Doc. no. P0003R2
Date: 2016-03-20
Project: Programming Language C++
Audience: Core Working Group
Reply to: Alisdair Meredith <>

Removing Deprecated Exception Specifications from C++17

Table of Contents

Revision History

Revision 0

Original version of the paper for the 2015 pre-Kona mailing.

Revision 1

Revised the wording to modify the proposed working draft after the Kona 2015 meeting, N4567. This accommodates renumbering a few clauses, notably Annex D where several other features have already been removed, and allowing for exception specifications in the type system, and the rewording of inheriting constructors.

Revision 2

Changes for Jacksonville wording review:


Dynamic exception specifications were deprecated in C++11. This paper formally proposes removing the feature from C++17, while retaining the (still) deprecated throw() specification strictly as an alias for noexcept(true).

A Brief History of Exception Specifications

Exception specifications were added as part of the original design of the exception language feature. However, the experience of using them was less than desired, with the general consensus by the time the 1998 standard was published being, generally, to not use them. The standard library made the explicit choice to not use exception specifications apart from a handful of places where guaranteeing an empty (no-throwing) specification seemed useful. N741 gives a brief summary of the LWG thoughts at the time.

By the time C++11 was published, the feeling against exception specifications had grown, the limited real-world use generally reported negative user experience, and the language feature, renamed as dynamic exception specifications, was deprecated, (N3051). A new language feature, noexcept, was introduced to describe the important case of knowing when a function could guarantee to not throw any exceptions.

Looking ahead to C++17, there is a desire to incorporate exception specifications into the type system, P0012R1. This solves a number of awkward corners that arise from exception specifications not being part of the type of a function, but still does nothing for the case of deprecated dynamic exception specifications, so the actual language does not get simpler, and we must still document and teach the awkward corners that arise from dynamic exception specifications outside the type system.

Proposed Changes

The recommendation of this paper is to remove dynamic exception specifications from the language. However, the syntax of the throw() specification should be retained, but no longer as a dynamic exception specification. Its meaning should become strictly an alias for noexcept(true), and its usage should remain deprecated.

To minimize the impact on the current standard wording, the grammar term exception-specification is retained, although it has only one production and could be replaced entirely with noexcept-specification. Alternatively, the grammar term noexcept-specification could be retired.

All wording dealing with compatible exception specifications, that used to be described in terms of sets of permitted exceptions, has been redrafted in terms of potentially-throwing and non-throwing operations and exception specifications. Eliminating the set-algebra significantly simplifies the wording.

Motivation for Change

Dynamic exception specifications are a failed experiment, but this is not immediately clear to novices, especially where the "novice" is an experienced developer coming from other languages such as Java, where exception specifications may be more widely used. Their continuing presence, especially in an important part of the language model (exceptions are key to understanding constructors, destructors, and RAII) is an impediment that must be explained. It remains embarrassing to explain to new developers that there are features of the language that conforming compilers are required to support, yet should never be used.

Exception specifications in general occupy an awkward corner of the grammar, where they do not affect the type system, yet critically affect how a virtual function can be overridden, or which functions can bind to certain function pointer variables. As noted above, P0012R1 goes a long way to resolving that problem for noexcept exception specifications, which makes the hole left for dynamic exception specifications even more awkward and unusual for the next generation of C++ developers.

C++17 is on schedule to be a release with several breaking changes for old code, with the standard library removing auto_ptr, the classic C++98 function binders, and more. Similarly, it is expected that the core language will lose trigraphs, the register keyword, and the increment operator for bool. This would be a good time to make a clean break with long discouraged (and actively deprecated) parts of the language.

The proposed change would resolve core issue 596 as no longer relevant (NAD), and should simplify core issue 1351, although that is marked as a Defect Report that was not yet applied to the working paper the proposed wording below was drafted from.

