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libstdc++: Simplify std::array accessors [PR104719]

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This removes the __array_traits::_S_ref and __array_traits::_S_ptr
accessors, which only exist to make the special case of std::array<T, 0>
syntactically well-formed.

By changing the empty type used as the std::array<T, 0>::_M_elems data
member to support operator[] and conversion to a pointer, we can write
code using the natural syntax. The indirection through _S_ref and
_S_ptr is removed for the common case, and a function call is only used
for the special case of zero-size arrays.

The invalid member access for zero-sized arrays is changed to use
__builtin_trap() instead of a null dereference. This guarantees a
runtime error if it ever gets called, instead of undefined behaviour
that is likely to get optimized out as unreachable.

libstdc++-v3/ChangeLog:

	PR libstdc++/104719
	* include/std/array (__array_traits::_S_ref): Remove.
	(__array_traits::_S_ptr): Remove.
	(__array_traits<T, 0>::_Type): Define operator[] and operator T*
	to provide an array-like API.
	(array::_AT_Type): Remove public typeef.
	(array::operator[], array::at, array::front, array::back): Use
	index operator to access _M_elems instead of _S_ref.
	(array::data): Use implicit conversion from _M_elems to pointer.
	(swap(array&, array&)): Use __enable_if_t helper.
	(get<I>): Use index operator to access _M_elems.
	* testsuite/23_containers/array/tuple_interface/get_neg.cc:
	Adjust dg-error line numbers.
This commit is contained in:
Jonathan Wakely 2022-03-25 10:28:28 +00:00
parent 9c6a4beeed
commit ef8d5ac08b
2 changed files with 36 additions and 46 deletions
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76
libstdc++-v3/include/std/array
View File

