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 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 news
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 Sect1 19.
Termination
Termination Handlers
Not many changes here to . 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:
Static objects are destroyed in reverse order of their creation.
Functions registered with atexit() are called in
reverse order of registration, once per registration call.
(This isn't actually new.)
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.