Two probably simple Qt4 Questions

[frenk_castle]

I have been studying the SDI/MDI example in my Qt4 and in my book Foundations of Qt development. I have two questions. I tried to provide enough details. If I wrote to much I apologize.

1.

In the Qt4 example there is a line of code

newAct->setShortcuts(QKeySequence::New);

newAct->setShortcuts(QKeySequence::New);

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The line with same functionality in the book is

newAction->setShortcut( tr("Ctrl+N") );

newAction->setShortcut( tr("Ctrl+N") );

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I have read the help provided with the Qt there is a whole table of values for enum QKeySequence::StandardKey. As far as I have managed to conclude the first version

QKeySequence::QKeySequence ( StandardKey key )

QKeySequence::QKeySequence ( StandardKey key )

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is platform dependent. It doesn't say, as far as I can see if second version

QKeySequence::QKeySequence ( const QString & key )

QKeySequence::QKeySequence ( const QString & key )

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is platform dependent.

Is there any real difference between these to versions and if there are some what are they?

2.

There was a thread about this but it didn't clarify it for me at all.

If I have type class B for instance and I want to use it as a member in my type class A. Code that I would use would look something like this.

class_B.h

#ifndef CLASS_B_H
#define CLASS_B_H
 
class B
{
//some code
}
 
#endif

#ifndef CLASS_B_H
#define CLASS_B_H

class B
{
//some code
}

#endif

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class_B.cpp

#include "class_B.h"
//some code

#include "class_B.h"
//some code

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class_A.h

#ifndef CLASS_A_H
#define CLASS_A_H
 
#include "class_B.h"
 
class A
{
    private:
        B b;
    //some code
}
 
#endif

#ifndef CLASS_A_H
#define CLASS_A_H

#include "class_B.h"

class A
{
    private:
        B b;
    //some code
}

#endif

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In the example in Qt class MainWindow derived from QMainWindow has QAction class member among others. But instead of including the QAction header the code is

QT_BEGIN_NAMESPACE
class QAction;
class QMenu;
class QTextEdit;
QT_END_NAMESPACE

QT_BEGIN_NAMESPACE
class QAction;
class QMenu;
class QTextEdit;
QT_END_NAMESPACE

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Will somebody explain to me what this code do exactly or where can I read it in details. I failed to find it in the Qt help. Why not just include all the headers? I know that namespace for instance can be a problem. When using for instance string from Standard template library even when you include the string header you need to write using std::string or to declare every string object with std:string sBuffer so that compiler can use the right type.

I would appreciate any help you can provide. Thanks in advance.