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    CWG Issue 703</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="703"></A><H4>703.
  
Narrowing for literals that cannot be exactly represented
</H4>
<B>Section: </B>9.5.5&#160; [<A href="https://wg21.link/dcl.init.list">dcl.init.list</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD2
 &#160;&#160;&#160;

 <B>Submitter: </B>Jason Merrill
 &#160;&#160;&#160;

 <B>Date: </B>2 July, 2008<BR>


<P>[Voted into WP at October, 2009 meeting.]</P>



<P>Both of the following initializations are ill-formed because of
narrowing, although they were previously well-formed:</P>

<PRE>
    struct A { int i; } a = { 1.0 };
    struct B { float f; } b = { 1.1 };
</PRE>

<P>The first one doesn't seem like a big problem, as there probably
isn't much code that has this kind of aggregate initialization.  The
second might be of more concern, because <TT>1.1</TT> is not
representable in either <TT>float</TT> or <TT>double</TT>.  Is the
resulting loss of precision a kind of narrowing that we want to
diagnose?</P>

<P><B>Notes from the September, 2008 meeting:</B></P>

<P>The CWG agreed that the second initialization should not be a
narrowing error; furthermore, this exemption should apply not
only to literals but to any floating-point constant expression.
Instead of the current formulation, requiring exact bidirectional
convertibility, the Standard should only require that the
initializer value be within the representable range of the target
type.</P>

<P><B>Proposed resolution (July, 2009):</B></P>

<P>Change 9.5.5 [<A href="https://wg21.link/dcl.init.list#6">dcl.init.list</A>] paragraph 6 as follows:</P>

<BLOCKQUOTE>

<P>A <I>narrowing conversion</I> is an implicit conversion</P>

<UL>
<LI><P>from a floating-point type to an integer type, or</P></LI>

<LI><P>from <TT>long double</TT> to <TT>double</TT> or <TT>float</TT>,
or from <TT>double</TT> to <TT>float</TT>, except where the source is
a constant expression and the actual value after conversion <DEL>will
fit into the target type and will produce the original value when
converted back to the original type</DEL> <INS>is within the range of
values that can be represented (even if it cannot be represented
exactly)</INS>, or</P></LI>

<LI><P>...</P></LI>

</UL>

</BLOCKQUOTE>

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