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    CWG Issue 2366</TITLE>
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<P><EM>This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21
  Core Issues List revision 118b.
  See http://www.open-std.org/jtc1/sc22/wg21/ for the official
  list.</EM></P>
<P>2025-09-28</P>
<HR>
<A NAME="2366"></A><H4>2366.
  
Can default initialization be constant initialization?
</H4>
<B>Section: </B>6.10.3.2&#160; [<A href="https://wg21.link/basic.start.static">basic.start.static</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD5
 &#160;&#160;&#160;

 <B>Submitter: </B>Geoffrey Romer
 &#160;&#160;&#160;

 <B>Date: </B>2017-11-01<BR>


<P>[Accepted as a DR at the July, 2019 meeting.]</P>

<P>According to 6.10.3.2 [<A href="https://wg21.link/basic.start.static#2">basic.start.static</A>] paragraph 2,</P>

<BLOCKQUOTE>

<I>Constant initialization</I>
is performed if a variable or temporary object with
static or thread storage duration is initialized by a constant initializer
for the entity. If constant initialization is not performed, a variable
with static storage duration (6.8.6.2 [<A href="https://wg21.link/basic.stc.static">basic.stc.static</A>]) or thread storage
duration (6.8.6.3 [<A href="https://wg21.link/basic.stc.thread">basic.stc.thread</A>]) is zero-initialized
(9.5 [<A href="https://wg21.link/dcl.init">dcl.init</A>]). Together, zero-initialization and constant
initialization are called static initialization; all other initialization
is <I>dynamic initialization</I>.

</BLOCKQUOTE>

<P>This appears to require an explicit initializer for constant
initialization and would preclude examples like:</P>

<PRE>
  struct S {
    int i = 1;
  };
  static constexpr S s;
</PRE>

<P>where there is no initializer.</P>

<P><B>Notes from the October, 2018 teleconference:</B></P>

<P>CWG agreed that the example should be well-formed.</P>

<P><B>Proposed resolution, June, 2019:</B></P>

<OL>
<LI><P>Change 6.10.3.2 [<A href="https://wg21.link/basic.start.static#2">basic.start.static</A>] paragraph 2 as follows:</P></LI>

<BLOCKQUOTE>

<I>Constant initialization</I>
is performed if a variable or
temporary object with static or thread storage duration is
<INS>constant-</INS>initialized <DEL>by a constant initializer</DEL>
(7.7 [<A href="https://wg21.link/expr.const">expr.const</A>]) <DEL>for the entity</DEL>. If constant
initialization is not performed, a variable with static
storage duration (6.8.6.2 [<A href="https://wg21.link/basic.stc.static">basic.stc.static</A>]) or thread
storage duration (6.8.6.3 [<A href="https://wg21.link/basic.stc.thread">basic.stc.thread</A>]) is
zero-initialized (9.5 [<A href="https://wg21.link/dcl.init">dcl.init</A>]). Together,
zero-initialization and constant initialization...

</BLOCKQUOTE>

<LI><P>Change 7.7 [<A href="https://wg21.link/expr.const#2">expr.const</A>] paragraph 2 as follows:</P></LI>

<BLOCKQUOTE>

<P>A <DEL><I>constant initializer</I> for a</DEL> variable or
temporary object <TT>o</TT> is <DEL>an initializer for which
interpreting its full-expression as
a <I>constant-expression</I> results in a constant
expression</DEL> <INS><I>constant-initialized</I> if</INS>
</P>

<UL>

<LI><P><INS>either it has an initializer or its
default-initialization results in some initialization being
performed, and</INS></P></LI>

<LI><P>
<INS>its initialization full-expression is a constant
expression when interpreted as
a <I>constant-expression</I></INS>, except that
if <TT>o</TT> is an object, <DEL>such an initializer</DEL>
<INS>the initialization full-expression</INS> may also
invoke constexpr constructors for <TT>o</TT> and its
subobjects even if those objects are of non-literal class
types. [<I>Note:</I> Such a class may have a non-trivial
destructor. Within this
evaluation, <TT>std::is_constant_evaluated()</TT>
(21.3.12 [<A href="https://wg21.link/meta.const.eval">meta.const.eval</A>])
returns <TT>true</TT>. &#8212;<I>end note</I>]</P></LI>

</UL>

</BLOCKQUOTE>

<LI><P>Change 7.7 [<A href="https://wg21.link/expr.const#3">expr.const</A>] paragraph 3 as follows:</P></LI>

<BLOCKQUOTE>

A variable is <I>usable in constant expressions</I> after
its initializing declaration is encountered if it is a
constexpr variable, or it is <INS>a constant-initialized
variable</INS> of reference type or of const-qualified
integral or enumeration type<DEL>, and its initializer is a
constant initializer</DEL>.

</BLOCKQUOTE>

<LI><P>Change 9.2.6 [<A href="https://wg21.link/dcl.constexpr#5">dcl.constexpr</A>] paragraph 5 as follows:</P></LI>

<BLOCKQUOTE>

For a constexpr function or constexpr constructor that is
neither defaulted nor a template, if no argument values
exist such that an invocation of the function or constructor
could be an evaluated subexpression of a core constant
expression (7.7 [<A href="https://wg21.link/expr.const">expr.const</A>]), or, for a
constructor, <DEL>a constant initializer for</DEL>
<INS>an evaluated subexpression of the initialization
full-expression of</INS>
some <INS>constant-initialized</INS> object
(6.10.3.2 [<A href="https://wg21.link/basic.start.static">basic.start.static</A>]), the program is ill-formed,
no diagnostic required. [<I>Example:</I>...

</BLOCKQUOTE>

</OL>

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