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%!TEX root = std.tex
\infannex{diff}{Compatibility}
\rSec1[diff.cpp20]{\Cpp{} and ISO \CppXX{}}
\rSec2[diff.cpp20.general]{General}
\pnum
\indextext{summary!compatibility with ISO \CppXX{}}%
Subclause \ref{diff.cpp20} lists the differences between \Cpp{} and
ISO \CppXX{} (ISO/IEC 14882:2020, \doccite{Programming Languages --- \Cpp{}}),
by the chapters of this document.
\rSec2[diff.cpp20.lex]{\ref{lex}: lexical conventions}
\diffref{lex.name}
\change
Previously valid identifiers containing characters
not present in UAX \#44 properties XID_Start or XID_Continue, or
not in Normalization Form C, are now rejected.
\rationale
Prevent confusing characters in identifiers.
Requiring normalization of names ensures consistent linker behavior.
\effect
Some identifiers are no longer well-formed.
\diffref{lex.string}
\change
Concatenated \grammarterm{string-literal}s can no longer have
conflicting \grammarterm{encoding-prefix}es.
\rationale
Removal of unimplemented conditionally-supported feature.
\effect
Concatenation of \grammarterm{string-literal}s
with different \grammarterm{encoding-prefix}es
is now ill-formed.
\begin{example}
\begin{codeblock}
auto c = L"a" U"b"; // was conditionally-supported; now ill-formed
\end{codeblock}
\end{example}
\rSec2[diff.cpp20.utilities]{\ref{utilities}: general utilities library}
\diffref{format}
\change
Signature changes: \tcode{format}, \tcode{format_to}, \tcode{vformat_to},
\tcode{format_to_n}, \tcode{formatted_size}.
Removal of \tcode{format_args_t}.
\rationale
Improve safety via compile-time format string checks,
avoid unnecessary template instantiations.
\effect
Valid \CppXX{} code that
contained errors in format strings or
relied on previous format string signatures or
\tcode{format_args_t} may become ill-formed.
For example:
\begin{codeblock}
auto s = std::format("{:d}", "I am not a number"); // ill-formed,
// previously threw \tcode{format_error}
\end{codeblock}
\rSec1[diff.cpp17]{\Cpp{} and ISO \CppXVII{}}
\rSec2[diff.cpp17.general]{General}
\pnum
\indextext{summary!compatibility with ISO \CppXVII{}}%
Subclause \ref{diff.cpp17} lists the differences between \Cpp{} and
ISO \CppXVII{} (ISO/IEC 14882:2017, \doccite{Programming Languages --- \Cpp{}}),
by the chapters of this document.
\rSec2[diff.cpp17.lex]{\ref{lex}: lexical conventions}
\diffref{lex.pptoken,module.unit,module.import,cpp.pre,cpp.module,cpp.import}
\change
New identifiers with special meaning.
\rationale
Required for new features.
\effect
Logical lines beginning with
\tcode{module} or \tcode{import} may
be interpreted differently
in this revision of \Cpp{}.
\begin{example}
\begin{codeblock}
class module {};
module m1; // was variable declaration; now \grammarterm{module-declaration}
module *m2; // variable declaration
class import {};
import j1; // was variable declaration; now \grammarterm{module-import-declaration}
::import j2; // variable declaration
\end{codeblock}
\end{example}
\diffref{lex.header}
\change
\grammarterm{header-name} tokens are formed in more contexts.
\rationale
Required for new features.
\effect
When the identifier \tcode{import}
is followed by a \tcode{<} character,
a \grammarterm{header-name} token may be formed.
\begin{example}
\begin{codeblock}
template<typename> class import {};
import<int> f(); // ill-formed; previously well-formed
::import<int> g(); // OK
\end{codeblock}
\end{example}
\diffref{lex.key}
\change
New keywords.
\rationale
Required for new features.
\begin{itemize}
\item
The \keyword{char8_t} keyword is added to differentiate
the types of ordinary and UTF-8 literals\iref{lex.string}.
\item
The \tcode{concept} keyword is
added to enable the definition of concepts\iref{temp.concept}.
