Keywords: ISO C++, library

Chapter 4. Support
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## Chapter 4. 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.

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