18 - Language support library [lib.language.support]

-1- The rounding mode for floating point arithmetic is characterized by the values:

• round_indeterminate if the rounding style is indeterminable

• round_toward_zero if the rounding style is toward zero

• round_to_nearest if the rounding style is to the nearest representable value

• round_toward_infinity if the rounding style is toward infinity

• round_toward_neg_infinity if the rounding style is toward negative infinity

-1- The presence or absence of denormalization (variable number of exponent bits) is characterized by the values:

• denorm_indeterminate if it cannot be determined whether or not the type allows denormalized values

• denorm_absent if the type does not allow denormalized values

• denorm_present if the type does allow denormalized values

-1- All members shall be provided for all specializations. However, many values are only required to be meaningful under certain conditions (for example, epsilon() is only meaningful if is_integer is false). Any value that is not ``meaningful'' shall be set to 0 or false.

-2- [Example:

namespace std {
  template<> class numeric_limits<float> {
  public:
    static const bool is_specialized = true;

    inline static float min() throw() { return 1.17549435E-38F; }
    inline static float max() throw() { return 3.40282347E+38F; }

    static const int digits   = 24;
    static const int digits10 =  6;

    static const bool is_signed  = true;
    static const bool is_integer = false;
    static const bool is_exact   = false;

    static const int radix = 2;
    inline static float epsilon() throw()     { return 1.19209290E-07F; }
    inline static float round_error() throw() { return 0.5F; }

    static const int min_exponent   = -125;
    static const int min_exponent10 = - 37;
    static const int max_exponent   = +128;
    static const int max_exponent10 = + 38;

    static const bool has_infinity             = true;
    static const bool has_quiet_NaN            = true;
    static const bool has_signaling_NaN        = true;
    static const float_denorm_style has_denorm = denorm_absent;
    static const bool has_denorm_loss          = false;

    inline static float infinity()      throw() { return ...; }
    inline static float quiet_NaN()     throw() { return ...; }
    inline static float signaling_NaN() throw() { return ...; }
    inline static float denorm_min()    throw() { return min(); }

    static const bool is_iec559  = true;
    static const bool is_bounded = true;
    static const bool is_modulo  = false;
    static const bool traps      = true;
    static const bool tinyness_before = true;

    static const float_round_style round_style = round_to_nearest;
  };
}

-1- Header <climits> (Table ??):

-2- The contents are the same as the Standard C library header <limits.h>.

-3- Header <cfloat> (Table ??):

-4- The contents are the same as the Standard C library header <float.h>.

See also ISO C subclause 7.1.5, 5.2.4.2.2, 5.2.4.2.1.

18.3 - Start and termination [lib.support.start.term]

-1- Header <cstdlib> (partial), Table ??:

-2- The contents are the same as the Standard C library header <stdlib.h>, with the following changes:

abort(void)

-3- The function abort() has additional behavior in this International Standard:

• The program is terminated without executing destructors for objects of automatic or static storage duration and without calling the functions passed to atexit() (basic.start.term).

-4- Effects: The atexit() functions register the function pointed to by f, to be called without arguments at normal program termination.

-5- For the execution of a function registered with atexit(), if control leaves the function because it provides no handler for a thrown exception, terminate() is called (lib.terminate).

-6- Implementation Limits: The implementation shall support the registration of at least 32 functions.

-7- Returns: The atexit() function returns zero if the registration succeeds, nozero if it fails.

exit(int  status )

-8- The function exit() has additional behavior in this International Standard:

• First, objects with static storage duration are destroyed and functions registered by calling atexit are called. Objects with static storage duration are destroyed in the reverse order of the completion of their constructor. (Automatic objects are not destroyed as a result of calling exit().)*

  [Footnote: Objects with automatic storage duration are all destroyed in a program whose function main() contains no automatic objects and executes the call to exit(). Control can be transferred directly to such a main() by throwing an exception that is caught in main(). --- end foonote]

  Functions registered with atexit are called in the reverse order of their registration.*

  [Footnote: A function is called for every time it is registered. --- end foonote]

  A function registered with atexit before an object obj1 of static storage duration is initialized will not be called until obj1's destruction has completed. A function registered with atexit after an object obj2 of static storage duration is initialized will be called before obj2's destruction starts.

