What's effect of QT_BEGIN_NAMESPACE? Thanks

[mihayinoviki]

Thank you!

[Antebios]

I still don't get it. This is one of those things in Qt that is not documented well.

[jpn]

It is possible to compile Qt inside any given namespace. See configure -help for more details. These macros ensure that Qt classes are properly declared inside the given namespace if any.

[PaceyIV]

What are the difference between

QT_FORWARD_DECLARE_CLASS(QTableView)

QT_FORWARD_DECLARE_CLASS(QTableView)

To copy to clipboard, switch view to plain text mode 

and

QT_BEGIN_NAMESPACE
class QTableView;
QT_END_NAMESPACE

QT_BEGIN_NAMESPACE
class QTableView;
QT_END_NAMESPACE

To copy to clipboard, switch view to plain text mode 

and

class QTableView;

class QTableView;

To copy to clipboard, switch view to plain text mode 

in a header file of a QtClass (like a MainWindow)?

[jpn]

Let's say one configures Qt with -qtnamespace Foo.

1)

QT_FORWARD_DECLARE_CLASS(QTableView)

QT_FORWARD_DECLARE_CLASS(QTableView)

To copy to clipboard, switch view to plain text mode 

Expands to:

namespace Foo { class QTableView; }
using ::Foo::QTableView;

namespace Foo { class QTableView; }
using ::Foo::QTableView;

To copy to clipboard, switch view to plain text mode 

2)

QT_BEGIN_NAMESPACE
class QTableView;
QT_END_NAMESPACE

QT_BEGIN_NAMESPACE
class QTableView;
QT_END_NAMESPACE

To copy to clipboard, switch view to plain text mode 

Expands to:

namespace Foo {
class QTableView;
}

namespace Foo {
class QTableView;
}

To copy to clipboard, switch view to plain text mode 

3)

class QTableView;

class QTableView;

To copy to clipboard, switch view to plain text mode 

Will cause a conflict because QTableView is forward declared outside the appropriate namespace, whereas including <QTableView> in the .cpp file brings in another QTableView declared inside the namespace and <QtGlobal> says

using namespace ::Foo;

using namespace ::Foo;

To copy to clipboard, switch view to plain text mode 

in order to make client code compile regardless of the Qt namespace usage. So QTableView becomes ambiguous and the compiler doesn't know which one to use, QTableView or Foo::QTableView.

Summary: Plain "class QTableView;" works as long as Qt is not compiled in a namespace. Using the macros will make your code more compatible. As far as I remember, for example the Eclipse integration requires Qt to be compiled in a certain namespace.

[Rockem]

how do you configure that option ?
where do I put it ? inside pro file ?

thanx

[faldzip]

just go to the QTDIR and run configure with -help option for deatails. You declare namespace name while use configure before building Qt

[Rockem]

oh .. so this means I will have to compile QT separate for every project ?

[nightghost]

if you want qt for every project in another namespace, then yes.