The new deleted constructors added by P2166R1 are a breaking change,
making previously valid code ill-formed in C++23. As a result, they
should only be defined for C++23 and not for C++11 and up.
libstdc++-v3/ChangeLog:
PR libstdc++/104099
* include/bits/basic_string.h (basic_string(nullptr_t)): Only
define for C++23.
(operator=(nullptr_t)): Likewise.
* include/bits/cow_string.h: Likewise.
* include/std/string_view (basic_string_view(nullptr_t)):
Likewise.
* testsuite/21_strings/basic_string/cons/char/nullptr.cc: Adjust
expected error. Add examples that become ill-formed in C++23.
* testsuite/21_strings/basic_string_view/cons/char/nonnull.cc:
Adjust expected errors.
* testsuite/21_strings/basic_string_view/cons/wchar_t/nonnull.cc:
Likewise.
The C++17 basic_string(const T&, size_t, size_t) constructor is
overconstrained, so it can't be used for a NTBS and a temporary string
gets constructed (potentially allocating memory). There is no
corresponding constructor taking an NTBS, so no need to disambiguate
from it. Accepting an NTBS avoids the temporary (and potential
allocation) and is what the standard requires.
libstdc++-v3/ChangeLog:
PR libstdc++/103919
* include/bits/basic_string.h (basic_string(const T&, size_t, size_t)):
Relax constraints on string_view parameter.
* include/bits/cow_string.h (basic_string(const T&, size_t, size_t)):
Likewise.
* testsuite/21_strings/basic_string/cons/char/103919.cc: New test.
When non-const references, pointers or iterators are obtained to the
contents of a COW std::basic_string, the implementation has to assume it
could result in a write to the contents. If the string was previously
shared, it does the "copy-on-write" step of creating a new copy of the
data that is not shared by another object. It also marks the string as
"leaked", so that no future copies of it will share ownership either.
However, if the string is empty then the only character in the sequence
is the terminating null, and modifying that is undefined behaviour. This
means that non-const references/pointers/iterators to an empty string
are effectively const. Since no direct modification is possible, there
is no need to "leak" the string, it can be safely shared with other
objects. This avoids unnecessary allocations to create new copies of
empty strings that can't be modified anyway.
We already did this optimization for strings that share ownership of the
static _S_empty_rep() object, but not for strings that have non-zero
capacity, and not for fully-dynamic-strings (where the _S_empty_rep()
object is never used).
With this change we avoid two allocations in the return statement:
std::string s;
s.reserve(1); // allocate
std::string s2 = s;
std::string s3 = s;
return s[0] + s2[0] + s3[0]; // leak+allocate twice
libstdc++-v3/ChangeLog:
* include/bits/cow_string.h (basic_string::_M_leak_hard): Do not
reallocate an empty string.
The relaxed load is already optimal, checking the __single_threaded
global before doing a non-atomic load isn't an optimization.
libstdc++-v3/ChangeLog:
* include/bits/cow_string.h (basic_string::_M_is_leaked()):
Revert change to check __is_single_threaded() before using
atomic load.
This adds std::__is_constant_evaluated() as a C++11 wrapper for
__builtin_is_constant_evaluated, but just returning false if the
built-in isn't supported by the compiler. This allows us to use it
throughout the library without checking __has_builtin every time.
Some uses in std::vector and std::string can only be constexpr when the
std::is_constant_evaluated() function actually works, so we might as
well guard them with a relevant macro and call that function directly,
rather than the built-in or std::__is_constant_evaluated().
The remaining checks of the __cpp_lib_is_constant_evaluated macro could
now be replaced by checking __cplusplus >= 202002 instead, but there's
no practical difference. We still need some kind of preprocessor check
there anyway.
libstdc++-v3/ChangeLog:
* doc/doxygen/user.cfg.in (PREDEFINED): Change macro name.
* include/bits/allocator.h (allocate, deallocate): Use
std::__is_constant_evaluated() unconditionally, instead of
checking whether std::is_constant_evaluated() (or the built-in)
can be used.
