules do not remove references.</td>
</tr>
<tr>
<td>Variable templates</td>
<td><code>template &lt;typename T&gt;<br>constexpr T tau = T(6.283185307179586476925286766559);</code></td>
<td>Allows templates that declare variables, rather than functions or classes.</td>
<td><a href="http://en.cppreference.com/w/cpp/language/variable_template">Variable template</a></td>
<td></td>
</tr>
</tbody>
</table>
<h2 id="blocklist_stdlib_review"><a name="library-review-14"></a>C++14 TBD Library Features</h2>
<p>The following C++14 library features are not allowed in the Chromium codebase. See the top of this page on how to propose moving a feature from this list into the allowed or banned sections.</p>
<table id="banned_stdlib" class="unlined striped">
<tbody>
<tr>
<th style='width:240px;'>Feature</th>
<th style='width:240px;'>Snippet</th>
<th style='width:240px;'>Description</th>
<th style='width:240px;'>Documentation Link</th>
<th style='width:240px;'>Notes</th>
</tr>
<tr>
<td><code>std::complex</code> literals</td>
<td><code>using namespace std::complex_literals;<br>std::complex&lt;double&gt; c = 2.0 + 0.5i;</code></td>
<td>Allows <code>std::complex</code> objects to be more easily constructed.</td>
<td><a href="http://en.cppreference.com/w/cpp/numeric/complex/operator%22%22i">std::literals::complex_literals</a></td>
<td></td>
</tr>
<td><code>std::string</code> literals</td>
<td><code>
#include &lt;string&gt;<br>
using std::literals::string_literals::operator"" s;<br>
auto s = "asdf"s; // deduces to std::string</code></td>
<td>Allows literals of type <code>std::string</code></td>
<td><a href="http://en.cppreference.com/w/cpp/string/basic_string/operator%22%22s">std::literals::string_literals::operator""s</a></td>
<td></td>
</tr>
</tbody>
</table>
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