# Chapter 18: Library Support

Chapter 18 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.

# Contents

- Types
- Implementation properties
- Start and Termination
- Dynamic memory management
- RTTI, the ABI, and demangling

## Types

All the types that you're used to in C are here in one form or another. 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.

In 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:

       const                             // this is a const object...
       class {
       public:
         template<class T>               // convertible to any type
           operator T*() const           // of null non-member
           { return 0; }                 // pointer...

         template<class C, class T>      // or any type of null
           operator T C::*() const       // member pointer...
           { return 0; }

       private:
         void operator&() const;         // whose address can't be
                                         // taken (see Item 27)...

       } NULL;                           // and whose name is NULL

(Cribbed from the published version of the Effective C++ CD, reproduced here with permission.)

If you aren't using g++ (why?), but you do have a compiler which supports member function templates, then you can use this definition of NULL (be sure to #undef any existing versions). It only helps if you actually use NULL in function calls, though; if you make a call of `foo(0);` instead of `foo(NULL);`, then you're back where you started.

Added Note: When we contacted Dr. Meyers to ask permission to print this stuff, it prompted him to run this code through current compilers to see what the state of the art is with respect to member template functions. He posted an article to Usenet after discovering that the code above is not valid! Even though it has no data members, it still needs a user-defined constructor (which means that the class needs a type name after all). The ctor can have an empty body; it just needs to be there. (Stupid requirement? We think so too, and this will probably be changed in the language itself.)

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## Implementation properties

### `<limits>`

This header mainly defines traits classes to give access to various implementation defined-aspects of the fundamental types. The traits classes -- fourteen in total -- are all specilizations of the template class `numeric_limits` 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;
       };

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## Start and Termination

Not many changes here to `<cstdlib>` (the old stdlib.h). 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.

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## Dynamic memory management

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.

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## RTTI, the ABI, and demangling

If you have read the source documentation for `namespace abi` then you are aware of the cross-vendor C++ ABI which we use. One of the exposed functions is the one which we use for demangling in programs like `c++filt`, and you can use it yourself as well.

(The function itself might use different demanglers, but that's the whole point of abstract interfaces. If we change the implementation, you won't notice.)

Probably the only times you'll be interested in demangling at runtime are when you're seeing `typeid` strings in RTTI, or when you're handling the runtime-support exception classes. For example:

    #include <exception>
    #include <iostream>
    #include <cxxabi.h>

    struct empty { };

    template <typename T, int N>
      struct bar { };

    int main()
    {
      int     status;
      char   *realname;

      // exception classes not in <stdexcept>, thrown by the implementation
      // instead of the user
      std::bad_exception  e;
      realname = abi::__cxa_demangle(e.what(), 0, 0, &status);
      std::cout << e.what() << "\t=> " << realname << "\t: " << status << '\n';
      free(realname);

      // typeid
      bar<empty,17>          u;
      const std::type_info  &ti = typeid(u);

      realname = abi::__cxa_demangle(ti.name(), 0, 0, &status);
      std::cout << ti.name() << "\t=> " << realname << "\t: " << status << '\n';
      free(realname);

      return 0;
    }

With GCC 3.1 and later, this prints

          St13bad_exception       => std::bad_exception   : 0
          3barI5emptyLi17EE       => bar<empty, 17>       : 0

The demangler interface is described in the source documentation linked to above. It is actually written in C, so you don't need to be writing C++ in order to demangle C++. (That also means we have to use crummy memory management facilities, so don't forget to free() the returned char array.)

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