* include/bits/basic_string.h: Check new macro. call
std::is_constant_evaluated() directly in C++20-only code that is
guarded by a suitable macro.
* include/bits/basic_string.tcc: Likewise.
* include/bits/c++config (__is_constant_evaluated): Define.
(_GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED): Replace with ...
(_GLIBCXX_HAVE_IS_CONSTANT_EVALUATED): New macro.
* include/bits/char_traits.h (char_traits): Replace conditional
calls to std::is_constant_evaluated with unconditional calls to
std::__is_constant_evaluated.
* include/bits/cow_string.h: Use new macro.
* include/bits/ranges_algobase.h (__copy_or_move): Replace
conditional calls to std::is_constant_evaluated with unconditional
calls to std::__is_constant_evaluated.
(__copy_or_move_backward, __fill_n_fn): Likewise.
* include/bits/ranges_cmp.h (ranges::less): Likewise.
* include/bits/stl_algobase.h (lexicographical_compare_three_way):
Likewise.
* include/bits/stl_bvector.h: Call std::is_constant_evaluated
directly in C++20-only code that is guarded by a suitable macro.
* include/bits/stl_construct.h (_Construct, _Destroy, _Destroy_n):
Replace is_constant_evaluated with __is_constant_evaluated.
* include/bits/stl_function.h (greater, less, greater_equal)
(less_equal): Replace __builtin_is_constant_evaluated and
__builtin_constant_p with __is_constant_evaluated.
* include/bits/stl_vector.h: Call std::is_constant_evaluated()
in C++20-only code.
* include/debug/helper_functions.h (__check_singular): Use
__is_constant_evaluated instead of built-in, or remove check
entirely.
* include/std/array (operator<=>): Use __is_constant_evaluated
unconditionally.
* include/std/bit (__bit_ceil): Likewise.
* include/std/type_traits (is_constant_evaluated): Define using
'if consteval' if possible.
* include/std/version: Use new macro.
* libsupc++/compare: Use __is_constant_evaluated instead of
__builtin_is_constant_evaluated.
* testsuite/23_containers/array/tuple_interface/get_neg.cc:
Adjust dg-error lines.
Most ref-count updates in the COW string are done via the functions in
<ext/atomicity.h>, which will use non-atomic ops when the program is
known to be single-threaded. The _M_is_leaked() and _M_is_shared()
functions use __atomic_load_n directly, because <ext/atomicity.h>
doesn't provide a load operation. Those functions can check the
__is_single_threaded() predicate to avoid using __atomic_load_n when not
needed.
The move constructor for the fully-dynamic-string increments the
ref-count by either 2 or 1, for leaked or non-leaked strings
respectively. That can be changed to use a non-atomic store of 1 for all
non-shared strings. It can be non-atomic because even if the program is
multi-threaded, conflicting access to the rvalue object while it's being
moved from would be data race anyway. It can store 1 directly for all
non-shared strings because it doesn't matter whether the initial
refcount was -1 or 0, it should be 1 after the move constructor creates
a second owner.
libstdc++-v3/ChangeLog:
* include/bits/cow_string.h (basic_string::_M_is_leaked): Use
non-atomic load when __is_single_threaded() is true.
(basic_string::_M_is_shared): Likewise.
(basic_string::(basic_string&&)) [_GLIBCXX_FULLY_DYNAMIC_STRING]:
Use non-atomic store when rvalue is not shared.
My last change to the fully-dynamic-string actually broke it. This fixes
the move constructor so it builds, and simplifies it slightly so that
more code is common between the fully-dynamic enabled/disabled cases.
libstdc++-v3/ChangeLog:
* include/bits/cow_string.h (basic_string(basic_string&&)): Fix
mem-initializer for _GLIBCXX_FULLY_DYNAMIC_STRING==0 case.
* testsuite/21_strings/basic_string/cons/char/noexcept_move_construct.cc:
Remove outdated comment.
* testsuite/21_strings/basic_string/cons/wchar_t/noexcept_move_construct.cc:
Likewise.
