pass "this" as parameter

[incapacitant]

I have this code as exemple that uses "this" :



ApplicationWindow::ApplicationWindow()
: QMainWindow( 0 )
{

// set background and size it correctly
( this )->resize( 569, 458 );



I would need to call a procedure with "this" as parameter so that I can display stuff in "this" from the procedure. Like for instance resize "this" inside another procedure to make my main proc shorter and cleaner.

How do I declare the function ? What is the the type of "this" ?

[jpn]

The keyword this represents a pointer to the object whose member function is being executed. It is a pointer to the object itself.

[jacek]

ApplicationWindow::ApplicationWindow()
: QMainWindow( 0 )
{

// set background and size it correctly
( this )->resize( 569, 458 );

You don't have to use "this" in above code. You can simply write:

ApplicationWindow::ApplicationWindow()
: QMainWindow( 0 )
{

// set background and size it correctly
resize( 569, 458 );



What is the the type of "this" ?
In this case "this" is this: ApplicationWindow*

[incapacitant]

sorry to be a pain, I understand now that the object is ApplicationWindow.
But in order to declare my function I need the type of ApplicationWindow !

have some trouble to declare it :
void aFunction ( type??? ApplicationWindow, type2 param )
{
// some code to define bool ok
if ( !ok ) QMessageBox::warning( ApplicationWindow, param );
}

Also in the function body instead of "this" I will just say ApplicationWindow, right ?

PS: why I am defined as Intermediate User, all this ranking should be based on the number of thanks people get. I never got any thanks, how could I ? But this is another story...
Thanks

[jacek]

sorry to be a pain, I understand now that the object is ApplicationWindow.
No, ApplicationWindow is a class, i.e. a kind of type.


But in order to declare my function I need the type of ApplicationWindow !

void aFunction ( ApplicationWindow *appWindow, type2 param )
{
// some code to define bool ok
if ( !ok ) QMessageBox::warning( appWindow, param );
}


why I am defined as Intermediate User, all this ranking should be based on the number of thanks people get. I never got any thanks, how could I ?
Because title is based on the number of posts. The "thanks" mechanism was added, so that people could say "Thank you" without interfering with the discussion.

[incapacitant]

slowly getting somewhere thanks, but :

function call:


s = qLoadIniStr( ApplicationWindow, "TxtDateEnvoi" ); // line 27



function definition


QString qLoadIniStr( ApplicationWindow *appWindow, const QString sParam );



compile error:
27 C:\Qt\test\sms\archiv.cpp expected primary-expression before ',' token

[jpn]

function call:


s = qLoadIniStr( ApplicationWindow, "TxtDateEnvoi" ); // line 27


Where do you have this function call?

- if inside ApplicationWindow:

s = qLoadIniStr( this, "TxtDateEnvoi" );
- if outside ApplicationWindow, you need to one way or another pass a pointer to the application window:


#include "applicationwindow.h"
...
ApplicationWindow* appWindow = ...
...
s = qLoadIniStr( appWindow, "TxtDateEnvoi" );

[incapacitant]

I am making the call outside of Application window so yes I need to pointer to the class.

Like you propose:


ApplicationWindow* appwindow = ...


What do you mean with "..." ?

[jpn]

I meant that you should fill in the gaps.
Hard, if not impossible, to advise you more without knowing a little more about your code..

[incapacitant]

thanks, i give up. will try on a simpler piece of code that I can post later...

[fullmetalcoder]

thanks, i give up. will try on a simpler piece of code that I can post later...

Why the hell would you give up??? It is the simplest piece of code possible! Are you sure you learnt C++, if I remember well, a guy here as a nice signature that may help you understand where lies your problem...:rolleyes:

A class, such as your mainWindow, is kinda perticular type, with its own data and structure while an object is an instance of a class.

To simplify, "class QSomeClass" would be much like "int", the only difference being that "QSomeClass" is an user-defined type while "int" is a built-in one.


int x;
QSomeClass y;

x and y are both objects, i.e. instance of a given type

To access an object or give it as parameter to a function you need a pointer or a reference to it. "this" is a perticular keyword that points to the instance of the object used for a function.


{

...

QSomeClass *class = new QSomeClass;

class->someFunction();

...

