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[except.pre] Clean-up pass to ensure consistency
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‎source/basic.tex

+266-5
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@@ -6760,7 +6760,14 @@
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\indextext{atomic!operation|)}%
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\indextext{threads!multiple|)}
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\rSec2[except.throw]{Throwing an exception}%
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\rSec2[except.flow]{Exceptional control flow}
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\rSec3[except.general]{General}
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\pnum
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In this Clause, ``before'' and ``after'' refer to the
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``sequenced before'' relation\iref{intro.execution}.
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\rSec3[except.throw]{Throwing an exception}%
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\indextext{exception handling!throwing}%
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\indextext{throwing|see{exception handling, throwing}}
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@@ -6951,7 +6958,7 @@
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\tcode{std::terminate} is invoked.
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\end{note}
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\rSec2[except.ctor]{Stack unwinding}%
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\rSec3[except.ctor]{Stack unwinding}%
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\indextext{exception handling!constructors and destructors}%
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\indextext{constructor!exception handling|see{exception handling, constructors and destructors}}%
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\indextext{destructor!exception handling|see{exception handling, constructors and destructors}}
@@ -7066,6 +7073,260 @@
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if any, is called to free the storage occupied by the object.
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\end{note}
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\rSec3[except.handle]{Handling an exception}
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\indextext{exception handling!handler|(}%
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\pnum
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The
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\grammarterm{exception-declaration}
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in a
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\grammarterm{handler}
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describes the type(s) of exceptions that can cause
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that
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\grammarterm{handler}
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to be entered.
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\indextext{exception handling!handler!incomplete type in}%
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\indextext{exception handling!handler!rvalue reference in}%
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\indextext{exception handling!handler!array in}%
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\indextext{exception handling!handler!pointer to function in}%
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The
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\grammarterm{exception-declaration}
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shall not denote an incomplete type, an abstract class type, or an rvalue reference type.
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The
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\grammarterm{exception-declaration}
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shall not denote a pointer or reference to an
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incomplete type, other than ``pointer to \cv{}~\keyword{void}''.
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\pnum
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A handler of type
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\indextext{array!handler of type}%
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``array of \tcode{T}'' or
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\indextext{function!handler of type}%
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function type \tcode{T}
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is adjusted to be of type
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``pointer to \tcode{T}''.
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\pnum
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\indextext{exception handling!handler!match|(}%
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A
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\grammarterm{handler}
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is a match for
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an exception object
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of type
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\tcode{E}
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if
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\begin{itemize}
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\item%
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The \grammarterm{handler} is of type \cv{}~\tcode{T} or
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\cv{}~\tcode{T\&} and
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\tcode{E} and \tcode{T}
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are the same type (ignoring the top-level \grammarterm{cv-qualifier}{s}), or
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\item%
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the \grammarterm{handler} is of type \cv{}~\tcode{T} or
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\cv{}~\tcode{T\&} and
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\tcode{T} is an unambiguous public base class of \tcode{E}, or
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\item%
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the \grammarterm{handler} is of type \cv{}~\tcode{T} or \tcode{const T\&}
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where \tcode{T} is a pointer or pointer-to-member type and
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\tcode{E} is a pointer or pointer-to-member type
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that can be converted to \tcode{T} by one or more of
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\begin{itemize}
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\item%
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a standard pointer conversion\iref{conv.ptr} not involving conversions
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to pointers to private or protected or ambiguous classes
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\item%
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a function pointer conversion\iref{conv.fctptr}
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\item%
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a qualification conversion\iref{conv.qual}, or
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\end{itemize}
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\item
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the \grammarterm{handler} is of type \cv{}~\tcode{T} or \tcode{const T\&} where \tcode{T} is a pointer or pointer-to-member type and \tcode{E} is \tcode{std::nullptr_t}.
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\end{itemize}
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\begin{note}
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A
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\grammarterm{throw-expression}
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whose operand is an integer literal with value zero does not match a handler of
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pointer or pointer-to-member type.
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A handler of reference to array or function type
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is never a match for any exception object\iref{expr.throw}.
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\end{note}
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7159+
\begin{example}
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\begin{codeblock}
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class Matherr { @\commentellip@ virtual void vf(); };
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class Overflow: public Matherr { @\commentellip@ };
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class Underflow: public Matherr { @\commentellip@ };
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class Zerodivide: public Matherr { @\commentellip@ };
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void f() {
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try {
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g();
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} catch (Overflow oo) {
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// ...
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} catch (Matherr mm) {
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// ...
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}
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}
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\end{codeblock}
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Here, the
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\tcode{Overflow}
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handler will catch exceptions of type
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\tcode{Overflow}
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and the
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\tcode{Matherr}
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handler will catch exceptions of type
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\tcode{Matherr}
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and of all types publicly derived from
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\tcode{Matherr}
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including exceptions of type
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\tcode{Underflow}
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and
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\tcode{Zerodivide}.
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\end{example}
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\pnum
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The handlers for a try block are tried in order of appearance.
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\begin{note}
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This makes it possible to write handlers that can never be
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executed, for example by placing a handler for a final derived class after
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a handler for a corresponding unambiguous public base class.
