P0898R3 Standard Library Concepts

source/compatibility.tex

@@ -1085,13 +1085,9 @@
 and
 \tcode{<unordered_set>}.
 In addition the following C compatibility headers are new:
-\tcode{<ccomplex>},
 \tcode{<cfenv>},
 \tcode{<cinttypes>},
-\tcode{<cstdalign>},
-\tcode{<cstdbool>},
 \tcode{<cstdint>},
-\tcode{<ctgmath>},
 and
 \tcode{<cuchar>}.
 Valid \CppIII{} code that \tcode{\#include}{s} headers with these names may be
@@ -1123,40 +1119,6 @@
 \rSec2[diff.cpp03.language.support]{\ref{language.support}:
 language support library}
 
-\diffref{new.delete.single}
-\change Linking \tcode{new} and \tcode{delete} operators.
-\rationale The two throwing single-object signatures of \tcode{operator new} and
-\tcode{operator delete} are now specified to form the base functionality for
-the other operators. This clarifies that replacing just these two signatures
-changes others, even if they are not explicitly changed.
-\effect
-Valid \CppIII{} code that replaces global \tcode{new} or \tcode{delete}
-operators may execute differently in this International Standard. For
-example, the following program should write \tcode{"custom deallocation"} twice,
-once for the single-object delete and once for the array delete.
-
-\begin{codeblock}
-#include <cstdio>
-#include <cstdlib>
-#include <new>
-
-void* operator new(std::size_t size) throw(std::bad_alloc) {
-  return std::malloc(size);
-}
-
-void operator delete(void* ptr) throw() {
-  std::puts("custom deallocation");
-  std::free(ptr);
-}
-
-int main() {
-  int* i = new int;
-  delete i;                     // single-object delete
-  int* a = new int[3];
-  delete [] a;                  // array delete
-}
-\end{codeblock}
-
 \diffref{new.delete.single}
 \change \tcode{operator new} may throw exceptions other than
 \tcode{std::bad_alloc}.
@@ -1165,6 +1127,10 @@
 Valid \CppIII{} code that assumes that global \tcode{operator new} only
 throws \tcode{std::bad_alloc} may execute differently in this International
 Standard.
+Valid \CppIII{} code that replaces the global replaceable \tcode{operator new}
+is ill-formed in this International Standard,
+because the exception specification of \tcode{throw(std::bad_alloc)}
+was removed.
 
 \rSec2[diff.cpp03.diagnostics]{\ref{diagnostics}: diagnostics library}
 
@@ -1186,7 +1152,7 @@
 have different runtime behavior.
 
 \diffref{refwrap}, \ref{arithmetic.operations}, \ref{comparisons},
-\ref{logical.operations}, \ref{bitwise.operations}, \ref{depr.negators}
+\ref{logical.operations}, \ref{bitwise.operations}
 \change Standard function object types no longer derived from
 \tcode{std::unary_function} or \tcode{std::binary_function}.
 \rationale Superseded by new feature; \tcode{unary_function} and
@@ -1800,6 +1766,7 @@
 
 \rSec2[diff.cpp14.depr]{\ref{depr}: compatibility features}
 
+\nodiffref
 \change
 The class templates
 \tcode{auto_ptr},
@@ -1818,6 +1785,7 @@
 \effect Valid \CppXIV{} code that uses these class templates
 and function templates may fail to compile in this International Standard.
 
+\nodiffref
 \change
 Remove old iostreams members [depr.ios.members].
 \rationale Redundant feature for compatibility with pre-standard code
@@ -1987,20 +1955,72 @@
 }
 \end{codeblock}
 
+\rSec2[diff.cpp17.except]{\ref{except}: exception handling}
+
+\diffref{except.spec}
+\change Remove \tcode{throw()} exception specification.
+\rationale
+Removal of obsolete feature that has been replaced by \tcode{noexcept}.
+\effect
+A valid \CppXVII{} function declaration, member function declaration, function
+pointer declaration, or function reference declaration that uses \tcode{throw()}
+for its exception specification will be rejected as ill-formed in this
+International Standard. It should simply be replaced with \tcode{noexcept} for no
+change of meaning since \CppXVII{}.
+\begin{note}
+There is no way to write a function declaration
+that is non-throwing in this International Standard
+and is also non-throwing in \CppIII{}
+except by using the preprocessor to generate
+a different token sequence in each case.
+\end{note}
+
 \rSec2[diff.cpp17.library]{\ref{library}: library introduction}
 
 \diffref{headers}
 \change New headers.
 \rationale New functionality.
 \effect
 The following \Cpp{} headers are new:
+\tcode{<bit>},
 \tcode{<compare>},
+\tcode{<concepts>},
+\tcode{<contract>},
 \tcode{<span>},
 \tcode{<syncstream>}, and
 \tcode{<version>}.
 Valid \CppXVII{} code that \tcode{\#include}{s} headers with these names may be
 invalid in this International Standard.
 