@ -49,36 +49,31 @@ namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
template<typename _Tp, std::size_t _Nm>
template<typename _Tp, size_t _Nm>
struct __array_traits
{
typedef _Tp _Type[_Nm];
typedef __is_swappable<_Tp> _Is_swappable;
typedef __is_nothrow_swappable<_Tp> _Is_nothrow_swappable;
static constexpr _Tp&
_S_ref(const _Type& __t, std::size_t __n) noexcept
{ return const_cast<_Tp&>(__t[__n]); }
static constexpr _Tp*
_S_ptr(const _Type& __t) noexcept
{ return const_cast<_Tp*>(__t); }
using _Type = _Tp[_Nm];
using _Is_swappable = __is_swappable<_Tp>;
using _Is_nothrow_swappable = __is_nothrow_swappable<_Tp>;
};
template<typename _Tp>
struct __array_traits<_Tp, 0>
{
struct _Type { };
typedef true_type _Is_swappable;
typedef true_type _Is_nothrow_swappable;
// Empty type used instead of _Tp[0] for std::array<_Tp, 0>.
struct _Type
{
// Indexing is undefined.
__attribute__((__always_inline__,__artificial__,__noreturn__))
_Tp& operator[](size_t) const noexcept { __builtin_trap(); }
static constexpr _Tp&
_S_ref(const _Type&, std::size_t) noexcept
{ return *static_cast<_Tp*>(nullptr); }
// Conversion to a pointer produces a null pointer.
__attribute__((__always_inline__,__artificial__))
operator _Tp*() const noexcept { return nullptr; }
};
static constexpr _Tp*
_S_ptr(const _Type&) noexcept
{ return nullptr; }
using _Is_swappable = true_type;
using _Is_nothrow_swappable = true_type;
};
/**
@ -111,8 +106,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
typedef std::reverse_iterator<const_iterator> const_reverse_iterator;
// Support for zero-sized arrays mandatory.
typedef __array_traits<_Tp, _Nm> _AT_Type;
typename _AT_Type::_Type _M_elems;
typename __array_traits<_Tp, _Nm>::_Type _M_elems;
// No explicit construct/copy/destroy for aggregate type.
@ -123,7 +117,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_GLIBCXX20_CONSTEXPR void
swap(array& __other)
noexcept(_AT_Type::_Is_nothrow_swappable::value)
noexcept(__array_traits<_Tp, _Nm>::_Is_nothrow_swappable::value)
{ std::swap_ranges(begin(), end(), __other.begin()); }
// Iterators.
@ -206,7 +200,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
operator[](size_type __n) noexcept
{
__glibcxx_requires_subscript(__n);
return _AT_Type::_S_ref(_M_elems, __n);
return _M_elems[__n];
}
[[__nodiscard__]]
@ -216,7 +210,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
#if __cplusplus >= 201402L
__glibcxx_requires_subscript(__n);
#endif
return _AT_Type::_S_ref(_M_elems, __n);
return _M_elems[__n];
}
_GLIBCXX17_CONSTEXPR reference
@ -226,7 +220,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
std::__throw_out_of_range_fmt(__N("array::at: __n (which is %zu) "
">= _Nm (which is %zu)"),
__n, _Nm);
return _AT_Type::_S_ref(_M_elems, __n);
return _M_elems[__n];
}
constexpr const_reference
@ -234,11 +228,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{
// Result of conditional expression must be an lvalue so use
// boolean ? lvalue : (throw-expr, lvalue)
return __n < _Nm ? _AT_Type::_S_ref(_M_elems, __n)
return __n < _Nm ? _M_elems[__n]
: (std::__throw_out_of_range_fmt(__N("array::at: __n (which is %zu) "
">= _Nm (which is %zu)"),
__n, _Nm),
_AT_Type::_S_ref(_M_elems, 0));
_M_elems[__n]);
}
[[__nodiscard__]]
@ -246,7 +240,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
front() noexcept
{
__glibcxx_requires_nonempty();
return *begin();
return _M_elems[0];
}
[[__nodiscard__]]
@ -256,7 +250,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
#if __cplusplus >= 201402L
__glibcxx_requires_nonempty();
#endif
return _AT_Type::_S_ref(_M_elems, 0);
return _M_elems[0];
}
[[__nodiscard__]]
@ -264,7 +258,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
back() noexcept
{
__glibcxx_requires_nonempty();
return _Nm ? *(end() - 1) : *end();
return _M_elems[_Nm - 1];
}
[[__nodiscard__]]
@ -274,19 +268,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
#if __cplusplus >= 201402L
__glibcxx_requires_nonempty();
#endif
return _Nm ? _AT_Type::_S_ref(_M_elems, _Nm - 1)
: _AT_Type::_S_ref(_M_elems, 0);
return _M_elems[_Nm - 1];
}
[[__gnu__::__const__, __nodiscard__]]
_GLIBCXX17_CONSTEXPR pointer
data() noexcept
{ return _AT_Type::_S_ptr(_M_elems); }
{ return _M_elems; }
[[__nodiscard__]]
_GLIBCXX17_CONSTEXPR const_pointer
data() const noexcept
{ return _AT_Type::_S_ptr(_M_elems); }
{ return _M_elems; }
};
#if __cpp_deduction_guides >= 201606
@ -371,9 +364,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
inline
#if __cplusplus > 201402L || !defined(__STRICT_ANSI__) // c++1z or gnu++11
// Constrained free swap overload, see p0185r1
typename enable_if<
__array_traits<_Tp, _Nm>::_Is_swappable::value
>::type
__enable_if_t<__array_traits<_Tp, _Nm>::_Is_swappable::value>
#else
void
#endif
@ -383,8 +374,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
#if __cplusplus > 201402L || !defined(__STRICT_ANSI__) // c++1z or gnu++11
template<typename _Tp, std::size_t _Nm>
typename enable_if<
!__array_traits<_Tp, _Nm>::_Is_swappable::value>::type
__enable_if_t<!__array_traits<_Tp, _Nm>::_Is_swappable::value>
swap(array<_Tp, _Nm>&, array<_Tp, _Nm>&) = delete;
#endif
@ -394,7 +384,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
get(array<_Tp, _Nm>& __arr) noexcept
{
static_assert(_Int < _Nm, "array index is within bounds");
return __array_traits<_Tp, _Nm>::_S_ref(__arr._M_elems, _Int);
return __arr._M_elems[_Int];
}
template<std::size_t _Int, typename _Tp, std::size_t _Nm>
@ -412,7 +402,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
get(const array<_Tp, _Nm>& __arr) noexcept
{
static_assert(_Int < _Nm, "array index is within bounds");
return __array_traits<_Tp, _Nm>::_S_ref(__arr._M_elems, _Int);
return __arr._M_elems[_Int];
}
template<std::size_t _Int, typename _Tp, std::size_t _Nm>

6
libstdc++-v3/testsuite/23_containers/array/tuple_interface/get_neg.cc
View File

@ -26,6 +26,6 @@ int n1 = std::get<1>(a);
int n2 = std::get<1>(std::move(a));
int n3 = std::get<1>(ca);
// { dg-error "static assertion failed" "" { target *-*-* } 396 }
// { dg-error "static assertion failed" "" { target *-*-* } 405 }
// { dg-error "static assertion failed" "" { target *-*-* } 414 }
// { dg-error "static assertion failed" "" { target *-*-* } 386 }
// { dg-error "static assertion failed" "" { target *-*-* } 395 }
// { dg-error "static assertion failed" "" { target *-*-* } 404 }