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 specificationotherwise 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 ([dcl.fct]).
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 ([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). The noexcept-specifier throw() is deprecated ([depr.except.spec]), 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 specfication is as specified below and no other declaration for that function shall have a noexcept-specifier. In an explicit instantiation ([temp.explicit]) 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 ([except.terminate]). [ 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 exception. — end example ]
An expression e is potentially-throwing if
e is a function call ([expr.call]) whose postfix-expression has a function type, or a pointer-to-function type, with a potentially-throwing exception specification, or
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 ([intro.execution])) that is potentially-throwing, or
e is a throw-expression ([expr.throw]), or
e is a dynamic_cast expression that casts to a reference type and requires a runtime check ([expr.dynamic.cast]), or
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
any of the immediate subexpressions ([intro.execution]) 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:
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 subojects 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 ([basic.stc.dynamic.deallocation]) 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 ([basic.def.odr]) 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.