+\diffref{headers}
+\change Remove vacuous \Cpp{} header files.
+\rationale
+The empty headers implied a false requirement to achieve C compatibility with the \Cpp{} headers.
+\effect
+A valid \CppXVII{} program that \tcode{\#include}{s} any of the following headers may fail to compile:
+\tcode{<ccomplex>},
+\tcode{<ciso646>},
+\tcode{<cstdalign>},
+\tcode{<cstdbool>}, and
+\tcode{<ctgmath>}.
+To retain the same behavior:
+\begin{itemize}
+\item
+a \tcode{\#include} of \tcode{<ccomplex>} can be replaced by
+a \tcode{\#include} of \tcode{<complex>}\iref{complex.syn},
+\item
+a \tcode{\#include} of \tcode{<ctgmath>} can be replaced by
+a \tcode{\#include} of \tcode{<cmath>}\iref{cmath.syn} and
+a \tcode{\#include} of \tcode{<complex>},
+and
+\item
+a \tcode{\#include} of
+\tcode{<ciso646>},
+\tcode{<cstdalign>}, or
+\tcode{<cstdbool>}
+can simply be removed.
+\end{itemize}
+
 \rSec2[diff.cpp17.containers]{\ref{containers}: containers library}
 
 \diffrefs{forwardlist}{list}
@@ -2013,6 +2033,89 @@
 Translation units compiled against this version of \Cpp{} may be incompatible with
 translation units compiled against \CppXVII{}, either failing to link or having undefined behavior.
 
+\rSec2[diff.cpp17.depr]{\ref{depr}: compatibility features}
+
+\nodiffref
+\change Remove \tcode{uncaught_exception}.
+\rationale
+The function did not have a clear specification when multiple exceptions were
+active, and has been superseded by \tcode{uncaught_exceptions}.
+\effect
+A valid \CppXVII{} program that calls \tcode{std::uncaught_exception} may fail
+to compile. It might be revised to use \tcode{std::uncaught_exceptions} instead,
+for clear and portable semantics.
+
+\nodiffref
+\change Remove support for adaptable function API.
+\rationale
+The deprecated support relied on a limited convention that could not be
+extended to support the general case or new language features. It has been
+superseded by direct language support with \tcode{decltype}, and by the
+\tcode{std::bind} and \tcode{std::not_fn} function templates.
+\effect
+A valid \CppXVII{} program that relies on the presence of \tcode{result_type},
+\tcode{argument_type}, \tcode{first_argument_type}, or
+\tcode{second_argument_type} in a standard library class may fail to compile. A
+valid \CppXVII{} program that calls \tcode{not1} or \tcode{not2}, or uses the
+class templates \tcode{unary_negate} or \tcode{binary_negate}, may fail to
+compile.
+
+\nodiffref
+\change Remove redundant members from \tcode{std::allocator}.
+\rationale
+\tcode{std::allocator} was overspecified, encouraging direct usage in user containers
+rather than relying on \tcode{std::allocator_traits}, leading to poor containers.
+\effect
+A valid \CppXVII{} program that directly makes use of the \tcode{pointer},
+\tcode{const_pointer}, \tcode{reference}, \tcode{const_reference},
+\tcode{rebind}, \tcode{address}, \tcode{construct}, \tcode{destroy}, or
+\tcode{max_size} members of \tcode{std::allocator}, or that directly calls
+\tcode{allocate} with an additional hint argument, may fail to compile.
+
+\nodiffref
+\change Remove \tcode{raw_memory_iterator}.
+\rationale
+The iterator encouraged use of algorithms that might throw exceptions, but did
+not return the number of elements successfully constructed that might need to
+be destroyed in order to avoid leaks.
+\effect
+A valid \CppXVII{} program that uses this iterator class may fail to compile.
+
+\nodiffref
+\change Remove temporary buffers API.
+\rationale
+The temporary buffer facility was intended to provide an efficient optimization
+for small memory requests, but there is little evidence this was achieved in
+practice, while requiring the user to provide their own exception-safe wrappers
+to guard use of the facility in many cases.
+\effect
+A valid \CppXVII{} program that calls \tcode{get_temporary_buffer} or
+\tcode{return_temporary_buffer} may fail to compile.
+
+\nodiffref
+\change Remove \tcode{shared_ptr::unique}.
+\rationale
+The result of a call to this member function is not reliable in the presence of
+multiple threads and weak pointers. The member function \tcode{use_count} is
+similarly unreliable, but has a clearer contract in such cases, and remains
+available for well defined use in single-threaded cases.
+\effect
+A valid \CppXVII{} program that calls \tcode{unique} on a \tcode{shared_ptr}
+object may fail to compile.
+
+\diffref{depr.meta.types}
+\change Remove deprecated type traits.
+\rationale
+The traits had unreliable or awkward interfaces. The \tcode{is_literal_type}
+trait provided no way to detect which subset of constructors and member
+functions of a type were declared \tcode{constexpr}. The \tcode{result_of}
+trait had a surprising syntax that could not report the result of a regular
+function type. It has been superseded by the \tcode{invoke_result} trait.
+\effect
+A valid \CppXVII{} program that relies on the \tcode{is_literal_type} or
+\tcode{result_of} type traits, on the \tcode{is_literal_type_v} variable template,
+or on the \tcode{result_of_t} alias template may fail to compile.
+
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 \rSec1[diff.library]{C standard library}
 \indextext{library!C standard}%
@@ -2031,27 +2134,17 @@
 in~\ref{depr.c.headers}, but their use is deprecated in \Cpp{}.
 