\item
The \keyword{consteval} keyword is added to
declare immediate functions\iref{dcl.constexpr}.
\item
The \keyword{constinit} keyword is added to
prevent unintended dynamic initialization\iref{dcl.constinit}.
\item
The \tcode{co_await}, \tcode{co_yield}, and \tcode{co_return} keywords are added
to enable the definition of coroutines \iref{dcl.fct.def.coroutine}.
\item
The \tcode{requires} keyword is added
to introduce constraints through a \grammarterm{requires-clause}\iref{temp.pre}
or a \grammarterm{requires-expression}\iref{expr.prim.req}.
\end{itemize}
\effectafteritemize
Valid \CppXVII{} code using
\keyword{char8_t},
\tcode{concept},
\keyword{consteval},
\keyword{constinit},
\tcode{co_await}, \tcode{co_yield}, \tcode{co_return},
or \tcode{requires}
as an identifier is not valid in this revision of \Cpp{}.
\diffref{lex.operators}
\change
New operator \tcode{<=>}.
\rationale
Necessary for new functionality.
\effect
Valid \CppXVII{} code that contains a \tcode{<=} token
immediately followed by a \tcode{>} token
may be ill-formed or have different semantics in this revision of \Cpp{}:
\begin{codeblock}
namespace N {
struct X {};
bool operator<=(X, X);
template<bool(X, X)> struct Y {};
Y<operator<=> y; // ill-formed; previously well-formed
}
\end{codeblock}
\diffref{lex.literal}
\change
Type of UTF-8 string and character literals.
\rationale
Required for new features.
The changed types enable function overloading, template specialization, and
type deduction to distinguish ordinary and UTF-8 string and character literals.
\effect
Valid \CppXVII{} code that depends on
UTF-8 string literals having type ``array of \tcode{const char}'' and
UTF-8 character literals having type ``\tcode{char}''
is not valid in this revision of \Cpp{}.
\begin{codeblock}
const auto *u8s = u8"text"; // \tcode{u8s} previously deduced as \tcode{const char*}; now deduced as \tcode{const char8_t*}
const char *ps = u8s; // ill-formed; previously well-formed
auto u8c = u8'c'; // \tcode{u8c} previously deduced as \tcode{char}; now deduced as \keyword{char8_t}
char *pc = &u8c; // ill-formed; previously well-formed
std::string s = u8"text"; // ill-formed; previously well-formed
void f(const char *s);
f(u8"text"); // ill-formed; previously well-formed
template<typename> struct ct;
template<> struct ct<char> {
using type = char;
};
ct<decltype(u8'c')>::type x; // ill-formed; previously well-formed.
\end{codeblock}
\rSec2[diff.cpp17.basic]{\ref{basic}: basics}
\diffref{basic.life}
\change
A pseudo-destructor call ends the lifetime of
the object to which it is applied.
\rationale
Increase consistency of the language model.
\effect
Valid ISO \CppXVII{} code may be ill-formed or
have undefined behavior in this revision of \Cpp{}.
\begin{example}
\begin{codeblock}
int f() {
int a = 123;
using T = int;
a.~T();
return a; // undefined behavior; previously returned 123
}
\end{codeblock}
\end{example}
\diffref{intro.races}
\change
Except for the initial release operation,
a release sequence consists solely of atomic read-modify-write operations.
\rationale
Removal of rarely used and confusing feature.
\effect
If a \tcode{memory_order_release} atomic store is followed
by a \tcode{memory_order_relaxed} store to the same variable by the same thread,
then reading the latter value with a \tcode{memory_order_acquire} load
no longer provides any ``happens before'' guarantees,
even in the absence of intervening stores by another thread.
\rSec2[diff.cpp17.expr]{\ref{expr}: expressions}
\diffref{expr.prim.lambda.capture}
\change
Implicit lambda capture may capture additional entities.
\rationale
Rule simplification, necessary to resolve interactions with constexpr if.
\effect
Lambdas with a \grammarterm{capture-default}
may capture local entities
that were not captured in \CppXVII{}
if those entities are only referenced in contexts
that do not result in an odr-use.
