NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c++-part2/17841

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From: Rob Stewart <[e-mail]>
Newsgroups: comp.lang.c++
Subject: Re: Constructor and enum Question...
Date: Wed, 17 Apr 1996 15:33:40 -0400
Organization: Datalytics, Inc
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Alexey Ruzin wrote:
>
> cwu wrote:
> >
> > Recently I am studying C++ and have a difficulty to understand
> > "constructor" and "enum".  I am very appreciated any comments.
>
> Hi, excuse me for my Engilsh.
>
> As for 'enum':
>
> In short, it looks like set of defines, which declare 'int' constants.
> These defines are grouped in type, but you don't need use it mnemonic
> names always. Example:
>
> ...
> enum {a,b,c} f;
> ...
>
> this is equal:
>
> ...
> #define a 0
> #define b 1
> #define c 2

Well, not exactly.  It declares a new type, compatible with int,
that gives names to the values.  (#define creates a number which
takes on a different type depending upon the expression in which
the expanded value--a literal number--appears.) By default, the
enumerations (the named values in the enumerated type) start at
zero and increase sequentially.  You can, however, assign
arbitrary values to any of them.  The compiler will increment,
by one, any that don't have values along the way.

enum
{
   a = 5,        // 5
   b,            // 6
   c = 0        // 0
};

The tag-name may be omitted, as above, if you only want to use
the enumerations and not declare a variable or parameter of the
enumerated type.  If you omit the tag-name, you are creating the
equivalent of a set of const ints.

With the tag-name, you can declare variables and parameters of
that enumerated type, so they can only assume values from the
enumerations in that type.  You cannot, however, assign a
variable of an enumerated type from anything other than the
available enumerations.

(Most of the preceding statements can be qualified if you start
casting!)

>
> As for 'constructor':
> [snip]

A constructor (abbreviated ctor or c'tor) is a member function
of a class that is charged with turning raw memory--whether on
the stack or heap--into an object.  Typically, this
transformation involves initializing data members (dms).  Prior
to entering the body of the ctor, while processing the
initializer list, if any, the compiler inserts code that calls
base class and dm ctors.  Thus, by the time you enter the ctor
body (after the opening brace), all base classes and dms have
been initialized in some fashion.  From this point forward, a
ctor is like any other member function.  You can put any
initialization code into it you like.

ctors are like any other member function (mf) except that ctors
have no return value (not even void), and they (only one per
object) are called before any other mf.  What makes ctors
special is that they share the class name.  This is how the
compiler recognizes a particular member function as a ctor.

Like other mfs, ctors may be overloaded, so each may take
different parameters.  In fact, if you don't create a ctor for
your class, the compiler will create two for you: the default
and copy ctors.  The former takes no parameters and simply
invokes base class and dm default ctors.  The latter does a
shallow (duplicates pointers rather than what they point to)
copy of one object to the new one.

BTW, ctors are matched by a destructor (dtor) (only one per
class).  The dtor is responsible for reverting the object back
into raw memory.  This can mean releasing resources (like memory
allocated on the heap), or notifying cooperating objects that
they are disappearing, etc.

It comes down to this: a ctor is the first member function
called on raw memory to turn it into an object, and the dtor is
the last member function called on an object to turn it back
into raw memory.

--
Robert Stewart        | My opinions are usually my own.
Datalytics, Inc.    | [e-mail]