This paper implement the changes from P2438R2 basic_string::substr() &&
paper into C++26. The additional substr and constructor overload are
implemented only for SSO string, as they require mutating the content
(if reused), and thus would require copy of the string anyway for COW
strings. (In consequence allocators implicitly constructible from int
remain ambiguous for C++11 ABI strings, see r16-7497-gfa1149534d8580).
In addition to cases when the allocators are not compatible (equal),
this patch does not reuse (transfer) allocator storage, if the selected
substring fits inside the SSO buffer, so we do not risk keeping large
chunk of memory for few characters. (This also covers cases when the
source stored the content in the local buffer).
As this additional overloads are meant to be optimization, in contrast
to move constructor, the source is left unmodified if the allocation
is not transferred. This avoid introducing a write (of null terminator)
to previously untouched, heap allocated, memory.
Separate overloads for substr(size_type __pos, size_type __n) and
substr(size_type __pos == 0), that delegate to corresponding constructor,
are provided to avoid the check of __n against the length() in the later
case.
Finally, the signatures of existing substr() overload are not modified
(no longer required since C++20), which avoid any impact on the ABI.
PR libstdc++/119745
libstdc++-v3/ChangeLog:
* include/bits/basic_string.h (basic_string::_M_construct)
[__cplusplus >= 202302L]: Declare.
(basic_string::basic_string(basic_string&&, size_type, const _Alloc&))
(basic_string(basic_string&&, size_type, size_type, const _Alloc&))
(basic_string::substr(size_type, size_type) &&)
(basic_string::substr(size_type) &&) [__cplusplus >= 202302L]: Define.
* include/bits/basic_string.tcc (basic_string::_M_construct)
[__cplusplus >= 202302L]: Define.
* testsuite/21_strings/basic_string/operations/substr/rvalue.cc: New test.
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Reviewed-by: Patrick Palka <ppalka@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>
716ec14c57