\rSec2[diff.cpp17.dcl.dcl]{\ref{dcl.dcl}: declarations}
\diffref{dcl.typedef}
\change
Unnamed classes with a typedef name for linkage purposes
can contain only C-compatible constructs.
\rationale
Necessary for implementability.
\effect
Valid \CppXVII{} code may be ill-formed in this revision of \Cpp{}.
\begin{codeblock}
typedef struct {
void f() {} // ill-formed; previously well-formed
} S;
\end{codeblock}
\diffref{dcl.fct.default}
\change
A function cannot have different default arguments
in different translation units.
\rationale
Required for modules support.
\effect
Valid \CppXVII{} code may be ill-formed in this revision of \Cpp{},
with no diagnostic required.
\begin{codeblock}
// Translation unit 1
int f(int a = 42);
int g() { return f(); }
// Translation unit 2
int f(int a = 76) { return a; } // ill-formed, no diagnostic required; previously well-formed
int g();
int main() { return g(); } // used to return 42
\end{codeblock}
\diffref{dcl.init.aggr}
\change
A class that has user-declared constructors is never an aggregate.
\rationale
Remove potentially error-prone aggregate initialization
which may apply notwithstanding the declared constructors of a class.
\effect
Valid \CppXVII{} code that aggregate-initializes
a type with a user-declared constructor
may be ill-formed or have different semantics
in this revision of \Cpp{}.
\begin{codeblock}
struct A { // not an aggregate; previously an aggregate
A() = delete;
};
struct B { // not an aggregate; previously an aggregate
B() = default;
int i = 0;
};
struct C { // not an aggregate; previously an aggregate
C(C&&) = default;
int a, b;
};
A a{}; // ill-formed; previously well-formed
B b = {1}; // ill-formed; previously well-formed
auto* c = new C{2, 3}; // ill-formed; previously well-formed
struct Y;
struct X {
operator Y();
};
struct Y { // not an aggregate; previously an aggregate
Y(const Y&) = default;
X x;
};
Y y{X{}}; // copy constructor call; previously aggregate-initialization
\end{codeblock}
\diffref{dcl.init.list}
\change
Boolean conversion from a pointer or pointer-to-member type
is now a narrowing conversion.
\rationale
Catches bugs.
\effect
Valid \CppXVII{} code may fail to compile
in this revision of \Cpp{}. For example:
\begin{codeblock}
bool y[] = { "bc" }; // ill-formed; previously well-formed
\end{codeblock}
\rSec2[diff.cpp17.class]{\ref{class}: classes}
\diffref{class.ctor,class.conv.fct}
\change
The class name can no longer be used parenthesized
immediately after an \keyword{explicit} \grammarterm{decl-specifier}
in a constructor declaration.
The \grammarterm{conversion-function-id} can no longer be used parenthesized
immediately after an \keyword{explicit} \grammarterm{decl-specifier}
in a conversion function declaration.
\rationale
Necessary for new functionality.
\effect
Valid \CppXVII{} code may fail to compile
in this revision of \Cpp{}. For example:
\begin{codeblock}
struct S {
explicit (S)(const S&); // ill-formed; previously well-formed
explicit (operator int)(); // ill-formed; previously well-formed
explicit(true) (S)(int); // OK
};
\end{codeblock}
\diffref{class.ctor,class.dtor}
\change
A \grammarterm{simple-template-id}
is no longer valid as the \grammarterm{declarator-id} of a constructor or destructor.
\rationale
Remove potentially error-prone option for redundancy.
\effect
Valid \CppXVII{} code may fail to compile
in this revision of \Cpp{}. For example:
\begin{codeblock}
template<class T>
struct A {
A<T>(); // error: \grammarterm{simple-template-id} not allowed for constructor
A(int); // OK, injected-class-name used
~A<T>(); // error: \grammarterm{simple-template-id} not allowed for destructor
};
\end{codeblock}
\diffref{class.copy.elision}
\change
A function returning an implicitly movable entity
may invoke a constructor taking an rvalue reference to a type
different from that of the returned expression.