• Next, all open C streams (as mediated by the function signatures declared in <cstdio>) with unwritten buffered data are flushed, all open C streams are closed, and all files created by calling tmpfile() are removed.*

  [Footnote: Any C streams associated with cin, cout, etc (lib.iostream.objects) are flushed and closed when static objects are destroyed in the previous phase. The function tmpfile() is declared in <cstdio>. --- end foonote]

• Finally, control is returned to the host environment. If status is zero or EXIT_SUCCESS, an implementation-defined form of the status successful termination is returned. If status is EXIT_FAILURE, an implementation-defined form of the status unsuccessful termination is returned. Otherwise the status returned is implementation-defined.*

  [Footnote: The macros EXIT_FAILURE and EXIT_SUCCESS are defined in <cstdlib>. --- end foonote]

-9- The function exit() never returns to its caller.

See also subclauses basic.start, basic.start.term, ISO C subclause 7.10.4.

18.4 - Dynamic memory management [lib.support.dynamic]

-1- The header <new> defines several functions that manage the allocation of dynamic storage in a program. It also defines components for reporting storage management errors.

Header <new> synopsis

namespace std {
  class bad_alloc;
  struct nothrow_t {};
  extern const nothrow_t nothrow;
  typedef void (*new_handler)();
  new_handler set_new_handler(new_handler new_p) throw();
}

  void* operator new(std::size_t size) throw(std::bad_alloc);
  void* operator new(std::size_t size, const std::nothrow_t&) throw();
  void  operator delete(void* ptr) throw();
  void  operator delete(void* ptr, const std::nothrow_t&) throw();
  void* operator new[](std::size_t size) throw(std::bad_alloc);
  void* operator new[](std::size_t size, const std::nothrow_t&) throw();
  void  operator delete[](void* ptr) throw();
  void  operator delete[](void* ptr, const std::nothrow_t&) throw();

  void* operator new  (std::size_t size, void* ptr) throw();
  void* operator new[](std::size_t size, void* ptr) throw();
  void  operator delete  (void* ptr, void*) throw();
  void  operator delete[](void* ptr, void*) throw();

See also intro.memory, basic.stc.dynamic, expr.new, expr.delete, class.free, lib.memory.

18.4.1 - Storage allocation and deallocation [lib.new.delete]

-1- Effects: The allocation function (basic.stc.dynamic.allocation) called by a new-expression (expr.new) to allocate size bytes of storage suitably aligned to represent any object of that size.

-2- Replaceable: a C++ program may define a function with this function signature that displaces the default version defined by the C++ Standard library.

-3- Required behavior: Return a non-null pointer to suitably aligned storage (basic.stc.dynamic), or else throw a bad_alloc exception. This requirement is binding on a replacement version of this function.

-4- Default behavior:

• Executes a loop: Within the loop, the function first attempts to allocate the requested storage. Whether the attempt involves a call to the Standard C library function malloc is unspecified.

• Returns a pointer to the allocated storage if the attempt is successful. Otherwise, if the last argument to set_new_handler() was a null pointer, throw bad_alloc.

• Otherwise, the function calls the current new_handler (lib.new.handler). If the called function returns, the loop repeats.

• The loop terminates when an attempt to allocate the requested storage is successful or when a called new_handler function does not return.

-5- Effects: Same as above, except that it is called by a placement version of a new-expression when a C++ program prefers a null pointer result as an error indication, instead of a bad_alloc exception.

-6- Replaceable: a C++ program may define a function with this function signature that displaces the default version defined by the C++ Standard library.

-7- Required behavior: Return a non-null pointer to suitably aligned storage (basic.stc.dynamic), or else return a null pointer. This nothrow version of operator new returns a pointer obtained as if acquired from the ordinary version. This requirement is binding on a replacement version of this function.

-8- Default behavior:

• Executes a loop: Within the loop, the function first attempts to allocate the requested storage. Whether the attempt involves a call to the Standard C library function malloc is unspecified.