Compatibility Concerns

There is certainly some body of existing code that has not heeded the existing best practice of discouraging dynamic exception specifications, and has not yet accounted for the feature being deprecated in C++11. It is not clear how much of such code would be expected to port unmodified into a C++17 (or beyond) world, and the change is relatively simple - just strike the (non-empty) dynamic exception specification to retain equivalent meaning. The key difference is that the unexpected handler will not now be called to translate unexpected exceptions. In rare cases, this would allow a new exception type to propagate, rather than calling terminate to abort. If enforcing that semantic is seen as important in production systems, there is a more intrusive workaround available:

void legacy() throw(something) try { // function body as before } catch(const something&) { throw; } catch(...) { terminate(); }

It is thought that the empty dynamic exception specification, throw(), was much more widely used in practice, often in the mistaken impression that compilers would use this information to optimize code generation where no exception could propagate, where in fact this is strictly a pessimization that forces stack unwinding to the function exit, and then calling the unexpected callback in a manner that is guaranteed to fail before the subsequent call to terminate. This paper proposes treating such (still deprecated) exception specifications as synonyms for noexcept(true), yielding the performance benefit many were originally expecting.

It should also be noted that at least one widely distributed compiler has still not implemented this feature in 2015, and at least one vendor has expressed a desire to never implement the deprecated feature (while that vendor has implemented the noexcept form of exception specification). Code on that platform would not be adversely impacted by the proposed removal, and portable code must always have allowed for the idiosyncrasies of this platform.

One remaining task is to survey popular open source libraries and see what level of usage, if any, remains in large, easily accessible codebases.


Evolution Working Group Review

The first version of this paper was presented to the Evolution Working Group at the Kona 2015 meeting, and was recommended to advance to Core 'as written'. Specifically, that means retaining the empty exception specification as an alternate spelling of noexcept(true):

12 13 6 0 0

Design Choices

By choosing to keep the throw() specification, we look to retain source compatibility with a large body of existing code that has used this specification to indicate code that should not throw. However, we also change the meaning of that specification to be exactly the same as a plain noexcept specification, which means that an implementation is no longer permitted to call unexpected when a violation is detected (which was required in all previous standards) and the previously mandated stack unwinding might no longer occur. This means that a conforming C++17 implementation is not permitted to simply retain the C++14 semantics while issuing an "extension" diagnostic.

Drafting Choices

As the author is not a regular drafter of Core wording, a quick rationale for some of the most obvious wording decisions seems helpful, especially when drawing attention to changes that were deliberately not made.

Where there is some question over these decisions, it would probably be more efficient to have direct feedback here, than sowing repeating detailed comments throughout the wording - at least for the first round of review.

Retain the grammar production exception-specification

To minimize the impact on the existing grammar and wording, the term exception-specification is retained at exactly the same place in the grammar. However, it loses its ability to describe anything other than a noexcept-specification. The empty throw specification becomes a (deprecated) synonym for noexcept(true), and the noexcept-specification production is retired.

An alternative formulation might have been to retire the original exception-specification itself, forcing us to find and address each existing use in the text - either to simply replace it with noexcept-specification, or rewrite that part of the document to no longer refer to exception specifications at all.

The author has gone with the first option as it will be a less disruptive set of changes, and the Core working group has the expertise to spot and remove any remaining redundancies as a later clean-up (if desired).

Simplify the specification from algebra on sets of types

The current text regarding exception specifications is written in terms of sets of types that might pass through the specification, including the fictitious notion of an "any" type. With the removal of dynamic exception specifications we can distill that down to a single bit. Functions (and expressions) either allow exceptions to propagate, or they do not.

With the removal of dynamic exception specifications, why not remove the text for unwinding when an specification is violated?

The simple answer is that would be a technical change that needs to be separately blessed by EWG. The wording demanding unwinding at a violated exception specification can be cleanly removed with the whole of sub-clause 15.5.2. However, the implementation-defined choice on whether to unwind through an exception-specification should remain, to support the variety of exception unwinders deployed in production compilers today.

Exception specification vs. exception-specification

During the wording review with the Core Working Group, it was confimed that we wanted to be much more precise in the usage of the similar looking terms, "exception specification" (in plain English), refers to the notion of an exception specification, which may be implicit, while the italic term exception-specification denotes exactly that grammar production and its usage as such in source files.

Considerations for Drafting Review

Does the exception specification predicate tie into the grammar?

There is some concern that the new 15.4p1 does not quite tie in the new definition for an exception specification predicate, to the grammar term that is intended to produce the value for that predicate.

