must explicitly initialize the base class

#import <string>


class A
{
};

class B
{
};

class C
{
public:
    C(const std::string& inS);
    C(const std::string& inS, A* inA);

    virtual ~C();
};




class D : public virtual C, public virtual B
{
public:
    D(const std::string& inS, B* inB)
        :
        C(inS)
    {
    }
};




class E : public virtual D, public virtual A
{
public:
    E(const std::string& inS, B* inB)
        :
        D(inS, inB)
    {
    }
};


-----------------

Why does the above code get this error:


$ clang Test.cpp
Test.cpp:40:5: error: constructor for 'E' must explicitly initialize the base class 'C' which does not have a default constructor
    E(const std::string& inS, B* inB)
    ^
Test.cpp:12:7: note: 'C' declared here
class C
      ^
1 error generated.