• Returns a pointer to the allocated storage if the attempt is successful. Otherwise, if the last argument to set_new_handler() was a null pointer, return a null pointer.

• Otherwise, the function calls the current new_handler (lib.new.handler). If the called function returns, the loop repeats.

• The loop terminates when an attempt to allocate the requested storage is successful or when a called new_handler function does not return. If the called new_handler function terminates by throwing a bad_alloc exception, the function returns a null pointer.

-9- [Example:

T* p1 = new T;                  //  throws  bad_alloc  if it fails
T* p2 = new(nothrow) T;         //  returns  0  if it fails

-10- Effects: The deallocation function (basic.stc.dynamic.deallocation) called by a delete-expression to render the value of ptr invalid.

-11- Replaceable: a C++ program may define a function with this function signature that displaces the default version defined by the C++ Standard library.

-12- Required behavior: accept a value of ptr that is null or that was returned by an earlier call to the default operator new(std::size_t) or operator new(std::size_t,const std::nothrow_t&).

-13- Default behavior:

• For a null value of ptr , do nothing.

• Any other value of ptr shall be a value returned earlier by a call to the default operator new, which was not invalidated by an intervening call to operator delete(void*) (lib.res.on.arguments). For such a non-null value of ptr , reclaims storage allocated by the earlier call to the default operator new.

-14- Notes: It is unspecified under what conditions part or all of such reclaimed storage is allocated by a subsequent call to operator new or any of calloc, malloc, or realloc, declared in <cstdlib>.

18.4.1.2 - Array forms [lib.new.delete.array]

-1- Effects: The allocation function (basic.stc.dynamic.allocation) called by the array form of a new-expression (expr.new) to allocate size bytes of storage suitably aligned to represent any array object of that size or smaller.*

[Footnote: It is not the direct responsibility of operator new[](std::size_t) or operator delete[](void*) to note the repetition count or element size of the array. Those operations are performed elsewhere in the array new and delete expressions. The array new expression, may, however, increase the size argument to operator new[](std::size_t) to obtain space to store supplemental information. --- end foonote]

-2- Replaceable: a C++ program can define a function with this function signature that displaces the default version defined by the C++ Standard library.

-3- Required behavior: Same as for operator new(std::size_t). This requirement is binding on a replacement version of this function.

-4- Default behavior: Returns operator new( size ).

void* operator new[](std::size_t size, const std::nothrow_t&) throw();

-5- Effects: Same as above, except that it is called by a placement version of a new-expression when a C++ program prefers a null pointer result as an error indication, instead of a bad_alloc exception.

-6- Replaceable: a C++ program can define a function with this function signature that displaces the default version defined by the C++ Standard library.

-7- Required behavior: Same as for operator new(std::size_t,const std::nothrow_t&). This nothrow version of operator new[] returns a pointer obtained as if acquired from the ordinary version.

-8- Default behavior: Returns operator new( size ,nothrow).

void operator delete[](void* ptr) throw();
void operator delete[](void* ptr, const std::nothrow_t&) throw();

-9- Effects: The deallocation function (basic.stc.dynamic.deallocation) called by the array form of a delete-expression to render the value of ptr invalid.

-10- Replaceable: a C++ program can define a function with this function signature that displaces the default version defined by the C++ Standard library.

-11- Required behavior: accept a value of ptr that is null or that was returned by an earlier call to operator new[](std::size_t) or operator new[](std::size_t,const std::nothrow_t&).

-12- Default behavior:

• For a null value of ptr , does nothing.

• Any other value of ptr shall be a value returned earlier by a call to the default operator new[](std::size_t).*

  [Footnote: The value must not have been invalidated by an intervening call to operator delete[](void*) (lib.res.on.arguments). --- end foonote]

  For such a non-null value of ptr , reclaims storage allocated by the earlier call to the default operator new[].

-13- It is unspecified under what conditions part or all of such reclaimed storage is allocated by a subsequent call to operator new or any of calloc, malloc, or realloc, declared in <cstdlib>.

18.4.1.3 - Placement forms [lib.new.delete.placement]

-1- These functions are reserved, a C++ program may not define functions that displace the versions in the Standard C++ library (lib.constraints).

void* operator new(std::size_t size, void* ptr) throw();

-2- Returns: ptr .

