[except.spec] Exception specifications are function declarators

Author: AlisdairM

source/declarations.tex

@@ -4109,6 +4109,272 @@
 \end{footnote}
 \indextext{declarator!function|)}
 
+\rSec2[except.spec]{Exception specifications}%
+\indextext{exception specification|(}
+
+\pnum
+The predicate indicating whether a function cannot exit via an exception
+is called the \defn{exception specification} of the function.
+If the predicate is false,
+the function has a
+\indextext{exception specification!potentially-throwing}%
+\defnx{potentially-throwing exception specification}%
+{potentially-throwing!exception specification},
+otherwise it has a
+\indextext{exception specification!non-throwing}%
+\defn{non-throwing exception specification}.
+The exception specification is either defined implicitly,
+or defined explicitly
+by using a \grammarterm{noexcept-specifier}
+as a suffix of a function declarator\iref{dcl.fct}.
+
+\begin{bnf}
+\nontermdef{noexcept-specifier}\br
+    \keyword{noexcept} \terminal{(} constant-expression \terminal{)}\br
+    \keyword{noexcept}\br
+\end{bnf}
+
+\pnum
+\indextext{exception specification!noexcept!constant expression and}%
+In a \grammarterm{noexcept-specifier}, the \grammarterm{constant-expression},
+if supplied, shall be a contextually converted constant expression
+of type \keyword{bool}\iref{expr.const};
+that constant expression is the exception specification of
+the function type in which the \grammarterm{noexcept-specifier} appears.
+A \tcode{(} token that follows \keyword{noexcept} is part of the
+\grammarterm{noexcept-specifier} and does not commence an
+initializer\iref{dcl.init}.
+The \grammarterm{noexcept-specifier} \keyword{noexcept}
+without a \grammarterm{constant-expression}
+is
+equivalent to the \grammarterm{noexcept-specifier}
+\tcode{\keyword{noexcept}(\keyword{true})}.
+\begin{example}
+\begin{codeblock}
+void f() noexcept(sizeof(char[2])); // error: narrowing conversion of value 2 to type \keyword{bool}
+void g() noexcept(sizeof(char));    // OK, conversion of value 1 to type \keyword{bool} is non-narrowing
+\end{codeblock}
+\end{example}
+
+\pnum
+If a declaration of a function
+does not have a \grammarterm{noexcept-specifier},
+the declaration has a potentially throwing exception specification
+unless it is a destructor or a deallocation function
+or is defaulted on its first declaration,
+in which cases the exception specification
+is as specified below
+and no other declaration for that function
+shall have a \grammarterm{noexcept-specifier}.
+In an explicit instantiation\iref{temp.explicit}
+a \grammarterm{noexcept-specifier} may be specified,
+but is not required.
+If a \grammarterm{noexcept-specifier} is specified
+in an explicit instantiation,
+the exception specification shall be the same as
+the exception specification of all other declarations of that function.
+A diagnostic is required only if the
+exception specifications are not the same
+within a single translation unit.
+
+\pnum
+\indextext{exception specification!virtual function and}%
+If a virtual function has a
+non-throwing exception specification,
+all declarations, including the definition, of any function
+that overrides that virtual function in any derived class
+shall have a non-throwing
+exception specification,
+unless the overriding function is defined as deleted.
+\begin{example}
+\begin{codeblock}
+struct B {
+  virtual void f() noexcept;
+  virtual void g();
+  virtual void h() noexcept = delete;
+};
+
+struct D: B {
+  void f();                     // error
+  void g() noexcept;            // OK
+  void h() = delete;            // OK
+};
+\end{codeblock}
+
+The declaration of
+\tcode{D::f}
+is ill-formed because it
+has a potentially-throwing exception specification,
+whereas
+\tcode{B::f}
+has a non-throwing exception specification.
+\end{example}
+
+\pnum
+An expression $E$ is
+\defnx{potentially-throwing}{potentially-throwing!expression} if
+\begin{itemize}
+\item
+$E$ is a function call\iref{expr.call}
+whose \grammarterm{postfix-expression}
+has a function type,
+or a pointer-to-function type,
+with a potentially-throwing exception specification,
+or
+\item
+$E$ implicitly invokes a function
+(such as an overloaded operator,
+an allocation function in a \grammarterm{new-expression},
+a constructor for a function argument,
+or a destructor if $E$ is a full-expression\iref{intro.execution})
+that has a potentially-throwing exception specification,
+or
+\item
+$E$ is a \grammarterm{throw-expression}\iref{expr.throw},
+or
+\item
+$E$ is a \keyword{dynamic_cast} expression that casts to a reference type and
+requires a runtime check\iref{expr.dynamic.cast},
+or
+\item
+$E$ is a \keyword{typeid} expression applied to a
+(possibly parenthesized) built-in unary \tcode{*} operator
+applied to a pointer to a
+polymorphic class type\iref{expr.typeid},
+or
+\item
+any of the immediate subexpressions\iref{intro.execution}
