cpp_faq.txt

国外网站上的一些精典的C程序

/* ==== CPP_ECHO.FAQ ========================== release 95-08-16 =========

This is the Frequently Asked Question list of the fidonet c_plusplus echo.
Suggestions, enhancements, changes and comments will be appreciated.

 _____          Archivist/Editor: Auke Reitsma, Delft, The Netherlands.
 /_|__|         Netmail         : 2:281/527
/  | \  --------------------------------------------------------------- */

This CPP_echo FAQ list currently consists of four sections: "FAQ's", "New
FAQ's", "Abbreviations and Terminology" and "Contributors". Each FAQ
consists of a number, the question and a date on a single line -- if
possible -- the answer, one or more lines listing the contributor and the
(last) submission date and a 'last edit date'.
The number uniquely identifies the question and will NEVER change. The
question date should change when the question is asked again -- or used as
answer -- in the cplusplus echo. In a relatively short time this will
identify the unimportant questions, which will make it easier to delete
these items.
Three question marks indicate something that is at least somewhat doubtful
and may need to be changed. Again, contributions are welcome.
The "Abbreviations and Terminology" section is included to supply less
common abbreviations and terms related to C++ and also to 'explain'
abbreviations used in this list. It is also used for indexing and cross-
referencing.

Auke Reitsma 95-07-15.

/* ---- Frequently Asked Questions ------------------------------------ */

000     What is the topic of the C++ FAQ list?                  (95-04-15)

Any C++ related subject occurring 'frequently' in the cplusplus echo.
There must be no doubts _at_all_ that it is on topic in the echo. This
means that normal (ANSI) C subjects are not on topic for this list. Also
the coverage of subjects specific to a platform, compiler or library will
be limited.

95-05-01


001     What is the difference between C and C++?               (95-04-15)

The essential difference is the "Object orientation" in C++.

95-04-15


002     What normal C constructs work differently in C++?       (95-04-15)

- Assigning int's to enum's.
- Assigning void pointers to other types of pointers.
- Function declaration foo() without parameters.
- Character constants are of type char in C++. They are of type int in C.
- ALL functions MUST be prototyped in C++, which is not required in C.
- In C++:
        struct A { /* ... */ };
  is equivalent to:
        typedef struct A { /* ... */ } A;
  in C.

See C++PL2, "Notes to the reader" (p. 11/12) and "C++ and ANSI-C" section
r.18.2 (p.628/629) for more items and details.

Auke Reitsma [95-05-01], Jerry Coffin [95-05-06], David Nugent [95-04-16],
Jari Laaksonen [95-04-15], 95-05-16


003     Why and when is a virtual destructor needed?            (95-04-15)

Any class that may act as the base of another class should have a virtual
destructor. This ensures that when an object of the derived class is
destroyed that the derived class dtor will be invoked to destroy it.
If the destructor is not virtual, under some common circumstances, only
the base class' destructor will be invoked, regardless of the class
actually being destroyed.
For practical purposes this means that a class which does, could or should
have virtual member functions, should also have a virtual destructor.

Jerry Coffin [95-06-09], 95-06-16


004     How do I open binary files?                             (95-04-15)

Yeah, the various manuals and books are usually not clear that subject.
With BC and MSC, you do a bitwise or of the file open mode with
ios::binary, as in:

    ifstream mystream( "filename.ext", ios::in | ios::binary );

The default is ios::text for BC and MSC.
However, this is NOT supported by Symantec 6.1. SC 6.1 doesn't have a flag
to designate binary - it has ios::translated for text, and apparently to
work with binary files you simply designate ios::in or ios::out without
or'ing in ios::translated.

Jerry Coffin [95-05-06], 95-06-16


005     Why seem interrupt handlers as member functions to be impossible?
                                                                (95-04-15)
Interrupt handlers as member functions _are_ possible. But they must be
static member functions. Static member functions don't make use of the
implicit 'this' pointer required by normal member functions. The caller of
the interrupt handler doesn't know anything about objects and 'this'
pointers, so it can't pass a value of such a pointer.

Auke Reitsma [95-04-15], 95-04-15


006     Is there a new/delete equivalent of realloc?            (95-05-01)

No. It would have a number of difficult problems. Some of these are:
- Should the old instances in the memory be deleted and re-instantiated?
- How about contained classes?
- How about instances owned by the classes to be reallocated? These often
  have a pointer to their owner.

Origin Unknown [??-??-??], 95-05-01


007     Floating point representation and output seems to be compiler
        dependent.                                              (95-05-01)

Regrettably, yes. The action of ios::setprecision() varies among com-
pilers.