-3- Notes: Intentionally performs no other action.

-4- [Example: This can be useful for constructing an object at a known address:

char place[sizeof(Something)];
Something* p = new (place) Something();

-5- Returns: ptr .

-6- Notes: Intentionally performs no other action.

void operator delete(void* ptr, void*) throw();

-7- Effects: Intentionally performs no action.

-8- Notes: Default function called when any part of the initialization in a placement new expression that invokes the library's non-array placement operator new terminates by throwing an exception (expr.new).

void operator delete[](void* ptr, void*) throw();

-9- Effects: Intentionally performs no action.

-10- Notes: Default function called when any part of the initialization in a placement new expression that invokes the library's array placement operator new terminates by throwing an exception (expr.new).

18.4.2 - Storage allocation errors [lib.alloc.errors]

namespace std {
  class bad_alloc : public exception {
  public:
    bad_alloc() throw();
    bad_alloc(const bad_alloc&) throw();
    bad_alloc& operator=(const bad_alloc&) throw();
    virtual ~bad_alloc() throw();
    virtual const char* what() const throw();
  };
}

-1- The class bad_alloc defines the type of objects thrown as exceptions by the implementation to report a failure to allocate storage.

bad_alloc() throw();

-2- Effects: Constructs an object of class bad_alloc.

-3- Notes: The result of calling what() on the newly constructed object is implementation-defined.

    bad_alloc(const bad_alloc&) throw();
    bad_alloc& operator=(const bad_alloc&) throw();

-4- Effects: Copies an object of class bad_alloc.

virtual const char* what() const throw();

-5- Returns: An implementation-defined NTBS.

18.4.2.2 - Type new_handler [lib.new.handler]

-1- The type of a handler function to be called by operator new() or operator new[]() (lib.new.delete) when they cannot satisfy a request for additional storage.

-2- Required behavior: A new_handler shall perform one of the following:

• make more storage available for allocation and then return;

• throw an exception of type bad_alloc or a class derived from bad_alloc;

• call either abort() or exit();

-1- Effects: Establishes the function designated by new_p as the current new_handler .

-2- Returns: 0 on the first call, the previous new_handler on subsequent calls.

18.5 - Type identification [lib.support.rtti]

-1- The header <typeinfo> defines a type associated with type information generated by the implementation. It also defines two types for reporting dynamic type identification errors.

Header <typeinfo> synopsis

namespace std {
  class type_info;
  class bad_cast;
  class bad_typeid;
}

See also expr.dynamic.cast, expr.typeid.

18.5.1 - Class type_info [lib.type.info]

namespace std {
  class type_info {
  public:
    virtual ~type_info();
    bool operator==(const type_info&  rhs ) const;
    bool operator!=(const type_info&  rhs ) const;
    bool before(const type_info&  rhs ) const;
    const char* name() const;
  private:
    type_info(const type_info&  rhs );
    type_info& operator=(const type_info&  rhs );
  };
}

-1- The class type_info describes type information generated by the implementation. Objects of this class effectively store a pointer to a name for the type, and an encoded value suitable for comparing two types for equality or collating order. The names, encoding rule, and collating sequence for types are all unspecified and may differ between programs.

bool operator==(const type_info&  rhs ) const;

-2- Effects: Compares the current object with rhs .

-3- Returns: true if the two values describe the same type.

bool operator!=(const type_info&  rhs ) const;

-4- Returns: !(*this == rhs ).

bool before(const type_info&  rhs ) const;

-5- Effects: Compares the current object with rhs .

-6- Returns: true if *this precedes rhs in the implementation's collation order.

const char* name() const;

-7- Returns: an implementation-defined NTBS.

-8- Notes: The message may be a null-terminated multibyte string (lib.multibyte.strings), suitable for conversion and display as a wstring (lib.string.classes, lib.locale.codecvt)

type_info(const type_info&  rhs );
type_info& operator=(const type_info&  rhs );

-9- Effects: Copies a type_info object.

-10- Notes: Since the copy constructor and assignment operator for type_info are private to the class, objects of this type cannot be copied.

