17 Language support library [support]

17.9 Exception handling [support.exception]

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

17.9.1 Header <exception> synopsis [exception.syn]

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

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

  int uncaught_exceptions() noexcept;

  using exception_ptr = unspecified;

  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);
}

17.9.2 Class exception [exception]

namespace std {
  class exception {
  public:
    exception() noexcept;
    exception(const exception&) noexcept;
    exception& operator=(const exception&) noexcept;
    virtual ~exception();
    virtual const char* what() const noexcept;
  };
}
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The class exception defines the base class for the types of objects thrown as exceptions by C++ standard library components, and certain expressions, to report errors detected during program execution.
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Each standard library class T that derives from class exception has the following publicly accessible member functions, each of them having a non-throwing exception specification ([except.spec]):
  • (2.1)
    default constructor (unless the class synopsis shows other constructors)
  • (2.2)
    copy constructor
  • (2.3)
    copy assignment operator
The copy constructor and the copy assignment operator meet the following postcondition: If two objects lhs and rhs both have dynamic type T and lhs is a copy of rhs, then strcmp(lhs.what(), rhs.what()) is equal to 0.
The what() member function of each such T satisfies the constraints specified for exception​::​what() (see below).
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exception(const exception& rhs) noexcept; exception& operator=(const exception& rhs) noexcept;
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Postconditions: If *this and rhs both have dynamic type exception then the value of the expression strcmp(what(), rhs.what()) shall equal 0.
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virtual ~exception();
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Effects: Destroys an object of class exception.
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virtual const char* what() const noexcept;
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Returns: An implementation-defined ntbs.
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Remarks: The message may be a null-terminated multibyte string, suitable for conversion and display as a wstring ([string.classes], [locale.codecvt]).
The return value remains valid until the exception object from which it is obtained is destroyed or a non-const member function of the exception object is called.

17.9.3 Class bad_­exception [bad.exception]

namespace std {
  class bad_exception : public exception {
  public:
    // see [exception] for the specification of the special member functions
    const char* what() const noexcept override;
  };
}
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The class bad_­exception defines the type of the object referenced by the exception_­ptr returned from a call to current_­exception ([propagation]) when the currently active exception object fails to copy.
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const char* what() const noexcept override;
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Returns: An implementation-defined ntbs.

17.9.4 Abnormal termination [exception.terminate]

17.9.4.1 Type terminate_­handler [terminate.handler]

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using terminate_handler = void (*)();
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The type of a handler function to be called by std​::​terminate() when terminating exception processing.
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Required behavior: A terminate_­handler shall terminate execution of the program without returning to the caller.
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Default behavior: The implementation's default terminate_­handler calls abort().

17.9.4.2 set_­terminate [set.terminate]

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terminate_handler set_terminate(terminate_handler f) noexcept;
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Effects: Establishes the function designated by f as the current handler function for terminating exception processing.
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Remarks: It is unspecified whether a null pointer value designates the default terminate_­handler.
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Returns: The previous terminate_­handler.

17.9.4.3 get_­terminate [get.terminate]

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terminate_handler get_terminate() noexcept;
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Returns: The current terminate_­handler.
[ Note
:
This may be a null pointer value.
— end note
 ]

17.9.4.4 terminate [terminate]

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[[noreturn]] void terminate() noexcept;
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Remarks: Called by the implementation when exception handling must be abandoned for any of several reasons ([except.terminate]).
May also be called directly by the program.
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Effects: Calls a terminate_­handler function.
It is unspecified which terminate_­handler function will be called if an exception is active during a call to set_­terminate.
Otherwise calls the current terminate_­handler function.
[ Note
:
A default terminate_­handler is always considered a callable handler in this context.
— end note
 ]

17.9.5 uncaught_­exceptions [uncaught.exceptions]

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int uncaught_exceptions() noexcept;
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Returns: The number of uncaught exceptions.
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Remarks: When uncaught_­exceptions() > 0, throwing an exception can result in a call of the function std​::​terminate.

