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8252582: HotSpot Style Guide should permit variable templates

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Reviewed-by: dholmes, stefank, kvn
This commit is contained in:
Kim Barrett 2025-09-16 04:42:50 +00:00
parent 90e81c2bee
commit 0fbae8050b
2 changed files with 43 additions and 35 deletions
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45
doc/hotspot-style.html
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@ -86,8 +86,9 @@ values</a></li>
<li><a href="#thread_local" id="toc-thread_local">thread_local</a></li>
<li><a href="#nullptr" id="toc-nullptr">nullptr</a></li>
<li><a href="#atomic" id="toc-atomic">&lt;atomic&gt;</a></li>
<li><a href="#inline-variables" id="toc-inline-variables">Inline
Variables</a></li>
<li><a href="#variable-templates-and-inline-variables"
id="toc-variable-templates-and-inline-variables">Variable Templates and
Inline Variables</a></li>
<li><a href="#initializing-variables-with-static-storage-duration"
id="toc-initializing-variables-with-static-storage-duration">Initializing
variables with static storage duration</a></li>
@ -937,12 +938,18 @@ differ from what the Java compilers implement.</p>
"conservative" memory ordering, which may differ from (may be stronger
than) sequentially consistent. There are algorithms in HotSpot that are
believed to rely on that ordering.</p>
<h3 id="inline-variables">Inline Variables</h3>
<p>Variables with static storage duration may be declared
<code>inline</code> (<a href="https://wg21.link/p0386r2">p0386r2</a>).
This has similar effects as for declaring a function inline: it can be
defined, identically, in multiple translation units, must be defined in
every translation unit in which it is <a
<h3 id="variable-templates-and-inline-variables">Variable Templates and
Inline Variables</h3>
<p>The use of variable templates (including static data member
templates) (<a href="https://wg21.link/N3651">N3651</a>) is permitted.
They provide parameterized variables and constants in a simple and
direct form, instead of requiring the use of various workarounds.</p>
<p>Variables with static storage duration and variable templates may be
declared <code>inline</code> (<a
href="https://wg21.link/p0386r2">p0386r2</a>), and this usage is
permitted. This has similar effects as for declaring a function inline:
it can be defined, identically, in multiple translation units, must be
defined in every translation unit in which it is <a
href="https://en.cppreference.com/w/cpp/language/definition"
title="One Definition Rule">ODR used</a>, and the behavior of the
program is as if there is exactly one variable.</p>
@ -955,16 +962,17 @@ initializations can make initialization order problems worse. The few
ordering constraints that exist for non-inline variables don't apply, as
there isn't a single program-designated translation unit containing the
definition.</p>
<p>A <code>constexpr</code> static data member is implicitly
<code>inline</code>. As a consequence, an <a
<p>A <code>constexpr</code> static data member or static data member
template is implicitly <code>inline</code>. As a consequence, an <a
href="https://en.cppreference.com/w/cpp/language/definition"
title="One Definition Rule">ODR use</a> of such a variable doesn't
require a definition in some .cpp file. (This is a change from
pre-C++17. Beginning with C++17, such a definition is considered a
duplicate definition, and is deprecated.)</p>
<p>Declaring a <code>thread_local</code> variable <code>inline</code> is
forbidden for HotSpot code. <a href="#thread_local">The use of
<code>thread_local</code></a> is already heavily restricted.</p>
title="One Definition Rule">ODR use</a> of such a member doesn't require
a definition in some .cpp file. (This is a change from pre-C++17.
Beginning with C++17, such a definition is considered a duplicate
definition, and is deprecated.)</p>
<p>Declaring a <code>thread_local</code> variable template or
<code>inline</code> variable is forbidden in HotSpot code. <a
href="#thread_local">The use of <code>thread_local</code></a> is already
heavily restricted.</p>
<h3
id="initializing-variables-with-static-storage-duration">Initializing
variables with static storage duration</h3>
@ -1853,9 +1861,6 @@ Features</h3>
<ul>
<li><p>Trailing return type syntax for functions (<a
href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2541.htm">n2541</a>)</p></li>
<li><p>Variable templates (<a
href="https://isocpp.org/files/papers/N3651.pdf">n3651</a>, <a
href="http://wg21.link/p0127r2">p0127r2</a>)</p></li>
<li><p>Member initializers and aggregates (<a
href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3653.html">n3653</a>)</p></li>
<li><p>Rvalue references and move semantics</p></li>

33
doc/hotspot-style.md
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@ -856,14 +856,19 @@ ordering, which may differ from (may be stronger than) sequentially
consistent. There are algorithms in HotSpot that are believed to rely
on that ordering.
### Inline Variables
### Variable Templates and Inline Variables
Variables with static storage duration may be declared `inline`
([p0386r2](https://wg21.link/p0386r2)). This has similar effects as for
declaring a function inline: it can be defined, identically, in multiple
translation units, must be defined in every translation unit in which it is
[ODR used][ODR], and the behavior of the program is as if there is exactly one
variable.
The use of variable templates (including static data member templates)
([N3651](https://wg21.link/N3651)) is permitted. They provide parameterized
variables and constants in a simple and direct form, instead of requiring the
use of various workarounds.
Variables with static storage duration and variable templates may be declared
`inline` ([p0386r2](https://wg21.link/p0386r2)), and this usage is
permitted. This has similar effects as for declaring a function inline: it can
be defined, identically, in multiple translation units, must be defined in
every translation unit in which it is [ODR used][ODR], and the behavior of the
program is as if there is exactly one variable.
Declaring a variable inline allows the complete definition to be in a header
file, rather than having a declaration in a header and the definition in a
@ -874,13 +879,15 @@ make initialization order problems worse. The few ordering constraints
that exist for non-inline variables don't apply, as there isn't a single
program-designated translation unit containing the definition.
A `constexpr` static data member is implicitly `inline`. As a consequence, an
[ODR use][ODR] of such a variable doesn't require a definition in some .cpp
A `constexpr` static data member or static data member template
is implicitly `inline`. As a consequence, an
[ODR use][ODR] of such a member doesn't require a definition in some .cpp
file. (This is a change from pre-C++17. Beginning with C++17, such a
definition is considered a duplicate definition, and is deprecated.)
Declaring a `thread_local` variable `inline` is forbidden for HotSpot code.
[The use of `thread_local`](#thread_local) is already heavily restricted.
Declaring a `thread_local` variable template or `inline` variable is forbidden
in HotSpot code. [The use of `thread_local`](#thread_local) is already
heavily restricted.
### Initializing variables with static storage duration
@ -1849,10 +1856,6 @@ See Object Lifetime: C++17 6.8/8, C++20 6.7.3/8
* Trailing return type syntax for functions
([n2541](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2541.htm))
* Variable templates
([n3651](https://isocpp.org/files/papers/N3651.pdf),
[p0127r2](http://wg21.link/p0127r2))
* Member initializers and aggregates
([n3653](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3653.html))