18.5.2 - Class bad_cast [lib.bad.cast]

namespace std {
  class bad_cast : public exception {
  public:
    bad_cast() throw();
    bad_cast(const bad_cast&) throw();
    bad_cast& operator=(const bad_cast&) throw();
    virtual ~bad_cast() throw();
    virtual const char* what() const throw();
  };
}

-1- The class bad_cast defines the type of objects thrown as exceptions by the implementation to report the execution of an invalid dynamic-cast expression (expr.dynamic.cast).

bad_cast() throw();

-2- Effects: Constructs an object of class bad_cast.

-3- Notes: The result of calling what() on the newly constructed object is implementation-defined.

    bad_cast(const bad_cast&) throw();
    bad_cast& operator=(const bad_cast&) throw();

-4- Effects: Copies an object of class bad_cast.

virtual const char* what() const throw();

-5- Returns: An implementation-defined NTBS.

-6- Notes: The message may be a null-terminated multibyte string (lib.multibyte.strings), suitable for conversion and display as a wstring (lib.string.classes, lib.locale.codecvt)

18.5.3 - Class bad_typeid [lib.bad.typeid]

namespace std {
  class bad_typeid : public exception {
  public:
    bad_typeid() throw();
    bad_typeid(const bad_typeid&) throw();
    bad_typeid& operator=(const bad_typeid&) throw();
    virtual ~bad_typeid() throw();
    virtual const char* what() const throw();
  };
}

-1- The class bad_typeid defines the type of objects thrown as exceptions by the implementation to report a null pointer in a typeid expression (expr.typeid).

bad_typeid() throw();

-2- Effects: Constructs an object of class bad_typeid.

-3- Notes: The result of calling what() on the newly constructed object is implementation-defined.

    bad_typeid(const bad_typeid&) throw();
    bad_typeid& operator=(const bad_typeid&) throw();

-4- Effects: Copies an object of class bad_typeid.

virtual const char* what() const throw();

-5- Returns: An implementation-defined NTBS.

-6- Notes: The message may be a null-terminated multibyte string (lib.multibyte.strings), suitable for conversion and display as a wstring (lib.string.classes, lib.locale.codecvt)

18.6 - Exception handling [lib.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;

  typedef void (*unexpected_handler)();
  unexpected_handler set_unexpected(unexpected_handler  f ) throw();
  void unexpected();

  typedef void (*terminate_handler)();
  terminate_handler set_terminate(terminate_handler  f ) throw();
  void terminate();

  bool uncaught_exception();
}

See also except.special.

18.6.1 - Class exception [lib.exception]

namespace std {
  class exception {
  public:
    exception() throw();
    exception(const exception&) throw();
    exception& operator=(const exception&) throw();
    virtual ~exception() throw();
    virtual const char* what() const throw();
  };
}

-1- 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.

exception() throw();

-2- Effects: Constructs an object of class exception.

-3- Notes: Does not throw any exceptions.

exception(const exception&) throw();
exception& operator=(const exception&) throw();

-4- Effects: Copies an exception object.

-5- Notes: The effects of calling what() after assignment are implementation-defined.

virtual ~exception() throw();

-6- Effects: Destroys an object of class exception.

-7- Notes: Does not throw any exceptions.

virtual const char* what() const throw();

-8- Returns: An implementation-defined NTBS.

-9- Notes: The message may be a null-terminated multibyte string (lib.multibyte.strings), suitable for conversion and display as a wstring (lib.string.classes, lib.locale.codecvt)

18.6.2 - Violating exception-specifications [lib.exception.unexpected]

namespace std {
  class bad_exception : public exception {
  public:
    bad_exception() throw();
    bad_exception(const bad_exception&) throw();
    bad_exception& operator=(const bad_exception&) throw();
    virtual ~bad_exception() throw();
    virtual const char* what() const throw();
  };
}

-1- The class bad_exception defines the type of objects thrown as described in (except.unexpected).

bad_exception() throw();

-2- Effects: Constructs an object of class bad_exception.

