18 Exception handling [except]

18.4 Exception specifications [except.spec]

The predicate indicating whether a function cannot exit via an exception is called the exception specification of the function.
If the predicate is false, the function has a potentially-throwing exception specification, otherwise it has a non-throwing exception specification.
The exception specification is either defined implicitly, or defined explicitly by using a noexcept-specifier as a suffix of a function declarator.
noexcept-specifier:
	noexcept ( constant-expression )
	noexcept
	throw ( )
In a noexcept-specifier, the constant-expression, if supplied, shall be a contextually converted constant expression of type bool; that constant expression is the exception specification of the function type in which the noexcept-specifier appears.
A ( token that follows noexcept is part of the noexcept-specifier and does not commence an initializer.
The noexcept-specifier noexcept without a constant-expression is equivalent to the noexcept-specifier noexcept(true).
The noexcept-specifier throw() is deprecated, and equivalent to the noexcept-specifier noexcept(true).
If a declaration of a function does not have a 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 noexcept-specifier.
In an explicit instantiation a noexcept-specifier may be specified, but is not required.
If a noexcept-specifier is specified in an explicit instantiation directive, 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.
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.
[Example
:
struct B {
  virtual void f() noexcept;
  virtual void g();
  virtual void h() noexcept = delete;
};

struct D: B {
  void f();                     // ill-formed
  void g() noexcept;            // OK
  void h() = delete;            // OK
};
The declaration of D​::​f is ill-formed because it has a potentially-throwing exception specification, whereas B​::​f has a non-throwing exception specification.
end example
]
Whenever an exception is thrown and the search for a handler ([except.handle]) encounters the outermost block of a function with a non-throwing exception specification, the function std​::​terminate() is called.
[Note
:
An implementation shall not reject an expression merely because, when executed, it throws or might throw an exception from a function with a non-throwing exception specification.
end note
]
[Example
:
extern void f();                // potentially-throwing

void g() noexcept {
  f();                          // valid, even if f throws
  throw 42;                     // valid, effectively a call to std​::​terminate
}
The call to f is well-formed even though, when called, f might throw an exception.
end example
]
An expression e is potentially-throwing if
An implicitly-declared constructor for a class X, or a constructor without a 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:
  • a constructor selected by overload resolution in the implicit definition of the constructor for class X to initialize a potentially constructed subobject, or
  • a subexpression of such an initialization, such as a default argument expression, or,
  • for a default constructor, a default member initializer.
[Note
:
Even though destructors for fully-constructed subobjects are invoked when an exception is thrown during the execution of a constructor ([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 std​::​terminate rather than escape the constructor ([except.throw], [except.terminate]).
end note
]
The exception specification for an implicitly-declared destructor, or a destructor without a noexcept-specifier, is potentially-throwing if and only if any of the destructors for any of its potentially constructed subobjects is potentially throwing.
The exception specification for an implicitly-declared assignment operator, or an assignment-operator without a 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.
A deallocation function with no explicit noexcept-specifier has a non-throwing exception specification.
[Example
:
struct A {
  A(int = (A(5), 0)) noexcept;
  A(const A&) noexcept;
  A(A&&) noexcept;
  ~A();
};
struct B {
  B() throw();
  B(const B&) = default;        // implicit exception specification is noexcept(true)
  B(B&&, int = (throw Y(), 0)) noexcept;
  ~B() noexcept(false);
};
int n = 7;
struct D : public A, public B {
    int * p = new int[n];
    // D​::​D() potentially-throwing, as the new operator may throw bad_­alloc or bad_­array_­new_­length
    // D​::​D(const D&) non-throwing
    // D​::​D(D&&) potentially-throwing, as the default argument for B's constructor may throw
    // D​::​~D() potentially-throwing
};
Furthermore, if 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
]
An exception specification is considered to be needed when:
  • in an expression, the function is the unique lookup result or the selected member of a set of overloaded functions ([basic.lookup], [over.match], [over.over]);
  • the function is odr-used or, if it appears in an unevaluated operand, would be odr-used if the expression were potentially-evaluated;
  • the exception specification is compared to that of another declaration (e.g., an explicit specialization or an overriding virtual function);
  • the function is defined; or
  • the exception specification is needed for a defaulted special member function that calls the function.
    [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 noexcept-specifier needs the implicit exception specification to compare against.
    end note
    ]
The exception specification of a defaulted special member function is evaluated as described above only when needed; similarly, the noexcept-specifier of a specialization of a function template or member function of a class template is instantiated only when needed.