What's effect of QT_BEGIN_NAMESPACE? Thanks

[mihayinoviki]

What's effect of QT_BEGIN_NAMESPACE/QT_END_NAMESPACE pair? Thanks

[jpn]

It is possible to compile whole Qt inside a user-defined namespace. See configure options for more details. These macros expand as:

# define QT_BEGIN_NAMESPACE namespace QT_NAMESPACE {
# define QT_END_NAMESPACE }

# define QT_BEGIN_NAMESPACE namespace QT_NAMESPACE {
# define QT_END_NAMESPACE }

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[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)

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and

QT_BEGIN_NAMESPACE
class QTableView;
QT_END_NAMESPACE

QT_BEGIN_NAMESPACE
class QTableView;
QT_END_NAMESPACE

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and

class QTableView;

class QTableView;

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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)

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Expands to:

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

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

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2)

QT_BEGIN_NAMESPACE
class QTableView;
QT_END_NAMESPACE

QT_BEGIN_NAMESPACE
class QTableView;
QT_END_NAMESPACE

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Expands to:

namespace Foo {
class QTableView;
}

namespace Foo {
class QTableView;
}

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3)

class QTableView;

class QTableView;

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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;

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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.