Error passing shared_ptr<Derived>& as shared_ptr<Base>& without const

I got compile error when passing shared_ptr<Derived>& as shared_ptr<Base>&, see the below code and detailed question.

Note: this question is similar to "Passing shared_ptr<Derived> as shared_ptr<Base>" but not duplicate.

#include <memory> class TBase { public: virtual ~TBase() {} }; class TDerived : public TBase { public: virtual ~TDerived() {} }; void FooRef(std::shared_ptr<TBase>& b) { // Do something } void FooConstRef(const std::shared_ptr<TBase>& b) { // Do something } void FooSharePtr(std::shared_ptr<TBase> b) { // Do something } int main() { std::shared_ptr<TDerived> d; FooRef(d); // *1 Error: invalid initialization of reference of type ‘std::shared_ptr<TBase>&’ from expression of type ‘std::shared_ptr<TDerived>’ FooConstRef(d); // *2 OK, just pass by const reference FooSharePtr(d); // *3 OK, construct a new shared_ptr<> return 0; } 

Compiled by g++ -std=c++11 -o shared_ptr_pass_by_ref shared_ptr_pass_by_ref.cpp

Env: Ubuntu 14.04, g++ (Ubuntu 4.8.2-19ubuntu1) 4.8.2

Detailed question: Why is it OK to pass by const reference (*2), but not OK to pass by reference (*1)?

Note: I know the best practice is to pass by const reference, but just want to know why the compile error occurs.