http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53248
Bug #: 53248
Summary: std::array<T,0> doesn't work when T is not
default-constructible
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: dwalke...@yahoo.com
(I'm using GCC 4.7.0 from MacPorts on my Mac OS X 10.4.11/Tiger (32-bit
PowerPC).) Here's a reduced example of my code:
//==================================
#include <array>
#include <typeindex>
#include <typeinfo>
template < typename ...Types >
union super_union;
template < >
union super_union<>
{
static auto optioned_types() -> std::array<std::type_index, 0>
{ return std::array<std::type_index, 0>{ {} }; }
};
template < typename Head, typename ...Tail >
union super_union<Head, Tail...>
{
static
auto optioned_types() -> std::array<std::type_index, 1 + sizeof...(Tail)>
{
using std::type_index;
return { {type_index(typeid(Head)), type_index(typeid(Tail))...} };
}
Head data;
super_union<Tail...> rest;
};
int main() { return 0; }
//==================================
When I compile it, I get:
//==================================
[daryle]$ g++-mp-4.7 -std=c++11 test_array.cpp
test_array.cpp: In static member function 'static std::array<std::type_index,
0ul> super_union<>::optioned_types()':
test_array.cpp:12:49: error: could not convert '<brace-enclosed initializer
list>()' from '<brace-enclosed initializer list>' to
'std::array<std::type_index, 0ul>::value_type {aka std::type_index}'
//==================================
When I change the returned initialized value on line 12 to
"std::array<std::type_index, 0>{}", I get the same result. But when I change
it to "std::array<std::type_index, 0>()", I get:
//==================================
[daryle]$ g++-mp-4.7 -std=c++11 test_array.cpp
test_array.cpp: In static member function 'static std::array<std::type_index,
0ul> super_union<>::optioned_types()':
test_array.cpp:12:45: error: use of deleted function
'std::array<std::type_index, 0ul>::array()'
In file included from test_array.cpp:1:0:
/opt/local/include/gcc47/c++/array:61:12: note: 'std::array<std::type_index,
0ul>::array()' is implicitly deleted because the default definition would be
ill-formed:
/opt/local/include/gcc47/c++/array:61:12: error: no matching function for call
to 'std::type_index::type_index()'
/opt/local/include/gcc47/c++/array:61:12: note: candidates are:
In file included from test_array.cpp:2:0:
/opt/local/include/gcc47/c++/typeindex:51:5: note:
std::type_index::type_index(const std::type_info&)
/opt/local/include/gcc47/c++/typeindex:51:5: note: candidate expects 1
argument, 0 provided
/opt/local/include/gcc47/c++/typeindex:49:10: note: constexpr
std::type_index::type_index(const std::type_index&)
/opt/local/include/gcc47/c++/typeindex:49:10: note: candidate expects 1
argument, 0 provided
/opt/local/include/gcc47/c++/typeindex:49:10: note: constexpr
std::type_index::type_index(std::type_index&&)
/opt/local/include/gcc47/c++/typeindex:49:10: note: candidate expects 1
argument, 0 provided
//==================================
Looking at <array> on the web at your documentation pages, there is no
specialization of "std::array<T,N>" for "std::array<T,0>"; you just nudge the
element count to 1 when it would have been zero. That secret element still
needs to be specified, since std::type_index doesn't have a default
constructor, although no initializers are logically necessary for an empty
array (i.e. the work-around abstraction leaks).
The first way I see around it is to make an explicit specialization that's
data-less.