Function and catch-clause parameters can be thrown using move constructors.
\rationale
Side effect of making it easier to write
more efficient code that takes advantage of moves.
\effect
Valid \CppXVII{} code may fail to compile or have different semantics
in this revision of \Cpp{}.
For example:
\begin{codeblock}
struct base {
base();
base(base const &);
private:
base(base &&);
};
struct derived : base {};
base f(base b) {
throw b; // error: \tcode{base(base \&\&)} is private
derived d;
return d; // error: \tcode{base(base \&\&)} is private
}
struct S {
S(const char *s) : m(s) { }
S(const S&) = default;
S(S&& other) : m(other.m) { other.m = nullptr; }
const char * m;
};
S consume(S&& s) { return s; }
void g() {
S s("text");
consume(static_cast<S&&>(s));
char c = *s.m; // undefined behavior; previously ok
}
\end{codeblock}
\rSec2[diff.cpp17.over]{\ref{over}: overloading}
\diffref{over.match.oper}
\change
Equality and inequality expressions can now find
reversed and rewritten candidates.
\rationale
Improve consistency of equality with three-way comparison
and make it easier to write the full complement of equality operations.
\effect
Equality and inequality expressions between two objects of different types,
where one is convertible to the other,
could invoke a different operator.
Equality and inequality expressions between two objects of the same type
could become ambiguous.
\begin{codeblock}
struct A {
operator int() const;
};
bool operator==(A, int); // \#1
// \#2 is built-in candidate: \tcode{bool operator==(int, int);}
// \#3 is built-in candidate: \tcode{bool operator!=(int, int);}
int check(A x, A y) {
return (x == y) + // ill-formed; previously well-formed
(10 == x) + // calls \#1, previously selected \#2
(10 != x); // calls \#1, previously selected \#3
}
\end{codeblock}
\rSec2[diff.cpp17.temp]{\ref{temp}: templates}
\diffref{temp.names}
\change
An \grammarterm{unqualified-id}
that is followed by a \tcode{<}
and for which name lookup
finds nothing or finds a function
will be treated as a \grammarterm{template-name}
in order to potentially cause argument dependent lookup to be performed.
\rationale
It was problematic to call a function template
with an explicit template argument list
via argument dependent lookup
because of the need to have a template with the same name
visible via normal lookup.
\effect
Previously valid code that uses a function name
as the left operand of a \tcode{<} operator
would become ill-formed.
\begin{codeblock}
struct A {};
bool operator<(void (*fp)(), A);
void f() {}
int main() {
A a;
f < a; // ill-formed; previously well-formed
(f) < a; // still well formed
}
\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 \keyword{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
revision of \Cpp{}. It should simply be replaced with \keyword{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 revision of \Cpp{}
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:
\libheaderref{barrier},
\libheaderref{bit},
\libheaderref{charconv},
\libheaderref{compare},
\libheaderref{concepts},
\libheaderref{coroutine},
\libheaderref{format},
\libheaderref{latch},
\libheaderref{numbers},
\libheaderref{ranges},
\libheaderref{semaphore},
\libheaderrefx{source_location}{source.location.syn},
\libheaderref{span},
\libheaderrefx{stop_token}{thread.stoptoken.syn},
\libheaderref{syncstream}, and
\libheaderrefx{version}{support.limits.general}.
Valid \CppXVII{} code that \tcode{\#include}{s} headers with these names may be
invalid in this revision of \Cpp{}.
\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:
\libnoheader{ccomplex},
\libnoheader{ciso646},
\libnoheader{cstdalign},
\libnoheader{cstdbool}, and
\libnoheader{ctgmath}.
To retain the same behavior:
\begin{itemize}
\item
a \tcode{\#include} of \libnoheader{ccomplex} can be replaced by
a \tcode{\#include} of \libheaderref{complex},
\item
a \tcode{\#include} of \libnoheader{ctgmath} can be replaced by
a \tcode{\#include} of \libheaderref{cmath} and
a \tcode{\#include} of \libheader{complex},
and
\item
a \tcode{\#include} of
\libnoheader{ciso646},
\libnoheader{cstdalign}, or
\libnoheader{cstdbool}
can simply be removed.
