Derived classes may access protected members of their base class only on themselves or other instances of the same type or derived type. Here is a link to the relevant section in the draft of the C++ standard:
http://www.csci.csusb.edu/dick/c++std/cd2/access.html#class.protected I don't have access to the actual standard, but I figure you do :-). The bug occurs when the protected member is a template member function. If the protected member is a simple data member the code fails to compile as it should. BTW, VC++ 2005 catches this bug and refuses to compile the test program. Test Program (bug.cpp) ---------------------- struct B { protected: template <typename T> T getX() { return T(x); } protected: int x; }; struct D : public B { D(B * b) { //x = b->x; // fails to compile (correct) x = b->getX<int>(); // doesn't fail to compile (bug) } }; int main (int argc, char * const argv[]) { B b; D d(&b); } Output of gcc -v ---------------- Using built-in specs. Target: x86_64-unknown-linux-gnu Configured with: ../src/gcc-4.0.2/configure : (reconfigured) ../src/gcc-4.0.2/configure Thread model: posix gcc version 4.0.2 Build command ------------- g++ -Wall bug.cpp -o bug -- Summary: g++ allows access to protected template member functions of base class Product: gcc Version: 4.0.2 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: gsayfan at numenta dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32519