David Nugent [95-04-16], 95-05-01


008     What is a "Copy Constructor"?                           (95-07-16)

A Copy Constructor constructs a new object as a copy of an existing object
of the same type. Frequently copy constructors do a 'deep copy' of the
object. X( const X& X_object ){...}; is a copy constructor for class X.

Deep Copy vs. Shallow Copy: a shallow copy simply copies the contents of
an object directly - if the object contains pointers, both the old copy
and the new copy contain pointers to the same actual item. In a deep copy,
when an object contains a pointer, a new copy of whatever the pointer
points AT is created and the new object contains a pointer to the newly
created copy of the item.
Why are deep copies important? If you carry out a shallow copy you end up
with two pointers to the same item. If that item is an object with a
destructor, this generally means you'll end up calling the destructor for
that item twice, which will generally cause problems.
Unfortunately, most don't know to ask this question directly: the symptom
is generally heap corruption which is hard to track down directly since
there it has many possible causes.

Jerry Coffin [95-07-04], 95-07-16


009     Why a "operator=(...)" when there is a copy ctor?       (95-07-16)

You use the assignment operator (operator = ()) whenever an existing
object is to be replaced with a different object. The copy constructor
X(const X&) is used to create a new instance of an X-object exactly like
another.

Notice the subtle difference. Assignment changes an existing object while
construction creates a new object. You can view assignment as the applica-
tion of a destructor, to flush away the existing object, followed by a
copy construction, to make an exact copy of the assigned object.

Cliff Rhodes [95-07-08], 95-07-16


010     What is an ABC: an "Abstract Base Class"?               (95-08-01)

An Abstract Base Class is a class that is not intended to be instantiated
itself. Rather, it is intended strictly for use as a base for other
classes. To prevent instantiation, an ABC will typically contain at least
one pure virtual function.

The point of an ABC is to separate the interface of a group of classes
from the implementation of the functions that make up the interface. This
allows other code to ignore differences in how these functions are carried
out. An ABC creates a contract between its descendants and any other code
that uses them. The descendants must implement a certain set of functions.
Code that uses them must use those functions to access whatever it is the
object involved represents.

Jerry Coffin [95-07-14], 95-08-01


011     How is an ABC related to an "Abstract Data Type" (ADT)  (95-08-01)

An ADT is a concept - the basic idea of a data type that doesn't specify
how the data type is implemented. An ABC is a method C++ provides for
creating an ADT.

Jerry Coffin [95-07-26], 95-08-01


012     What is a 'pure' virtual function and what's its use?   (95-08-01)

A pure virtual function is signified by using `=0;' in place of the body
of the function. The presence of a pure virtual function prevents
instantiation of the class which contains it. For this to be of any use, a
derived class must implement the pure virtual function. I.e. the derived
class must provide a function with the same name which includes a function
body.
The basic reason for pure virtual functions is to specify something that
a class can do without specifying how the class will do it.

Jerry Coffin [95-07-26], 95-08-01


/* ---- New since previous release -- provisionally numbered ---------- */

None.


/* ---- Abbreviations and Terminology --------------------------------- */

This section also serves as an index.

ABC     Abstract Base Class, see FAQ #010 and #011

ADT     Abstract Data Type, see FAQ #011

ARM     "The Annotated C++ Reference Manual"
        by B. Stroustrup and M.A. Ellis. ISBN 0-201-51459-1

assignment operator
        See FAQ #009

BC      Borland C. Usually including the Turbo products.

binary files
        See FAQ #004

C++PL2  "The C++ Programming Language" second edition
        by B. Stroustrup, ISBN 0-201-53992-6

copy constructor
        See FAQ #008

ctor    constructor

deep copy
        See FAQ #008

dtor    destructor

interrupt handlers
        See FAQ #005

MSC     Microsoft C.

pure virtual function
        See FAQ #012

SC      Symantec C.

shallow copy
        See FAQ #008

static member functions
        See FAQ #005

subclass
        Not a correct C++ term at all. Use "derived class". See ARM p.197

superclass
        Not a correct C++ term at all. Use "base class". See ARM p.197

virtual destructor
        See FAQ #003

virtual function, pure
        See FAQ #012

/* ---- Contributors -------------------------------------------------- */

Jerry Coffin
Jari Laaksonen
David Nugent
Auke Reitsma
Cliff Rhodes

/* ==== CPP_ECHO.FAQ end ============================================== */