How to handle namespaces in C++?

Question

I have created a DialogBox in QtDesigner

//---------- *.H
namespace Ui {
   class MyDialog;
}

class MyDialog : public QDialog {
   Q_OBJECT
public:
   explicit MyDialog(QWidget* parent = 0);
   ~MyDialog();

 private:
    Ui::MyDialog* ui;  
};

and its source

// --------- *.CPP
MyDialog::MyDialog(QWidget* parent = 0) :
    QDialog(parent),
    ui(new Ui::MyDialog)
{
    ui->setupUi(this);
    //..
}

I would like to structure that in a certain namespace encapsulation, so that I have com::example::mydialogs::MyDialog like:

//---------- *.H
namespace com {
namespace example {
namespace mydialogs {

namespace Ui {
   class MyDialog;
}

class MyDialog : public QDialog {
   Q_OBJECT
public:
   explicit MyDialog(QWidget* parent = 0);
   ~MyDialog();

 private:
    Ui::MyDialog* ui;  
};
}}} //namespace closing

and its source

// --------- *.CPP
namespace com {
namespace example {
namespace mydialogs {

MyDialog::MyDialog(QWidget* parent = 0) :
    QDialog(parent),
    ui(new Ui::MyDialog)
{
    ui->setupUi(this);
    //..
}
}}} //namespace closing

but my compiler complains about that with a message:

C:\MyProjects\Test\com\example\mydialogs\MyDialog.h:29: Error: forward declaration of 'class com::example::mydialogs::Ui::MyDialog' class MyDialog;

The namespace Ui was added by Qt automatically. How can I use my namespace structure properly?

Answer 1

In order to forward declare a namespace, you need to use the following canonical syntax:

namespace ns1
{
   namespace ns2
   {
   //....
     namespace nsN
     {
     class a;
     }
  //....    
  }
}

The above code snippet was taken from here. So in your case, you need to close the namespaces where the forward declaration is.

namespace com {
namespace example {
namespace mydialogs {

namespace Ui {
   class MyDialog;
}

} // mydialogs
} // example
} // com

namespace com {
namespace example {
namespace mydialogs {

class MyDialog : public QDialog {
   Q_OBJECT
public:
   explicit MyDialog(QWidget* parent = 0);
   ~MyDialog();

private:
    Ui::MyDialog* ui;  
};
}}} //namespace closing

Your implementation code looks correct. Make those slight changes to the header file and it should compile.

I have not tested this code. Use at your own risk.

Answer 2

Just leave outside Ui::MyDialog

namespace Ui {
   class MyDialog;
}

namespace com {
namespace example {
namespace mydialogs {
...

Because the forward declaration Ui::MyDialog is created by uic tool when processing your ui file, it cannot being included in your declaration.