Thread: why default assignment can NOT be used for const reference?

The following code is incorrect. Compiler says "The default argument given for parameter 1 in foo"
What's wrong?

Code:

class Base{
virtual void foo(string str ="" )=0;
};

class Derived:public Base{
void foo(string str ="" ){}
};

Anther question is that

Code:

void foo(const std::string& str = "")

The compiler says it's not allowed, why is that?