problem with virtual function in a .h

[mickey]

Hello,
I attach you the class; It works now but I want understand where exactly is the origin of the problem...
If I do the same thing inside a .cpp, the "inline" keyword isn't necessary; then, I don't understand the compiler message (.net) ("already defined in main.obj")

error LNK2005: "public: virtual struct Tree::Node * __thiscall Tree::getRoot(void)const " (?getRoot@Tree@@UBEPAUNode@1@XZ) already defined in main.obj

error LNK2005: "public: virtual struct Tree::Node * __thiscall Tree::getRoot(void)const " (?getRoot@Tree@@UBEPAUNode@1@XZ) already defined in main.obj

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Thanks