\end{itemize}
\rSec2[diff.cpp17.containers]{\ref{containers}: containers library}
\diffref{forward.list,list}
\change
Return types of \tcode{remove}, \tcode{remove_if}, and \tcode{unique}
changed from \keyword{void} to \tcode{container::size_type}.
\rationale
Improve efficiency and convenience of finding number of removed elements.
\effect
Code that depends on the return types might have different semantics in this revision of \Cpp{}.
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.iterators]{\ref{iterators}: iterators library}
\diffref{iterator.traits}
\change
The specialization of \tcode{iterator_traits} for \tcode{void*} and
for function pointer types no longer contains any nested typedefs.
\rationale
Corrects an issue misidentifying pointer types that are not incrementable
as iterator types.
\effect
A valid \CppXVII{} program that relies on the presence of the typedefs
may fail to compile, or have different behavior.
\rSec2[diff.cpp17.alg.reqs]{\ref{algorithms}: algorithms library}
\diffref{algorithms.requirements}
\change
The number and order of deducible template parameters for algorithm declarations
is now unspecified, instead of being as-declared.
\rationale
Increase implementor freedom and allow some function templates
to be implemented as function objects with templated call operators.
\effect
A valid \CppXVII{} program that passes explicit template arguments to
algorithms not explicitly specified to allow such in this version of \Cpp{}
may fail to compile or have undefined behavior.
\rSec2[diff.cpp17.input.output]{\ref{input.output}: input/output library}
\diffref{istream.extractors}
\change
Character array extraction only takes array types.
\rationale
Increase safety via preventing buffer overflow at compile time.
\effect
Valid \CppXVII{} code may fail to compile in this revision of \Cpp{}:
\begin{codeblock}
auto p = new char[100];
char q[100];
std::cin >> std::setw(20) >> p; // ill-formed; previously well-formed
std::cin >> std::setw(20) >> q; // OK
\end{codeblock}
\diffref{ostream.inserters.character}
\change
Overload resolution for ostream inserters used with UTF-8 literals.
\rationale
Required for new features.
\effect
Valid \CppXVII{} code that passes UTF-8 literals to
\tcode{basic_ostream<char, ...>::operator<<} or
\tcode{basic_ostream<wchar_t, ...>::operator<<} is now ill-formed.
\begin{codeblock}
std::cout << u8"text"; // previously called \tcode{operator<<(const char*)} and printed a string;
// now ill-formed
std::cout << u8'X'; // previously called \tcode{operator<<(char)} and printed a character;
// now ill-formed
\end{codeblock}
\diffref{ostream.inserters.character}
\change
Overload resolution for ostream inserters
used with \keyword{wchar_t}, \keyword{char16_t}, or \keyword{char32_t} types.
\rationale
Removal of surprising behavior.
\effect
Valid \CppXVII{} code that passes
\keyword{wchar_t}, \keyword{char16_t}, or \keyword{char32_t} characters or strings
to \tcode{basic_ostream<char, ...>::operator<<} or
that passes \keyword{char16_t} or \keyword{char32_t} characters or strings
to \tcode{basic_ostream<wchar_t, ...>::operator<<} is now ill-formed.
\begin{codeblock}
std::cout << u"text"; // previously formatted the string as a pointer value;
// now ill-formed
std::cout << u'X'; // previously formatted the character as an integer value;
// now ill-formed
\end{codeblock}
\diffref{fs.class.path}
\change
Return type of filesystem path format observer member functions.
\rationale
Required for new features.
