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Message: [QUOTE="toton, post: 2577183"] Hi, I have two namespace contains class InkFrame and PrefDialog respectively. PrefDialog needs InkFrame to show the dialog over the frame. It also stores a pointer to InkFrame inside it. Now I want InkFrame to be forward declared in the PrefDialog header file rather than to be included. I want to include it in the cpp file instead. There is no harm including it in PrefDialog header, and it works. But I want to save some compilation time. That is why I thought to forward declare it (and all the other classes whenever possible) instead of include. They works perfectly if both of them are from same namespace. But fails to work if they are from different namespace. To make it little clear, the headers are InkFrame.hpp => namespace ui{ class InkFrame : public Frame{ ... } ; } here Frame, is a library class and necessary headers are already there. PrefDialog.hpp => #include "InkFrame.hpp" //and other necessary headers. using ui::InkFrame namespace client{ namespace ui{ class PrefDialog : public Dialog{ InkFrame* _parent; public: PrefDialog(InkFrame* parent); }; } } This code works. Next I removed include & have forward declaration. The include and using statement is shifted to cpp file for PrefDialog. PrefDialog.hpp => //#include "InkFrame.hpp" //and other necessary headers. //using ui::InkFrame namespace client{ namespace ui{ class InkFrame; //forward declaration. class PrefDialog : public Dialog{ InkFrame* _parent; public: PrefDialog(InkFrame* parent); }; } } Here, the code assumes InkFrame is from client::ui (which is quite reasonable, I think) and fails to compile the code as cpp gets it from ui namespace. declaring at top class ui::InkFrame also do not work, as the ui namespace is not defined here! also declaring like namespace ui{ class InkFrame; } at the top is not working. These two cases the header do not see the declaration even, while in the firest case it see and find a mismatch at the cpp. Thus, what is the common way to forward declare a class from a different namespace? Thanks for any advice. abir [/QUOTE]

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