 \pnum
-There are no \Cpp{} headers for the C headers
+There are no \Cpp{} headers for the C standard library's headers
 \tcode{<stdatomic.h>}\indextext{\idxhdr{stdatomic.h}!absence thereof},
 \tcode{<stdnoreturn.h>}\indextext{\idxhdr{stdnoreturn.h}!absence thereof},
 and \tcode{<threads.h>}\indextext{\idxhdr{threads.h}!absence thereof},
-nor are the C headers themselves part of \Cpp{}.
+nor are these headers from the C standard library headers themselves part of \Cpp{}.
 
 \pnum
-The \Cpp{} headers \tcode{<ccomplex>}\indexhdr{ccomplex}\iref{depr.ccomplex.syn}
-and \tcode{<ctgmath>}\indexhdr{ctgmath}\iref{depr.ctgmath.syn}, as well
-as their corresponding C headers \tcode{<complex.h>}\indexhdr{complex.h}
-and \tcode{<tgmath.h>}\indexhdr{tgmath.h}, do not contain any of the
-content from the C standard library and instead merely include other headers
-from the \Cpp{} standard library.
-
-\pnum
-The headers \tcode{<ciso646>}\indexhdr{ciso646},
-\tcode{<cstdalign>}\indexhdr{cstdalign}\iref{depr.cstdalign.syn},
-and \tcode{<cstdbool>}\indexhdr{cstdbool}\iref{depr.cstdbool.syn}
-are meaningless in \Cpp{}. Use of
-the \Cpp{} headers \tcode{<ccomplex>}, \tcode{<cstdalign>}, \tcode{<cstdbool>},
-and \tcode{<ctgmath>} is deprecated\iref{depr.c.headers}.
+The C headers \tcode{<complex.h>}\indexhdr{complex.h} and
+\tcode{<tgmath.h>}\indexhdr{tgmath.h} do not contain any of the content from
+the C standard library and instead merely include other headers from the \Cpp{}
+standard library.
 
 \rSec2[diff.mods.to.definitions]{Modifications to definitions}
 
@@ -2103,27 +2196,27 @@
 and
 \tcode{xor_eq}
 are keywords in this International
-Standard\iref{lex.key}.
-They do not appear as macro names defined in
-\tcode{<ciso646>}.
-\indexhdr{ciso646}%
+Standard\iref{lex.key},
+and are not introduced as macros
+by \tcode{<iso646.h>}\iref{depr.iso646.h.syn}.
 
 \rSec3[diff.header.stdalign.h]{Header \tcode{<stdalign.h>}}
 \indexhdr{stdalign.h}%
 
 \pnum
 The token \tcode{alignas} is a keyword in this International
-Standard\iref{lex.key}. It does not appear as a macro name defined
-in \tcode{<cstdalign>}\indexhdr{cstdalign}\iref{depr.cstdalign.syn}.
+Standard\iref{lex.key},
+and is not introduced as a macro
+by \tcode{<stdalign.h>}\iref{depr.stdalign.h.syn}.
 
 \rSec3[diff.header.stdbool.h]{Header \tcode{<stdbool.h>}}
 \indexhdr{stdbool.h}%
 
 \pnum
 The tokens \tcode{bool}, \tcode{true}, and \tcode{false}
-are keywords in this International Standard\iref{lex.key}.
-They do not appear as macro names defined in
-\tcode{<cstdbool>}\indexhdr{cstdbool}\iref{depr.cstdbool.syn}.
+are keywords in this International Standard\iref{lex.key},
+and are not introduced as macros
+by \tcode{<stdbool.h>}\iref{depr.stdbool.h.syn}.
 
 \rSec3[diff.null]{Macro \tcode{NULL}}