P0619R4 Review of deprecated facilities

Author: AlisdairM

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
@@ -1186,7 +1182,7 @@
 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 \rationale Superseded by new feature; \tcode{unary_function} and
@@ -1987,6 +1983,20 @@
 }
 \end{codeblock}
 
+\rSec2[diff.cpp17.except]{\ref{except}: exception handling}
+
+\diffref{except.spec}
+\change Remove \tcode{throw()} exception specification.
+\rationale
+The empty dynamic exception specification was retained for one additional \Cpp{}
+Standard to ease the transition away from this feature.
+\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{}.
+
 \rSec2[diff.cpp17.library]{\ref{library}: library introduction}
 
 \diffref{headers}
@@ -2001,6 +2011,19 @@
 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>}.
+The \tcode{\#include} directives can simply be removed with no loss of meaning.
+
 \rSec2[diff.cpp17.containers]{\ref{containers}: containers library}
 
 \diffrefs{forwardlist}{list}
@@ -2013,6 +2036,83 @@
 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}
+
+\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.
+
+\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.
+
+\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.
+
+\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.
+
+\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.
+
+\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}%
@@ -2038,20 +2138,10 @@
 nor are the C 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}
 
@@ -2104,26 +2194,20 @@
 \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}%
 
 \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}.
 
 \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}.
 
 \rSec3[diff.null]{Macro \tcode{NULL}}
 

source/containers.tex

@@ -5628,6 +5628,7 @@
       friend class vector;
       reference() noexcept;
     public:
+      reference(const reference&) = default;
       ~reference();
       operator bool() const noexcept;
       reference& operator=(const bool x) noexcept;

source/exceptions.tex

@@ -705,7 +705,6 @@
 \nontermdef{noexcept-specifier}\br
     \terminal{noexcept} \terminal{(} constant-expression \terminal{)}\br
     \terminal{noexcept}\br
-    \terminal{throw} \terminal{(} \terminal{)}
 \end{bnf}
 
 \pnum
@@ -723,10 +722,6 @@
 is
 equivalent to the \grammarterm{noexcept-specifier}
 \tcode{noexcept(true)}.
-The \grammarterm{noexcept-specifier} \tcode{throw()}
-is deprecated\iref{depr.except.spec}, and
-equivalent to the \grammarterm{noexcept-specifier}
-\tcode{noexcept(true)}.
 
 \pnum
 If a declaration of a function
@@ -920,7 +915,7 @@
   ~A();
 };
 struct B {
-  B() throw();
+  B() noexcept;
   B(const B&) = default;        // implicit exception specification is \tcode{noexcept(true)}
   B(B&&, int = (throw Y(), 0)) noexcept;
   ~B() noexcept(false);