Re: Creating an instance of a class

From:
"=?iso-8859-1?q?Erik_Wikstr=F6m?=" <eriwik@student.chalmers.se>
Newsgroups:
comp.lang.c++
Date:
19 Feb 2007 01:46:45 -0800
Message-ID:
<1171878405.064914.181020@s48g2000cws.googlegroups.com>
On Feb 19, 10:06 am, "jimbo" <joachim.zett...@googlemail.com> wrote:

On 19 Feb., 09:55, Piyo <cybermax...@yahoo.com> wrote:

jimbo wrote:

Dear all,

I am more or less new to c++ programing and therefore still have
problems with some fundamentals :(

At the moment I try to build a GUI-Application with Qt4. Sometimes I
have seen in the tutorials I have to instance a class with the new
command. Like QWidget QLabel *label = new QLabel("Hello World"); Th=

en

I have created a pointer to a QLabel Object. The other way I have seen
is to just use QLabel label("Hello World"); Without a pointer and
without the new statement.

So to access the functions of the QLabel class I have to use the "."
for the part without pointer like label.show and for the pointer part:
label->show.

Can somebody explain me, why it is possible to use the two ways and
what is the difference in general.

Thank you a lot in advance.

jimbo


Given an object and a pointer to that object, you can access members
like so:

class foo
{
public:
    int bar;

};

int
main()
{
    foo* baz1 = new foo();
    foo baz2;

    baz2.bar; // cool
    baz2->bar; // not cool
    baz1.bar; // not cool
    (*baz1).bar; // cool but too long
    (baz1)->bar; // short hand version of the previous line
    baz1->bar; // even shorter version of the previous line

}

In summary, "->" is a short hand notation to dereference the
pointer first and then access its member.

Good Luck!


Hi to all of you,

first of all, thanks a lot for the explanation. I didn=B4t know that
this whole topic is related to where the memory is allocated. Now with
heap and stack I can imagine a little bit better what I am doing :)

But to come back to my own little example. When i am already in a
main() function of my app and i create the instances there without the
new operator then i will have no problems concerning the use of the
instance in a different scope because i am already in main and main is
not ended yet, or?


Yes, the scope of main is a superset of almost all other scopes
(static being the only exception that I can think of right now).

I think i need to take it more carefully when i implement an own class
and use functions there.


It's not so much when implementing a class as when using it. But since
we are on the topic of implementing classes and dynamic memory
allocation I can say that if you implementation uses new instantiate
members (perhaps in the constructor) you should probably delete them
in the destructor, like so:

class Foo {
  int* arr; // An array
public:
  Foo(int i) : arr(new int[i] { } // * allocate an array of size i
  ~Foo() { delete[] arr; } // Free the memory used
};

This way a user of the class will never have to worry about how the
class manages its resources, it will just work.

* If you don't understand the ': arr(new int[i] { }'-bit it's an
initializer-list and does the same thing as '{ arr = new int[i]; }',
for a more detailed explanation see http://www.parashift.com/c++-faq-lite/c=
tors.html#faq-10.6

--
Erik Wikstr=F6m

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