\effect
Valid \CppXVII{} code that depends on the \tcode{u8string()} and
\tcode{generic_u8string()} member functions of \tcode{std::filesystem::path}
returning \tcode{std::string} is not valid in this revision of \Cpp{}.
\begin{codeblock}
std::filesystem::path p;
std::string s1 = p.u8string(); // ill-formed; previously well-formed
std::string s2 = p.generic_u8string(); // ill-formed; previously well-formed
\end{codeblock}
\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 can 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 \keyword{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_storage_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 \keyword{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.cpp14]{\Cpp{} and ISO \CppXIV{}}
\rSec2[diff.cpp14.general]{General}
\pnum
\indextext{summary!compatibility with ISO \CppXIV{}}%
Subclause \ref{diff.cpp14} lists the differences between \Cpp{} and
ISO \CppXIV{} (ISO/IEC 14882:2014, \doccite{Programming Languages --- \Cpp{}}),
in addition to those listed above,
by the chapters of this document.
\rSec2[diff.cpp14.lex]{\ref{lex}: lexical conventions}
\diffref{lex.phases}
\indextext{trigraph sequence}%
\change
Removal of trigraph support as a required feature.
\rationale
Prevents accidental uses of trigraphs in non-raw string literals and comments.
\effect
Valid \CppXIV{} code that uses trigraphs may not be valid or may have different
semantics in this revision of \Cpp{}. Implementations may choose to
translate trigraphs as specified in \CppXIV{} if they appear outside of a raw
string literal, as part of the \impldef{mapping from physical source file characters
to basic source character set} mapping from physical source file characters to
the basic source character set.
\diffref{lex.ppnumber}
\change
\grammarterm{pp-number} can contain \tcode{p} \grammarterm{sign} and
\tcode{P} \grammarterm{sign}.
\rationale
Necessary to enable \grammarterm{hexadecimal-floating-point-literal}s.
\effect
Valid \CppXIV{} code may fail to compile or produce different results in
this revision of \Cpp{}. Specifically, character sequences like \tcode{0p+0}
and \tcode{0e1_p+0} are three separate tokens each in \CppXIV{}, but one single token
in this revision of \Cpp{}.
For example:
\begin{codeblock}
#define F(a) b ## a
int b0p = F(0p+0); // ill-formed; equivalent to ``\tcode{int b0p = b0p + 0;}\!'' in \CppXIV{}
\end{codeblock}
\rSec2[diff.cpp14.expr]{\ref{expr}: expressions}
\diffref{expr.post.incr,expr.pre.incr}
\change
Remove increment operator with \tcode{bool} operand.
\rationale
Obsolete feature with occasionally surprising semantics.
\effect
A valid \CppXIV{} expression utilizing the increment operator on
a \tcode{bool} lvalue is ill-formed in this revision of \Cpp{}.
Note that this might occur when the lvalue has a type given by a template
parameter.
\diffref{expr.new,expr.delete}
\change
Dynamic allocation mechanism for over-aligned types.
\rationale
Simplify use of over-aligned types.
\effect
In \CppXIV{} code that uses a \grammarterm{new-expression}
to allocate an object with an over-aligned class type,
where that class has no allocation functions of its own,
\tcode{::operator new(std::size_t)}
is used to allocate the memory.
In this revision of \Cpp{},
\tcode{::operator new(std::size_t, std::align_val_t)}
is used instead.
\rSec2[diff.cpp14.dcl.dcl]{\ref{dcl.dcl}: declarations}
\diffref{dcl.stc}
\indextext{\idxcode{register} storage class}%
\change
Removal of \keyword{register} \grammarterm{storage-class-specifier}.
\rationale
Enable repurposing of deprecated keyword in future revisions of \Cpp{}.
\effect
A valid \CppXIV{} declaration utilizing the \keyword{register}
\grammarterm{storage-class-specifier} is ill-formed in this revision of \Cpp{}.
The specifier can simply be removed to retain the original meaning.
\diffref{dcl.spec.auto}
\change
\keyword{auto} deduction from \grammarterm{braced-init-list}.
\rationale
More intuitive deduction behavior.
\effect
Valid \CppXIV{} code may fail to compile or may change meaning
in this revision of \Cpp{}. For example:
\begin{codeblock}
auto x1{1}; // was \tcode{std::initializer_list<int>}, now \tcode{int}
auto x2{1, 2}; // was \tcode{std::initializer_list<int>}, now ill-formed
\end{codeblock}
\diffref{dcl.fct}
\change
Make exception specifications be part of the type system.
