value initialization and array data member

value initialization and array data member

[Gabriel Dos Reis]

Hi Jason,

I'm looking into std::bitset with respect to constexpr issue.
My understanding has always been that one can use the
syntax `member()' to value-initialize an array data member
`member'.  However, std/biset uses the notation
`member({ })'.  Is that required?  Especially for a mem-initializer
in a constexpr constructor definition?

-- Gaby

Re: value initialization and array data member

[Jason Merrill]

On 11/05/2010 09:03 PM, Gabriel Dos Reis wrote:
> I'm looking into std::bitset with respect to constexpr issue.
> My understanding has always been that one can use the
> syntax `member()' to value-initialize an array data member
> `member'.

I believe so.

Jason

Re: value initialization and array data member

[Paolo Carlini]

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Hi,
> On 11/05/2010 09:03 PM, Gabriel Dos Reis wrote:
>> I'm looking into std::bitset with respect to constexpr issue.
>> My understanding has always been that one can use the
>> syntax `member()' to value-initialize an array data member
>> `member'.
> I believe so.
Thanks Gaby and Jason.

I wanted to apply the below patch, which seems correct to me in the
light of your exchange and triggered ICEs, for
23_containers/bitset/cons/constexpr.cc for example I got:

In file included from
/home/paolo/Gcc/svn-dirs/trunk/libstdc++-v3/testsuite/23_containers/bitset/cons/constexpr.cc:21:0:

/home/paolo/Gcc/svn-dirs/trunk-build/x86_64-unknown-linux-gnu/libstdc++-v3/include/bitset:
In constructor 'constexpr std::_Base_bitset<_Nw>::_Base_bitset() [with
long unsigned int _Nw = 4ul]':
/home/paolo/Gcc/svn-dirs/trunk-build/x86_64-unknown-linux-gnu/libstdc++-v3/include/bitset:814:7:  
instantiated from 'constexpr std::bitset<_Nb>::bitset() [with long
unsigned int _Nb = 256ul]'
/home/paolo/Gcc/svn-dirs/trunk/libstdc++-v3/testsuite/util/testsuite_common_types.h:627:20:  
instantiated from 'void
__gnu_test::constexpr_default_constructible::operator()()::_Concept::__constraint()
[with _Tp = std::bitset<256ul>]'
/home/paolo/Gcc/svn-dirs/trunk/libstdc++-v3/testsuite/util/testsuite_common_types.h:631:17:  
instantiated from 'void
__gnu_test::constexpr_default_constructible::operator()() [with _Tp =
std::bitset<256ul>]'
/home/paolo/Gcc/svn-dirs/trunk/libstdc++-v3/testsuite/23_containers/bitset/cons/constexpr.cc:29:38:  
instantiated from here
/home/paolo/Gcc/svn-dirs/trunk-build/x86_64-unknown-linux-gnu/libstdc++-v3/include/bitset:76:18:
internal compiler error: in build_data_member_initialization, at
cp/semantics.c:5499

Can you have a look?

Thanks,
Paolo.

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Index: include/std/bitset
===================================================================
--- include/std/bitset	(revision 166384)
+++ include/std/bitset	(working copy)
@@ -72,9 +72,10 @@
       /// 0 is the least significant word.
       _WordT 		_M_w[_Nw];
 
-#ifdef __GXX_EXPERIMENTAL_CXX0X__
-      constexpr _Base_bitset() : _M_w({ }) { }
+      _GLIBCXX_CONSTEXPR _Base_bitset()
+      : _M_w() { }
 
+#ifdef __GXX_EXPERIMENTAL_CXX0X__
       constexpr _Base_bitset(unsigned long long __val)
       : _M_w({ _WordT(__val)
 #if __SIZEOF_LONG_LONG__ > __SIZEOF_LONG__
@@ -82,14 +83,9 @@
 #endif
        }) { }
 #else
-      _Base_bitset()
-      { _M_do_reset(); }
-
       _Base_bitset(unsigned long __val)
-      {
-	_M_do_reset();
-	_M_w[0] = __val;
-      }
+      : _M_w()
+      { _M_w[0] = __val; }
 #endif
 
       static _GLIBCXX_CONSTEXPR size_t

Re: value initialization and array data member

[Paolo Carlini]

... oh well, this is enough:

struct A
{
  int arr[1];

  constexpr A()
  : arr() { }
};

Paolo.