}

void QSomeClass::someFunction()
{
...
}

This C++ code performs something like that (C equivalent, QSomeClass being a structure in that case) :



{

...

QSomeClass *class = new QSomeClass;

someFunction(class);

...

}

void someFunction(QSomeClass *this)
{
...
}


Does that help??? If not you may consider having a look there : What are your favorite C++ book (http://www.qtcentre.org/forum/showthread.php?t=29)s?
I hope it'll help you learning that wonderful programming language C++ is!

[incapacitant]

ok, i don't give up...

let me summarize the problem, I have defined a simple procedure outside classes as follows qloadini.cpp :


#include <QtCore>
#include <QtGui>

//************************************************** **************
QStringList qLoadIniVecStr( const QString sParam )
{
QString s;
QStringList param;
QStringList ret;
QString fileName = "efrei.ini";

QFile file( fileName );
if (file.open(QIODevice::ReadOnly)) {
QTextStream is(&file);
while (!is.atEnd()) {
s = is.readLine();
if (s[0]!=QChar('#'))
{
param = s.split( "=" );
if ( param[0] == sParam ) ret.push_back( param[1] );
}
}
}
if (ret.size()==0) ret.push_back( "Error loading: " + param[0] );
return ret;
file.close();
}

//************************************************** **************
QString qLoadIniStr( const QString sParam )
{
QString s;
QStringList param;
QString fileName = "efrei.ini";

QFile file( fileName );
if (file.open(QIODevice::ReadOnly)) {
QTextStream is(&file);
while (!is.atEnd()) {
s = is.readLine();
if (s[0]!=QChar('#'))
{
param = s.split( "=" );
if ( param[0] == sParam )
{
file.close();
return param[1];
}
}
}
}
file.close();
return "Error loading: " + sParam;
}



The idea is to externalize constants and load them from an ini file when I need them.


Then I have have the qloadini.h :


#ifndef QLOADINI_H
#define QLOADINI_H

#include <QStringList>

QString qLoadIniStr( const QString sParam );
QStringList qLoadIniVecStr( const QString sParam );

#endif


Finally I call this procedure from various GUI classes with :


TxtError = qLoadIniStr( "Unknown Error" );
ErrHttp = qLoadIniStr( "ErrHttp" );
QMessageBox::warning( this, ErrHttp, TxtError );
// here for instance i don't even bother to check what is returned, the QMessageBox
// displays the default error parameter if the retrieve filed.


As the qloadini procs are defined they return some message that tells me whether I was able to retrieve the constant or not. The problem is that I then have to handle the errors in the calling procedure which is cluttering the code. What I would have liked is to be able to display a QMessageBox inside the qloadini procedures themselves.
It would be a generic message telling that a constant parameter is not retrieved and that the ini file has something wrong with it.

But I don't know how to pass this "this" parameter so that the qloadini QMessageBox can address the current window. I could dupplicate qloadini in all classes that call it but that would hardly be an improvement.

[wysota]

Declare the function parameter as a base class of all Qt window classes. As you're learning C++ right now (:cool: ), I won't tell you which is it and I suggest others do the same. Find what is the base class of all Qt window classes and why does it work when you pass a pointer to the base class and then call the function with a subclass as a parameter.

[incapacitant]

Thanks for the hint which as it stands sounds chinese.
So am banned from newbie.

Are there plans for new newbies ? :)

Thanks wysota.

[wysota]

Thanks for the hint which as it stands sounds chinese.
So am banned from newbie.

Are there plans for new newbies ? :)

Learn C++ :)

[incapacitant]

I've got the books. Just started reading. I'll be back. ;)

[incapacitant]

Declare the function parameter as a base class of all Qt window classes. As you're learning C++ right now (:cool: ), I won't tell you which is it and I suggest others do the same. Find what is the base class of all Qt window classes and why does it work when you pass a pointer to the base class and then call the function with a subclass as a parameter.


Nice hint that I store carefully in a text file, however given my poor knowledge could you say the same thing in more words, in newbie terms ?
Thanks

[wysota]

It's as newbie as it can get. Just learn about classes, base classes (superclasses) and subclasses and look into Qt documentation and find the base class of all Qt top level windows.