ISO C++ library

# Support

Support

##

This part deals with the functions called and objects created automatically during the course of a program's existence.

While we can't reproduce the contents of the Standard here (you need to get your own copy from your nation's member body; see our homepage for help), we can mention a couple of changes in what kind of support a C++ program gets from the Standard Library.

## Types

### Fundamental Types

C++ has the following builtin types:

- char

- signed char

- unsigned char

- signed short

- signed int

- signed long

- unsigned short

- unsigned int

- unsigned long

- bool

- wchar_t

- float

- double

- long double

These fundamental types are always available, without having to include a header file. These types are exactly the same in either C++ or in C.

Specializing parts of the library on these types is prohibited: instead, use a POD.

### Numeric Properties

The header `limits` defines traits classes to give access to various implementation defined-aspects of the fundamental types. The traits classes -- fourteen in total -- are all specializations of the template class numeric_limits, documented here and defined as follows:

       template<typename T>
         struct class
         {
           static const bool is_specialized;
           static T max() throw();
           static T min() throw();

           static const int digits;
           static const int digits10;
           static const bool is_signed;
           static const bool is_integer;
           static const bool is_exact;
           static const int radix;
           static T epsilon() throw();
           static T round_error() throw();

           static const int min_exponent;
           static const int min_exponent10;
           static const int max_exponent;
           static const int max_exponent10;

           static const bool has_infinity;
           static const bool has_quiet_NaN;
           static const bool has_signaling_NaN;
           static const float_denorm_style has_denorm;
           static const bool has_denorm_loss;
           static T infinity() throw();
           static T quiet_NaN() throw();
           static T denorm_min() throw();

           static const bool is_iec559;
           static const bool is_bounded;
           static const bool is_modulo;

           static const bool traps;
           static const bool tinyness_before;
           static const float_round_style round_style;
         };

### NULL

The only change that might affect people is the type of `NULL`: while it is required to be a macro, the definition of that macro is not allowed to be `(void*)0`, which is often used in C.

For `g++`, `NULL` is

    #define

'd to be `__null`, a magic keyword extension of `g++`.

The biggest problem of #defining `NULL` to be something like 0L is that the compiler will view that as a long integer before it views it as a pointer, so overloading won't do what you expect. (This is why `g++` has a magic extension, so that `NULL` is always a pointer.)

In his book Effective C++, Scott Meyers points out that the best way to solve this problem is to not overload on pointer-vs-integer types to begin with. He also offers a way to make your own magic `NULL` that will match pointers before it matches integers.

See the Effective C++ CD example

## Dynamic Memory

There are six flavors each of `new` and `delete`, so make certain that you're using the right ones. Here are quickie descriptions of `new`:

- single object form, throwing a bad_alloc on errors; this is what most people are used to using

- Single object "nothrow" form, returning NULL on errors

- Array `new`, throwing bad_alloc on errors

- Array nothrow `new`, returning `NULL` on errors

- Placement `new`, which does nothing (like it's supposed to)

- Placement array `new`, which also does nothing

They are distinguished by the parameters that you pass to them, like any other overloaded function. The six flavors of `delete` are distinguished the same way, but none of them are allowed to throw an exception under any circumstances anyhow. (They match up for completeness' sake.)

Remember that it is perfectly okay to call `delete` on a NULL pointer! Nothing happens, by definition. That is not the same thing as deleting a pointer twice.

By default, if one of the throwing `new`s can't allocate the memory requested, it tosses an instance of a bad_alloc exception (or, technically, some class derived from it). You can change this by writing your own function (called a new-handler) and then registering it with `set_new_handler()`:

       typedef void (*PFV)(void);

       static char*  safety;
       static PFV    old_handler;

       void my_new_handler ()
       {
           delete[] safety;
           popup_window ("Dude, you are running low on heap memory.  You
                          should, like, close some windows, or something.
                          The next time you run out, we're gonna burn!");
           set_new_handler (old_handler);
           return;
       }

       int main ()
       {
           safety = new char[500000];
           old_handler = set_new_handler (&my_new_handler);
           ...
       }

bad_alloc is derived from the base exception class defined in Chapter 19.

## Termination

### Termination Handlers

Not many changes here to `cstdlib`. You should note that the `abort()` function does not call the destructors of automatic nor static objects, so if you're depending on those to do cleanup, it isn't going to happen. (The functions registered with `atexit()` don't get called either, so you can forget about that possibility, too.)

The good old `exit()` function can be a bit funky, too, until you look closer. Basically, three points to remember are:

1. Static objects are destroyed in reverse order of their creation.

2. Functions registered with `atexit()` are called in reverse order of registration, once per registration call. (This isn't actually new.)

3. The previous two actions are interleaved, that is, given this pseudocode:

         extern "C or C++" void  f1 (void);
         extern "C or C++" void  f2 (void);

         static Thing obj1;
         atexit(f1);
         static Thing obj2;
         atexit(f2);

   then at a call of `exit()`, `f2` will be called, then `obj2` will be destroyed, then `f1` will be called, and finally `obj1` will be destroyed. If `f1` or `f2` allow an exception to propagate out of them, Bad Things happen.

Note also that `atexit()` is only required to store 32 functions, and the compiler/library might already be using some of those slots. If you think you may run out, we recommend using the `xatexit`/`xexit` combination from `libiberty`, which has no such limit.

### Verbose Terminate Handler

If you are having difficulty with uncaught exceptions and want a little bit of help debugging the causes of the core dumps, you can make use of a GNU extension, the verbose terminate handler.

    #include <exception>

    int main()
    {
      std::set_terminate(__gnu_cxx::__verbose_terminate_handler);
      ...

      throw anything;
    }

The `__verbose_terminate_handler` function obtains the name of the current exception, attempts to demangle it, and prints it to stderr. If the exception is derived from exception then the output from `what()` will be included.

Any replacement termination function is required to kill the program without returning; this one calls abort.

For example:

    #include <exception>
    #include <stdexcept>

    struct argument_error : public std::runtime_error
    {
      argument_error(const std::string& s): std::runtime_error(s) { }
    };

    int main(int argc)
    {
      std::set_terminate(__gnu_cxx::__verbose_terminate_handler);
      if (argc > 5)
        throw argument_error(argc is greater than 5!);
      else
        throw argc;
    }

With the verbose terminate handler active, this gives:

       % ./a.out
       terminate called after throwing a `int'
       Aborted
       % ./a.out f f f f f f f f f f f
       terminate called after throwing an instance of `argument_error'
       what(): argc is greater than 5!
       Aborted

The 'Aborted' line comes from the call to `abort()`, of course.

This is the default termination handler; nothing need be done to use it. To go back to the previous silent death method, simply include `exception` and `cstdlib`, and call

         std::set_terminate(std::abort);

After this, all calls to `terminate` will use `abort` as the terminate handler.

Note: the verbose terminate handler will attempt to write to stderr. If your application closes stderr or redirects it to an inappropriate location, `__verbose_terminate_handler` will behave in an unspecified manner.