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\end{note}
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\pnum
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A
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\tcode{...}
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in a handler's
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\grammarterm{exception-declaration}
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specifies a match for any exception.
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If present, a
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\tcode{...}
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handler shall be the last handler for its try block.
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\pnum
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If no match is found among the handlers for a try block,
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the search for a matching
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handler continues in a dynamically surrounding try block
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of the same thread.
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\pnum
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\indextext{exception handling!terminate called@\tcode{terminate} called}%
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\indextext{\idxcode{terminate}!called}%
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If the search for a handler
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encounters the outermost block of a function with a
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non-throwing exception specification,
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the function \tcode{std::terminate}\iref{except.terminate} is invoked.
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\begin{note}
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An implementation is not permitted to reject an expression merely because, when
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executed, it throws or might
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throw an exception from a function with a non-throwing exception specification.
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\end{note}
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\begin{example}
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\begin{codeblock}
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extern void f(); // potentially-throwing
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7232+
void g() noexcept {
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f(); // valid, even if \tcode{f} throws
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throw 42; // valid, effectively a call to \tcode{std::terminate}
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}
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\end{codeblock}
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The call to
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\tcode{f}
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is well-formed despite the possibility for it to throw an exception.
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\end{example}
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\pnum
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If no matching handler is found,
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the function \tcode{std::terminate} is invoked;
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whether or not the stack is unwound before this invocation of
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\tcode{std::terminate} is
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\impldef{whether stack is unwound before invoking the function \tcode{std::terminate}
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when no matching handler is found}\iref{except.terminate}
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\pnum
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A handler is considered \defnx{active}{exception handling!handler!active} when
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initialization is complete for the parameter (if any) of the catch clause.
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\begin{note}
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The stack will have been unwound at that point.
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\end{note}
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Also, an implicit handler is considered active when
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the function \tcode{std::terminate}
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is entered due to a throw. A handler is no longer considered active when the
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catch clause exits.
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\pnum
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\indextext{currently handled exception|see{exception handling, currently handled exception}}%
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The exception with the most recently activated handler that is
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still active is called the
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\defnx{currently handled exception}{exception handling!currently handled exception}.
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\pnum
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Referring to any non-static member or base class of an object
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in the handler for a
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\grammarterm{function-try-block}
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of a constructor or destructor for that object results in undefined behavior.
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\pnum
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Exceptions thrown in destructors of objects with static storage duration or in
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constructors of objects associated with non-block variables with static storage duration are not caught by a
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\grammarterm{function-try-block}
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on
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the \tcode{main} function\iref{basic.start.main}.
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Exceptions thrown in destructors of objects with thread storage duration or in constructors of objects associated with non-block variables with thread storage duration are not caught by a
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\grammarterm{function-try-block}
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on the initial function of the thread.
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\pnum
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If a \keyword{return} statement\iref{stmt.return} appears in a handler of the
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\grammarterm{function-try-block}
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of a
7287+
constructor, the program is ill-formed.
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\pnum
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The currently handled exception
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is rethrown if control reaches the end of a handler of the
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\grammarterm{function-try-block}
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of a constructor or destructor.
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Otherwise, flowing off the end of
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the \grammarterm{compound-statement}
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of a \grammarterm{handler}
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of a \grammarterm{function-try-block}
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is equivalent to flowing off the end of
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the \grammarterm{compound-statement}
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of that function (see \ref{stmt.return}).
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\pnum
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The variable declared by the \grammarterm{exception-declaration}, of type
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\cv{}~\tcode{T} or \cv{}~\tcode{T\&}, is initialized from the exception object,
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of type \tcode{E}, as follows:
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\begin{itemize}
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\item
7308+
if \tcode{T} is a base class of \tcode{E},
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the variable is copy-initialized\iref{dcl.init}
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from an lvalue of type \tcode{T} designating the corresponding base class subobject
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of the exception object;
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\item otherwise, the variable is copy-initialized\iref{dcl.init}
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from an lvalue of type \tcode{E} designating the exception object.
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\end{itemize}
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7316+
The lifetime of the variable ends
7317+
when the handler exits, after the
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destruction of any objects with automatic storage duration initialized
7319+
within the handler.
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\pnum
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When the handler declares an object,
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any changes to that object will not affect the exception object.
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When the handler declares a reference to an object,
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any changes to the referenced object are changes to the
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exception object and will have effect should that object be rethrown.%
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\indextext{exception handling!handler!match|)}%
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\indextext{exception handling!handler|)}
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\rSec2[basic.start]{Start and termination}
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\rSec3[basic.start.main]{\tcode{main} function}
@@ -7603,9 +7864,9 @@
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\pnum
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\indextext{\idxcode{terminate}}%
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In the situation where no matching handler is found, it is
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\impldef{stack unwinding before invocation of \tcode{std::terminate}}
7607-
whether or not the stack is unwound
7608-
before \tcode{std::terminate} is invoked.
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\impldef{whether stack is unwound before invoking the function \tcode{std::terminate}
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when no matching handler is found}
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whether or not the stack is unwound before \tcode{std::terminate} is invoked.
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In the situation where the search for a handler\iref{except.handle} encounters the
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outermost block of a function
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with a non-throwing exception specification\iref{except.spec}, it is

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