Did not touch wording that refers to types in exception specifications

There are a few clauses that talk about types when used in exception specifications, such as describing the name look-up rules, or requirements for type-completeness. With the removal of dynamic-exception-specifications, the first attempt at drafting updated these clauses too. However, in most or all cases, the existing text remains valid, but more specific to exception specifications containing boolean constant expressions. The existing specification and notes in these paragraphs now refer only to the contents of such expressions.

For reference (and review by the diligent) the list of clauses affected by this decision to make no changes is:

Note that all cases above use the grammar term exception-specification rather than the plain english "exception specification". In the non-expert view of the author drafting the paper, that seems to be correct, but would definitely be worth double-checking by a Core-drafting expert.

Clean up definition of "immediate subexpression"

The notion of an "immediate subexpression" will be more accurately defined as part of the resolution for Core Issue 1343. If that issue is resolved at the same meeting that this paper is adopted, strike the two new paragraphs 15.4p7,8. The drafting below shows those paragraphs in a stike- through font, so that the wording remains available should issue 1343 not be ready to proceed at the same meeting.

Clean up C++03 Compatibility Annex

Several paragraphs in Annex C.2 refer to dynamic exception specifications in ways that made sense for C++14, but are either no longer relevant or no longer legal in C++17.

Interaction with issues

Proposed Wording

Frist, adopt the definition of immediate subexpression in clause 1.9 [intro.execution] from Core Issues 1343.

Replace the whole of clause 15.4 with:

15.4 Exception specifications [except.spec]

  1. 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 an exception-specification as a suffix of a function declaration's declarator (8.3.5).
  2.    exception-specification:
          noexcept ( constant-expression )
          throw ( )
  3. In an exception-specification, the constant-expression, if supplied, shall be a contextually converted constant expression of type bool (5.20 [expr.const]). A ( token that follows noexcept is part of the exception-specification and does not commence an initializer (8.5). The exception-specification noexcept without a constant-expression is equivalent to the exception-specification noexcept(true). The exception-specification throw() is deprecated (D.2see Annex D), and equivalent to the exception-specification noexcept(true).
  4. If a declaration of a function has an implicit exception specification, other declarations of the function shall not specify an exception-specification. In an explicit instantiation (14.7.2) an exception-specification may be specified, but is not required. If an exception-specification is specified in an explicit instantiation directive, it shall be a potentially-throwing exception specification only if the exception specifications of all other declarations of that function are potentially-throwing. A diagnostic is required only if the exception specifications are not the same within a single translation unit.
  5. 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:
  6.      struct B {
           virtual void f() noexcept;
           virtual void g();
           virtual void h() noexcept;
         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]

  7. Whenever an exception is thrown and the search for a handler (15.3) encounters the outermost block of a function with a non-throwing exception specification, the function std::terminate() is called (15.5.1).
  8. 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. [ 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 that g does not allow. — end example ]

  9. A subexpression e1 of an expression e is an immediate subexpression if there is no subexpression e2 of e such that e1 is a subexpression of e2.
  10. If e initializes an object of type D using an inherited constructor for a class of type B (12.6.3), the set of immediate subexpressions of e includes the implied constructor invocations for subobjects of D that are not subobjects of B (including default argument expressions used in such invocations) as selected by overload resolution, and the initialization of non-static data members from brace-or-equal-initializers (12.6.2).
  11. An expression e is potentially-throwing if

    1. e is function call whose postfix-expression (5.2.2) has a function type, or a pointer-to-function type, with a potentially-throwing exception specification, or
    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 (1.9)) that is potentially-throwing, or
    3. e is a throw-expression (5.17), or
    4. e is a dynamic_cast expression that casts to a reference type and requires a run-time check (5.2.7), or
    5. e is a typeid expression applied to a glvalue expression whose type is a polymorphic class type (5.2.8), or
    6. e is a new-expression with a non-constant expression in the noptr-new-declarator (5.3.4), or
    7. any of the immediate subexpressions (1.9) of e is potentially-throwing.
  12. An implicitly-declared constructor for a class X 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 direct base class or a non-static member, or, if X is not abstract, to initialize a virtual base class, or
    • a subexpression of such an initialization, such as a default argument expression, or
    • a brace-or-equal-initializer for a non-static data member.
  13. [ Note: Even though destructors for fully-constructed subobjects are invoked when an exception is thrown during the execution of a constructor (15.2), their exception specifications do not contribute to the exception specification of the constructor, because an exception thrown from such a destructor would terminate rather than escape the constructor (15.1, 15.5.1). — end note]
  14. [ Example:
       struct A {
         A(int = (A(5), 0)) noexcept;
         A(const A&) noexcept;
         A(A&&) noexcept;
       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 (std::nothrow) 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 exception specification of D::~D() would be potentially throwing and 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 ]

