::TCanvas vs ROOT::TCanvas

#include <TCanvas.h>
#include <ROOT/TCanvas.hxx>

using namespace ROOT;
TCanvas c; // which one?!

::TCanvas vs ROOT::TCanvas

// #include <TCanvas.h> - in PCH!
#include <ROOT/TCanvas.hxx>

using namespace ROOT;
TCanvas c; // which one?!

Both-World Interfaces

/// Hypothetical interface, taking both v6 and v7 objects
namespace ROOT {
template <...>
class TSomething {
public:
  void Write(::TDirectory&);  
  void Write(TDirectory&);  
};
}

Both-World Interfaces

/// Hypothetical interface, taking both v6 and v7 objects
namespace ROOT {
template <...>
class TSomething {
public:
  void Write(::TDirectory&);  
  void Write(TDirectory&);  
};
}

using namespace ROOT;
ROOT::TDirectory<...> d; // BOOM!
TSomething().Write(d);