17.9.6 Exception propagation [propagation]

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using exception_ptr = unspecified;
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The type exception_­ptr can be used to refer to an exception object.
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exception_­ptr meets the requirements of Cpp17NullablePointer (Table 33).
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Two non-null values of type exception_­ptr are equivalent and compare equal if and only if they refer to the same exception.
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The default constructor of exception_­ptr produces the null value of the type.
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exception_­ptr shall not be implicitly convertible to any arithmetic, enumeration, or pointer type.
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[ Note
:
An implementation might use a reference-counted smart pointer as exception_­ptr.
— end note
 ]
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For purposes of determining the presence of a data race, operations on exception_­ptr objects shall access and modify only the exception_­ptr objects themselves and not the exceptions they refer to.
Use of rethrow_­exception on exception_­ptr objects that refer to the same exception object shall not introduce a data race.
[ Note
:
If rethrow_­exception rethrows the same exception object (rather than a copy), concurrent access to that rethrown exception object may introduce a data race.
Changes in the number of exception_­ptr objects that refer to a particular exception do not introduce a data race.
— end note
 ]
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exception_ptr current_exception() noexcept;
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Returns: An exception_­ptr object that refers to the currently handled exception or a copy of the currently handled exception, or a null exception_­ptr object if no exception is being handled.
The referenced object shall remain valid at least as long as there is an exception_­ptr object that refers to it.
If the function needs to allocate memory and the attempt fails, it returns an exception_­ptr object that refers to an instance of bad_­alloc.
It is unspecified whether the return values of two successive calls to current_­exception refer to the same exception object.
[ Note
:
That is, it is unspecified whether current_­exception creates a new copy each time it is called.
— end note
 ]
If the attempt to copy the current exception object throws an exception, the function returns an exception_­ptr object that refers to the thrown exception or, if this is not possible, to an instance of bad_­exception.
[ Note
:
The copy constructor of the thrown exception may also fail, so the implementation is allowed to substitute a bad_­exception object to avoid infinite recursion.
— end note
 ]
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[[noreturn]] void rethrow_exception(exception_ptr p);
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Preconditions: p is not a null pointer.
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Throws: The exception object to which p refers.
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template<class E> exception_ptr make_exception_ptr(E e) noexcept;
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Effects: Creates an exception_­ptr object that refers to a copy of e, as if: 
try {
  throw e;
} catch(...) {
  return current_exception();
}
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[ Note
:
This function is provided for convenience and efficiency reasons.
— end note
 ]

17.9.7 nested_­exception [except.nested]

namespace std {
  class nested_exception {
  public:
    nested_exception() noexcept;
    nested_exception(const nested_exception&) noexcept = default;
    nested_exception& operator=(const nested_exception&) noexcept = default;
    virtual ~nested_exception() = default;

    // access functions
    [[noreturn]] void rethrow_nested() const;
    exception_ptr nested_ptr() const noexcept;
  };

  template<class T> [[noreturn]] void throw_with_nested(T&& t);
  template<class E> void rethrow_if_nested(const E& e);
}
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The class nested_­exception is designed for use as a mixin through multiple inheritance.
It captures the currently handled exception and stores it for later use.
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[ Note
:
nested_­exception has a virtual destructor to make it a polymorphic class.
Its presence can be tested for with dynamic_­cast.
— end note
 ]
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nested_exception() noexcept;
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Effects: The constructor calls current_­exception() and stores the returned value.
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[[noreturn]] void rethrow_nested() const;
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Effects: If nested_­ptr() returns a null pointer, the function calls the function std​::​terminate.
Otherwise, it throws the stored exception captured by *this.
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exception_ptr nested_ptr() const noexcept;
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Returns: The stored exception captured by this nested_­exception object.
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template<class T> [[noreturn]] void throw_with_nested(T&& t);
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Let U be decay_­t<T>.
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Preconditions: U meets the Cpp17CopyConstructible requirements.
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Throws: If is_­class_­v<U> && !is_­final_­v<U> && !is_­base_­of_­v<nested_­exception, U> is true, an exception of unspecified type that is publicly derived from both U and nested_­exception and constructed from std​::​forward<T>(t), otherwise std​::​forward<T>(t).
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template<class E> void rethrow_if_nested(const E& e);
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Effects: If E is not a polymorphic class type, or if nested_­exception is an inaccessible or ambiguous base class of E, there is no effect.
Otherwise, performs: 
if (auto p = dynamic_cast<const nested_exception*>(addressof(e)))
  p->rethrow_nested();