  15. [ Note: An instantiation of an inheriting constructor template has an implied exception specification as if it were a non-template inheriting constructor. — end note ]
  16. The exception specification for an implicitly-declared destructor is potentially-throwing if and only if any of the destructors for its direct or virtual base classes or non-variant non-static data members has a potentially-throwing exception specification.
  17. The exception specification for an implicitly-declared assignment operator f is potentially-throwing if and only if the implicit definition of f selects by overload resolution an assignment operator from a direct base class, or from a non-variant non-static data members, that is potentially-throwing.
  18. A function with an implied non-throwing exception specification, where the function’s type is declared to be T, is instead considered to be of type "noexcept T".
  19. A deallocation function ( with no explicit exception-specification has a non-throwing exception specification.
  20. An exception specification is considered to be needed when:
    1. — in an expression, the function is the unique lookup result or the selected member of a set of overloaded functions (3.4, 13.3, 13.4);
    2. — the function is odr-used (3.2) or, if it appears in an unevaluated operand, would be odr-used if the expression were potentially-evaluated;
    3. — the exception specification is compared to that of another declaration (e.g., an explicit specialization or an overriding virtual function);
    4. — the function is defined; or
    5. — 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 exception-specification 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 exception-specification of a specialization of a function template or member function of a class template is instantiated only when needed.

And make the following additional changes to the working draft:

4.12 Function pointer conversions [conv.fctptr]

1 A prvalue of type “pointer to noexcept function” can be converted to a prvalue of type “pointer to function”. The result is a pointer to the function. A prvalue of type “pointer to member of type noexcept function” can be converted to a prvalue of type “pointer to member of type function”. The result points to the member function. [ Example:

void (*p)() throw(int);
void (**pp)() noexcept = &p; // error: cannot convert to pointer to noexcept function

struct S { typedef void (*p)(); operator p(); };
void (*q)() noexcept = S(); // error: cannot convert to pointer to noexcept function
end example ]

5.3.7 noexcept operator [expr.unary.noexcept]

1 The noexcept operator determines whether the evaluation of its operand, which is an unevaluated operand (Clause 5), can throw an exception (15.1).

noexcept ( expression )

2 The result of the noexcept operator is a constant of type bool and is a prvalue.

3 The result of the noexcept operator is true unless the expression is potentially-throwing (15.4) if the set of potential exceptions of the expression (15.4) is empty, and false otherwise.

8.3.5 Functions [dcl.fct]

2 In a declaration T D where D has the form

D1 ( parameter-declaration-clause ) cv-qualifier-seqopt ref-qualifieropt exception-specificationopt attribute-specifier-seqopt trailing-return-type
and the type of the contained declarator-id in the declaration T D1 is “derived-declarator-type-list T”, T shall be the single type-specifier auto. The type of the declarator-id in D is “derived-declarator-type-list noexceptopt function of (parameter-declaration-clause) cv-qualifier-seqopt ref-qualifieropt returning U”, where U is the type specified by the trailing-return-type, and where the optional noexcept is present if and only if the exception-specificationexception specification (15.4) is non-throwing. The optional attribute-specifier-seq appertains to the function type.

8 The return type, the parameter-type-list, the ref-qualifier, the cv-qualifier-seq, and whether the function has a non-throwing exception-specificationexception specification, but not the default arguments (8.3.6) or the exception specification (15.4), are part of the function type. [Note: Function types are checked during the assignments and initializations of pointers to functions, references to functions, and pointers to member functions. — end note ]

8.4.2 Explicitly-defaulted functions [dcl.fct.def.default]

3 If a function that is explicitly defaulted is declared with an exception-specification that is not the same ascompatible (15.4) with the exception specification of the implicit declaration, then

(3.1) — if the function is explicitly defaulted on its first declaration, it is defined as deleted;

(3.2) — otherwise, the program is ill-formed.