This is only supported for the cxx11 ABI, not for COW strings.
libstdc++-v3/ChangeLog:
* include/bits/basic_string.h (basic_string, operator""s): Add
constexpr for C++20.
(basic_string::basic_string(basic_string&&)): Only copy
initialized portion of the buffer.
(basic_string::basic_string(basic_string&&, const Alloc&)):
Likewise.
* include/bits/basic_string.tcc (basic_string): Add constexpr
for C++20.
(basic_string::swap(basic_string&)): Only copy initialized
portions of the buffers.
(basic_string::_M_replace): Add constexpr implementation that
doesn't depend on pointer comparisons.
* include/bits/cow_string.h: Adjust comment.
* include/ext/type_traits.h (__is_null_pointer): Add constexpr.
* include/std/string (erase, erase_if): Add constexpr.
* include/std/version (__cpp_lib_constexpr_string): Update
value.
* testsuite/21_strings/basic_string/cons/char/constexpr.cc:
New test.
* testsuite/21_strings/basic_string/cons/wchar_t/constexpr.cc:
New test.
* testsuite/21_strings/basic_string/literals/constexpr.cc:
New test.
* testsuite/21_strings/basic_string/modifiers/constexpr.cc: New test.
* testsuite/21_strings/basic_string/modifiers/swap/char/constexpr.cc:
New test.
* testsuite/21_strings/basic_string/modifiers/swap/wchar_t/constexpr.cc:
New test.
* testsuite/21_strings/basic_string/version.cc: New test.
Implement this C++23 feature. Because construction from a null pointer
is undefined, we can implement it for C++11 and up, turning undefined
behaviour into a compilation error.
libstdc++-v3/ChangeLog:
* include/bits/basic_string.h (basic_string(nullptr_t)): Define
as deleted.
(operator=(nullptr_t)): Likewise.
* include/bits/cow_string.h (basic_string(nullptr_t)): Likewise.
(operator=(nullptr_t)): Likewise.
* include/std/string_view (basic_string_view(nullptr_t)):
Likewise.
* testsuite/21_strings/basic_string/cons/char/nullptr.cc: New test.
* testsuite/21_strings/basic_string_view/cons/char/nonnull.cc:
Change dg-warning to dg-error.
* testsuite/21_strings/basic_string_view/cons/wchar_t/nonnull.cc:
Likewise.
The move constructor for the "fully-dynamic" COW string is not noexcept,
because it allocates a new empty string rep for the moved-from string.
However, there is no need to do that, because the moved-from string does
not have to be left empty. Instead, implement move construction for the
fully-dynamic case as a reference count increment, so the string is
shared.
Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:
* include/bits/cow_string.h [_GLIBCXX_FULLY_DYNAMIC_STRING]
(basic_string(basic_string&&)): Add noexcept and avoid
allocation, by sharing rep with the rvalue string.
The new contains member of the COW string is defined for non-strict
gnu++20 mode as well as for C++23 modes. I think that was left in the
committed patch unintentionally. It is inconsistent with the SSO string,
and doesn't actually compile because it uses the
basic_string_view::contains member which only defined for C++23.
This makes it only defined for C++23.
Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:
* include/bits/cow_string.h (basic_string::contains): Do not
define for -std=gnu++20.
This moves the definitions of the COW string to a separate file, so that
they don't need to be preprocessed for the common case. We could also
move the SSO string definitions to a new file, so that they don't need
to be preprocessed for the old ABI case, but that would require more
shovel work because there are some parts of <bits/basic_string.h> and
<bits/basic_string.tcc> that are common to both definitions.
libstdc++-v3/ChangeLog:
* include/Makefile.am: Add new header.
* include/Makefile.in: Regenerate.
* include/bits/basic_string.h [!_GLIBCXX_USE_CXX11_ABI]
(basic_string): Move definition of Copy-on-Write string to
new file.
* include/bits/basic_string.tcc: Likewise.
* include/bits/cow_string.h: New file.