+of $E$ is potentially-throwing.
+\end{itemize}
+
+\pnum
+An implicitly-declared constructor for a class \tcode{X},
+or a constructor without a \grammarterm{noexcept-specifier}
+that is defaulted on its first declaration,
+has a potentially-throwing exception specification
+if and only if
+any of the following constructs is potentially-throwing:
+\begin{itemize}
+\item
+the invocation of a constructor selected by overload resolution
+in the implicit definition of the constructor
+for class \tcode{X}
+to initialize a potentially constructed subobject, or
+\item
+a subexpression of such an initialization,
+such as a default argument expression, or,
+\item
+for a default constructor, a default member initializer.
+\end{itemize}
+\begin{note}
+Even though destructors for fully-constructed subobjects
+are invoked when an exception is thrown
+during the execution of a constructor\iref{except.ctor},
+their exception specifications do not contribute
+to the exception specification of the constructor,
+because an exception thrown from such a destructor
+would call the function \tcode{std::terminate}
+rather than escape the constructor\iref{except.throw,except.terminate}.
+\end{note}
+
+\pnum
+The exception specification for an implicitly-declared destructor,
+or a destructor without a \grammarterm{noexcept-specifier},
+is potentially-throwing if and only if
+any of the destructors
+for any of its potentially constructed subobjects
+has a potentially-throwing exception specification or
+the destructor is virtual and the destructor of any virtual base class
+has a potentially-throwing exception specification.
+
+\pnum
+The exception specification for an implicitly-declared assignment operator,
+or an assignment-operator without a \grammarterm{noexcept-specifier}
+that is defaulted on its first declaration,
+is potentially-throwing if and only if
+the invocation of any assignment operator
+in the implicit definition is potentially-throwing.
+
+\pnum
+A deallocation function\iref{basic.stc.dynamic.deallocation}
+with no explicit \grammarterm{noexcept-specifier}
+has a non-throwing exception specification.
+
+\pnum
+The exception specification for a comparison operator function\iref{over.binary}
+without a \grammarterm{noexcept-specifier}
+that is defaulted on its first declaration
+is potentially-throwing if and only if
+any expression
+in the implicit definition is potentially-throwing.
+
+\pnum
+\begin{example}
+\begin{codeblock}
+struct A {
+  A(int = (A(5), 0)) noexcept;
+  A(const A&) noexcept;
+  A(A&&) noexcept;
+  ~A();
+};
+struct B {
+  B() noexcept;
+  B(const B&) = default;        // implicit exception specification is \tcode{\keyword{noexcept}(\keyword{true})}
+  B(B&&, int = (throw 42, 0)) noexcept;
+  ~B() noexcept(false);
+};
+int n = 7;
+struct D : public A, public B {
+    int * p = new int[n];
+    // \tcode{D::D()} potentially-throwing, as the \keyword{new} operator may throw \tcode{bad_alloc} or \tcode{bad_array_new_length}
+    // \tcode{D::D(const D\&)} non-throwing
+    // \tcode{D::D(D\&\&)} potentially-throwing, as the default argument for \tcode{B}'s constructor may throw
+    // \tcode{D::\~D()} potentially-throwing
+};
+\end{codeblock}
+Furthermore, if
+\tcode{A::\~{}A()}
+were virtual,
+the program would be ill-formed since a function that overrides a virtual
+function from a base class
+shall not have a potentially-throwing exception specification
+if the base class function has a non-throwing exception specification.
+\end{example}
+
+\pnum
+An exception specification is considered to be \defnx{needed}{needed!exception specification} when:
+\begin{itemize}
+\item in an expression, the function is selected by
+overload resolution\iref{over.match,over.over};
+
+\item the function is odr-used\iref{term.odr.use} or, if it appears in an
+unevaluated operand, would be odr-used if the expression were
+potentially-evaluated;
+
+\item the exception specification is compared to that of another
+declaration (e.g., an explicit specialization or an overriding virtual
+function);
+
+\item the function is defined; or
+
+\item the exception specification is needed for a defaulted
+function that calls the function.
+\begin{note}
+A defaulted declaration does not require the
+exception specification of a base member function to be evaluated
+until the implicit exception specification of the derived
+function is needed, but an explicit \grammarterm{noexcept-specifier} needs
+the implicit exception specification to compare against.
+\end{note}
+\end{itemize}
+The exception specification of a defaulted
+function is evaluated as described above only when needed; similarly, the
+\grammarterm{noexcept-specifier} of a specialization of a function
+template or member function of a class template is instantiated only when
+needed.
+%
+\indextext{exception specification|)}
+
 \rSec3[dcl.fct.default]{Default arguments}%
 \indextext{declaration!default argument|(}