4 [ Example:

   struct S {
     constexpr S() = default;            // ill-formed: implicit S() is not constexpr
     S(int a = 0) = default;             // ill-formed: default argument
     void operator=(const S&) = default; // ill-formed: non-matching return type
     ~S() noexcept(false)throw(int) = default;          // deleted: exception specification does not match
     int i;
     S(S&);                              // OK: private copy constructor
   S::S(S&) = default;                   // OK: defines copy constructor

end example ]

14.5.3 Variadic templates [temp.variadic]

(4.7) — In a dynamic-exception-specification (15.4); the pattern is a type-id.

15.3 Handling an exception [except.handle]

7 A handler is considered active when initialization is complete for the parameter (if any) of the catch clause. [ Note: The stack will have been unwound at that point. — end note ] Also, an implicit handler is considered active when std::terminate() or std::unexpected() is entered due to a throw. A handler is no longer considered active when the catch clause exits or when std::unexpected() exits after being entered due to a throw.

15.5 Special functions [except.special]

1 The functions std::terminate() (15.5.1) and std::unexpected() (15.5.2) areis used by the exception handling mechanism for coping with errors related to the exception handling mechanism itself. The function std::current_exception() (18.8.6) and the class std::nested_exception (18.8.7) can be used by a program to capture the currently handled exception.

15.5.1 The std::terminate() function [except.terminate]

1 In some situations exception handling must be abandoned for less subtle error handling techniques. [ Note: These situations are:

(1.1) — when the exception handling mechanism, after completing the initialization of the exception object but before activation of a handler for the exception (15.1), calls a function that exits via an exception, or

(1.2) — when the exception handling mechanism cannot find a handler for a thrown exception (15.3), or

(1.3) — when the search for a handler (15.3) encounters the outermost block of a function with a non-throwing exception specification noexcept-specification that does not allow the exception (15.4), or

(1.4) — when the destruction of an object during stack unwinding (15.2) terminates by throwing an exception, or

(1.5) — when initialization of a non-local variable with static or thread storage duration (3.6.3) exits via an exception, or

(1.6) — when destruction of an object with static or thread storage duration exits via an exception (3.6.4), or

(1.7) — when execution of a function registered with std::atexit or std::at_quick_exit exits via an exception (18.5), or

(1.8) — when a throw-expression (5.17) with no operand attempts to rethrow an exception and no exception is being handled (15.1), or

(1.9) — when std::unexpected exits via an exception of a type that is not allowed by the previously violated exception specification, and std::bad_exception is not included in that exception specification (15.5.2), or

(1.10) — when the implementation’s default unexpected exception handler is called (D.5.1), or

(1.11) — when the function std::nested_exception::rethrow_nested is called for an object that has captured no exception (18.8.7), or

(1.12) — when execution of the initial function of a thread exits via an exception (, or

(1.13) — when the destructor or the copy assignment operator is invoked on an object of type std::thread that refers to a joinable thread (,, or

(1.14) — when a call to a wait(), wait_until(), or wait_for() function on a condition variable (30.5.1, 30.5.2) fails to meet a postcondition.

end note ]

2 In such cases, std::terminate() is called (18.8.4). In the situation where no matching handler is found, it is implementation-defined whether or not the stack is unwound before std::terminate() is called. In the situation where the search for a handler (15.3) encounters the outermost block of a function with a non-throwing exception specification noexcept-specification that does not allow the exception (15.4), it is implementation-defined whether the stack is unwound, unwound partially, or not unwound at all before std::terminate() is called. In all other situations, the stack shall not be unwound before std::terminate() is called. An implementation is not permitted to finish stack unwinding prematurely based on a determination that the unwind process will eventually cause a call to std::terminate().

