14 Exception handling [except]

14.5 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

In a noexcept-specifier, the constant-expression, if supplied, shall be a contextually converted constant expression of type bool ([expr.const]); 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 ([dcl.init]).

The noexcept-specifier noexcept without a constant-expression is equivalent to the noexcept-specifier noexcept(true).

[Example 1: void f() noexcept(sizeof(char[2])); // error: narrowing conversion of value 2 to type bool void g() noexcept(sizeof(char)); // OK, conversion of value 1 to type bool is non-narrowing — end example]

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, 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 2: 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 };

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 invoked ([except.terminate]).

[Note 1:

An implementation is not permitted to 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 3: 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 despite the possibility for it to throw an exception.

— end example]

An expression E is potentially-throwing if

(6.1)
E is a function call whose postfix-expression has a function type, or a pointer-to-function type, with a potentially-throwing exception specification, or
(6.2)
E implicitly invokes a function (such as an overloaded operator, an allocation function in a new-expression, a constructor for a function argument, or a destructor if E is a full-expression) that has a potentially-throwing exception specification, or
(6.3)
E is a throw-expression ([expr.throw]), or
(6.4)
E is a dynamic_cast expression that casts to a reference type and requires a runtime check ([expr.dynamic.cast]), or
(6.5)
E is a typeid expression applied to a (possibly parenthesized) built-in unary * operator applied to a pointer to a polymorphic class type ([expr.typeid]), or
(6.6)
any of the immediate subexpressions of E is potentially-throwing.

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:

(7.1)
the invocation of a constructor selected by overload resolution in the implicit definition of the constructor for class X to initialize a potentially constructed subobject, or
(7.2)
a subexpression of such an initialization, such as a default argument expression, or,
(7.3)
for a default constructor, a default member initializer.

[Note 2:

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 the function 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 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.

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.

The exception specification for a comparison operator function ([over.binary]) without a 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.

[Example 4: 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 noexcept(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]; // 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:

(13.1)
in an expression, the function is selected by overload resolution ([over.match], [over.over]);
(13.2)
the function is odr-used ([basic.def.odr]) or, if it appears in an unevaluated operand, would be odr-used if the expression were potentially-evaluated;
(13.3)
the exception specification is compared to that of another declaration (e.g., an explicit specialization or an overriding virtual function);
(13.4)
the function is defined; or
(13.5)
the exception specification is needed for a defaulted function that calls the function.
[Note 3:
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 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.