\rationale
Improve type-safety.
\effect
Valid \CppXIV{} code may fail to compile or change meaning in this
revision of \Cpp{}.
For example:
\begin{codeblock}
void g1() noexcept;
void g2();
template<class T> int f(T *, T *);
int x = f(g1, g2); // ill-formed; previously well-formed
\end{codeblock}
\diffref{dcl.init.aggr}
\change
Definition of an aggregate is extended
to apply to user-defined types with base classes.
\rationale
To increase convenience of aggregate initialization.
\effect
Valid \CppXIV{} code may fail to compile or produce different results in this
revision of \Cpp{}; initialization from an empty initializer list will
perform aggregate initialization instead of invoking a default constructor
for the affected types.
For example:
\begin{codeblock}
struct derived;
struct base {
friend struct derived;
private:
base();
};
struct derived : base {};
derived d1{}; // error; the code was well-formed in \CppXIV{}
derived d2; // still OK
\end{codeblock}
\rSec2[diff.cpp14.class]{\ref{class}: classes}
\diffref{class.inhctor.init}
\change
Inheriting a constructor no longer injects a constructor into the derived class.
\rationale
Better interaction with other language features.
\effect
Valid \CppXIV{} code that uses inheriting constructors may not be valid
or may have different semantics. A \grammarterm{using-declaration}
that names a constructor now makes the corresponding base class constructors
visible to initializations of the derived class
rather than declaring additional derived class constructors.
\begin{codeblock}
struct A {
template<typename T> A(T, typename T::type = 0);
A(int);
};
struct B : A {
using A::A;
B(int);
};
B b(42L); // now calls \tcode{B(int)}, used to call \tcode{B<long>(long)},
// which called \tcode{A(int)} due to substitution failure
// in \tcode{A<long>(long)}.
\end{codeblock}
\rSec2[diff.cpp14.temp]{\ref{temp}: templates}
\diffref{temp.deduct.type}
\change
Allowance to deduce from the type of a non-type template argument.
\rationale
In combination with the ability to declare
non-type template arguments with placeholder types,
allows partial specializations to decompose
from the type deduced for the non-type template argument.
\effect
Valid \CppXIV{} code may fail to compile
or produce different results in this revision of \Cpp{}.
For example:
\begin{codeblock}
template <int N> struct A;
template <typename T, T N> int foo(A<N> *) = delete;
void foo(void *);
void bar(A<0> *p) {
foo(p); // ill-formed; previously well-formed
}
\end{codeblock}
\rSec2[diff.cpp14.except]{\ref{except}: exception handling}
\diffref{except.spec}
\change
Remove dynamic exception specifications.
\rationale
Dynamic exception specifications were a deprecated feature
that was complex and brittle in use.
They interacted badly with the type system,
which became a more significant issue in this revision of \Cpp{}
where (non-dynamic) exception specifications are part of the function type.
\effect
A valid \CppXIV{} function declaration,
member function declaration,
function pointer declaration,
or function reference declaration,
if it has a potentially throwing dynamic exception specification,
is rejected as ill-formed in this revision of \Cpp{}.
Violating a non-throwing dynamic exception specification
calls \tcode{terminate} rather than \tcode{unexpected},
and it is unspecified whether stack unwinding is performed
prior to such a call.
\rSec2[diff.cpp14.library]{\ref{library}: library introduction}
\diffref{headers}
\change
New headers.
\rationale
New functionality.
\effect
The following \Cpp{} headers are new:
\libheaderrefx{any}{any.synop},
\libheaderref{charconv},
\libheaderref{execution},
\libheaderrefx{filesystem}{fs.filesystem.syn},
\libheaderrefx{memory_resource}{mem.res.syn},
\libheaderref{optional},\\
\libheaderrefx{string_view}{string.view.synop},
and
\libheaderref{variant}.
Valid \CppXIV{} code that \tcode{\#include}{s} headers with these names may be
invalid in this revision of \Cpp{}.
\diffref{namespace.future}
\change
New reserved namespaces.