15.5.2 The std::unexpected() function [except.unexpected]

1 If a function with a dynamic-exception-specification exits via an exception of a type that is not allowed by its exception specification, the function std::unexpected() is called (D.5) immediately after completing the stack unwinding for the former function.

2 [ Note: By default, std::unexpected() calls std::terminate(), but a program can install its own handler function (D.5.2). In either case, the constraints in the following paragraph apply. — end note ]

3 The std::unexpected() function shall not return, but it can throw (or rethrow) an exception. If it throws a new exception which is allowed by the exception specification which previously was violated, then the search for another handler will continue at the call of the function whose exception specification was violated. If it exits via an exception of a type that the dynamic-exception-specification does not allow, then the following happens: If the dynamic-exception-specification does not include the class std::bad_exception (18.8.3) then the function std::terminate() is called, otherwise the thrown exception is replaced by an implementation-defined object of type std::bad_exception and the search for another handler will continue at the call of the function whose dynamic-exception-specification was violated.

4 [Note: Thus, a dynamic-exception-specification guarantees that a function exits only via an exception of one of the listed types. If the dynamic-exception-specification includes the type std::bad_exception then any exception type not on the list may be replaced by std::bad_exception within the function std::unexpected(). — end note] Zombie names [zombie.names]:

  1. In namespace std, the following names are reserved for previous standardization: auto_ptr, binary_function, bind1st, bind2nd, binder1st, binder2nd, const_mem_fun1_ref_t, const_mem_fun1_t, const_mem_fun_ref_t, const_mem_fun_t, get_unexpected, mem_fun1_ref_t, mem_fun1_t, mem_fun_ref, mem_fun_ref_t, mem_fun_t, mem_fun, pointer_to_binary_function, pointer_to_unary_function, ptr_fun, random_shuffle, set_unexpected, and unary_function, unexpected, and unexpected_handler. Handler functions [handler.functions]

1 The C++ standard library provides default versions of the following handler functions (Clause 18):

(1.1) — unexpected_handler

(1.2) — terminate_handler

2 A C++ program may install different handler functions during execution, by supplying a pointer to a function defined in the program or the library as an argument to (respectively):

(2.1) — set_new_handler

(2.2) — set_unexpected

(2.3) — set_terminate

See also: subclauses 18.6.3, Storage allocation errors, and 18.8, Exception handling.

3 A C++ program can get a pointer to the current handler function by calling the following functions:

(3.1) — get_new_handler

(3.2) — get_unexpected

(3.3) — get_terminate

4 Calling the set_* and get_* functions shall not incur a data race. A call to any of the set_* functions shall synchronize with subsequent calls to the same set_* function and to the corresponding get_* function. Other functions [res.on.functions]

(2.2) — for handler functions (,, D.5.1), if the installed handler function does not implement the semantics of the applicable Required behavior: paragraph. Restrictions on exception handling [res.on.exception.handling]

1 Any of the functions defined in the C++ standard library can report a failure by throwing an exception of a type described in its Throws: paragraph. An implementation may strengthen the exception specification for a non-virtual function by adding a non-throwing noexcept-specification.

2 A function may throw an object of a type not listed in its Throws clause if its type is derived from a type named in the Throws clause and would be caught by an exception handler for the base type.

3 Functions from the C standard library shall not throw exceptions188 except when such a function calls a program-supplied function that throws an exception.189

4 Destructor operations defined in the C++ standard library shall not throw exceptions. Every destructor in the C++ standard library shall behave as if it had a non-throwing exception specification. Any other functions defined in the C++ standard library that do not have an exception-specification may throw implementation-defined exceptions unless otherwise specified.190 An implementation may strengthen this implicit exception-specification by adding an explicit one.191

5 An implementation may strengthen the exception specification for a non-virtual function by adding a non-throwing exception specification.

191) That is, an implementation may provide an explicit exception-specification that defines the subset of "any" exceptions thrown by that function. This implies that the implementation may list implementation-defined types in such an exception-specification.