-3- Notes: The result of calling what() on the newly constructed object is implementation-defined.

    bad_exception(const bad_exception&) throw();
    bad_exception& operator=(const bad_exception&) throw();

-4- Effects: Copies an object of class bad_exception.

virtual const char* what() const throw();

-5- Returns: An implementation-defined NTBS.

-6- Notes: The message may be a null-terminated multibyte string (lib.multibyte.strings), suitable for conversion and display as a wstring (lib.string.classes, lib.locale.codecvt)

18.6.2.2 - Type unexpected_handler [lib.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 exception-specification.

-2- Required behavior: An unexpected_handler shall not return. See also except.unexpected.

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

18.6.2.3 - set_unexpected [lib.set.unexpected]

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

-2- Requires: f shall not be a null pointer.

-3- Returns: The previous unexpected_handler .

18.6.2.4 - unexpected [lib.unexpected]

-1- Called by the implementation when a function exits via an exception not allowed by its exception-specification (except.unexpected). May also be called directly by the program.

-2- Effects: Calls the unexpected_handler function in effect immediately after evaluating the throw-expression (lib.unexpected.handler), if called by the implementation, or calls the current unexpected_handler, if called by the program.

18.6.3 - Abnormal termination [lib.exception.terminate]

-1- The type of a handler function to be called by terminate() when terminating exception processing.

-2- Required behavior: A terminate_handler shall terminate execution of the program without returning to the caller.

-3- Default behavior: The implementation's default terminate_handler calls abort().

18.6.3.2 - set_terminate [lib.set.terminate]

-1- Effects: Establishes the function designated by f as the current handler function for terminating exception processing.

-2- Requires: f shall not be a null pointer.

-3- Returns: The previous terminate_handler .

18.6.3.3 - terminate [lib.terminate]

-1- 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.

-2- Effects: Calls the terminate_handler function in effect immediately after evaluating the throw-expression (lib.terminate.handler), if called by the implementation, or calls the current terminate_handler function, if called by the program.

18.6.4 - uncaught_exception [lib.uncaught]

-1- Returns: true after completing evaluation of a throw-expression until either completing initialization of the exception-declaration in the matching handler or entering unexpected() due to the throw; or after entering terminate() for any reason other than an explicit call to terminate(). [Note: This includes stack unwinding (except.ctor).
--- end note]

-2- Notes: When uncaught_exception() is true, throwing an exception can result in a call of terminate() (except.terminate).

18.7 - Other runtime support [lib.support.runtime]

-1- Headers <cstdarg> (variable arguments), <csetjmp> (nonlocal jumps), <ctime> (system clock clock(), time()), <csignal> (signal handling), and <cstdlib> (runtime environment getenv(), system()).

-2- The contents of these headers are the same as the Standard C library headers <stdarg.h>, <setjmp.h>, <time.h>, <signal.h>, and <stdlib.h> respectively, with the following changes:

-3- The restrictions that ISO C places on the second parameter to the va_start() macro in header <stdarg.h> are different in this International Standard. The parameter parmN is the identifier of the rightmost parameter in the variable parameter list of the function definition (the one just before the ...). If the parameter parmN is declared with a function, array, or reference type, or with a type that is not compatible with the type that results when passing an argument for which there is no parameter, the behavior is undefined.

See also ISO C subclause 4.8.1.1.

-4- The function signature longjmp(jmp_buf jbuf , int val ) has more restricted behavior in this International Standard. If any automatic objects would be destroyed by a thrown exception transferring control to another (destination) point in the program, then a call to longjmp( jbuf , val ) at the throw point that transfers control to the same (destination) point has undefined behavior.

See also ISO C subclause 7.10.4, 7.8, 7.6, 7.12.

-5- The common subset of the C and C++ languages consists of all declarations, definitions, and expressions that may appear in a well formed C++ program and also in a conforming C program. A POF (``plain old function'') is a function that uses only features from this common subset, and that does not directly or indirectly use any function that is not a POF. All signal handlers shall have C linkage. A POF that could be used as a signal handler in a conforming C program does not produce undefined behavior when used as a signal handler in a C++ program. The behavior of any other function used as a signal handler in a C++ program is implementation defined.*

[Footnote: In particular, a signal handler using exception handling is very likely to have problems --- end foonote]