18.8 Exception Handling [support.exception]

1 The header <exception> defines several types and functions related to the handling of exceptions in a C++ program.

Header <exception> synopsis

namespace std {
  class exception;
  class bad_exception;
  class nested_exception;

  typedef void (*unexpected_handler)();
  unexpected_handler get_unexpected() noexcept;
  unexpected_handler set_unexpected(unexpected_handler f) noexcept;
  [[noreturn]] void unexpected();

  typedef void (*terminate_handler)();
  terminate_handler get_terminate() noexcept;
  terminate_handler set_terminate(terminate_handler f) noexcept;
  [[noreturn]] void terminate() noexcept;

  int uncaught_exceptions() noexcept;
  // D.9X, uncaught_exception (deprecated)
  bool uncaught_exception() noexcept;

  typedef unspecified exception_ptr;

  exception_ptr current_exception() noexcept;
  [[noreturn]] void rethrow_exception(exception_ptr p);
  template <class E> exception_ptr make_exception_ptr(E e) noexcept;
  template <class T> [[noreturn]] void throw_with_nested(T&& t);
  template <class E> void rethrow_if_nested(const E& e);

18.8.3 Class bad_exception [bad.exception]

1 The class bad_exception defines the type of the objects referenced by the exception_ptr returned from a call to current_exception (18.8.6 [propagation]) when the currently active exception object fails to copythrown as described in (15.5.2). Zero sized arrays []

4 Member function swap() shall have a non-throwing exception specificationnoexcept-specification which is equivalent to noexcept(true).

Annex B (informative) Implementation quantities [implimits]

(2.40) — Throw specifications on a single function declaration [256].

C.2.6 Clause 12: special member functions [diff.cpp03.special]

12.4 (destructors)

Change: User-declared destructors have an implicit exception specification.

Rationale: Clarification of destructor requirements.

Effect on original feature:Valid C++ 2003 code may execute differently in this International Standard. In particular, destructors that throw exceptions will call std::terminate() (without calling std::unexpected()) if their exception specification is non-throwingis noexcept or noexcept(true). For a throwing virtual destructor of a derived class, std::terminate() can be avoided only if the base class virtual destructor has an exception specification that is not noexcept and not noexcept(true).

C.4.x Clause 15: Exception handling [diff.cpp14.except]


Change: Remove dynamic exception specifications.

Rationale: Dynamic exception specifications were a deprecated feature that was complex and brittle in use. They interacted badly with the type system, which is a more significant issue in this International Standard where exception specifications become part of the function type.

Effect on original feature: A valid C++ 2014 function declaration, member-function-declaration, function-pointer-declaration, or function-reference declaration, which has a potentially throwing dynamic exception specification will be rejected as ill-formed in this International Standard. Violating a non-throwing dynamic exception specification will call terminate rather than unexpected and might not perform stack unwinding prior to such a call.

D.2 Dynamic exception specifications [depr.except.spec]

1 The exception-specification throw()use of dynamic-exception-specifications is deprecated.

D.5 Violating exception-specifications [exception.unexpected]

D.5.1 Type unexpected_handler [unexpected.handler]

typedef void (*unexpected_handler)();

1 The type of a handler function to be called by unexpected() when a function attempts to throw an exception not listed in its dynamic-exception-specification.

2 Required behavior: An unexpected_handler shall not return. See also 15.5.2.

3 Default behavior: The implementation's default unexpected_handler calls std::terminate().

D.5.2 set_unexpected [set.unexpected]

unexpected_handler set_unexpected(unexpected_handler f) noexcept;

1 Effects: Establishes the function designated by f as the current unexpected_handler.

2 Remark: It is unspecified whether a null pointer value designates the default unexpected_handler.

3 Returns: The previous unexpected_handler.

D.5.3 get_unexpected [get.unexpected]

unexpected_handler get_unexpected() noexcept;

1 Returns: The current unexpected_handler. [ Note: This may be a null pointer value. — end note ]

D.5.4 unexpected [unexpected]

[[noreturn]] void unexpected();

1 Remarks: Called by the implementation when a function exits via an exception not allowed by its exception-specification (15.5.2), in effect after evaluating the throw-expression (D.5.1). May also be called directly by the program.

2 Effects: Calls the current unexpected_handler function. [ Note: A default unexpected_handler is always considered a callable handler in this context. — end note ]


Thanks to Jens Maurer for the initial review, identifying a number of proposed edits that had unintended consequences.

VIII. References