c++ - Oddity with multiple inheritance from two template parameters? -

#include <iostream>  using namespace std;  template <typename e1, typename e2> class mix : public e1, public e2 { public:     mix() : e1(1), e2(2)     {           // set nothing here         cerr << "this " << << " in mix" << endl;         print(cerr);     }      void print(ostream& os)     {         os << "e1: " << e1::e1 << ", e2: " << e2::e2 << endl;         // os << "e1: " << e1 << ", e2: " << e2 << endl; won't compile     } };  class element1 { public:     element1(unsigned int e) : e1(e)     {         cerr << "this " << << " in element1" << endl;     }      unsigned int e1; };  class element2 { public:     element2(unsigned int e) : e2(e)     {         cerr << "this " << << " in element2" << endl;     }      unsigned int e2; };   int main(int argc, char** argv) {    mix<element1, element2> m;  } 

now, since we're equally inheriting 2 template parameter classes, expect this same in 2 constructors, not case. here run log:

this 0x7fff6c04aa70 in element1 0x7fff6c04aa74 in element2 0x7fff6c04aa70 in mix e1: 1, e2: 2 

as can see, while this same in element1 , mix, not true element2. why that? also, expect have access e1 , e2 base classes. can explain behavior?

the element mix contains element1 , element2. these - perhaps implementation aligned - written after 1 in memory. if use mix element1, point first of 2 (with size of element1), if use element2 point second (with size of element2) , if use mix point base address, same element1s base address, has differenz size (at least size of element1 + size of element2).

edit: can verify outputting size too:

#include

using namespace std;  template <typename e1, typename e2> class mix : public e1, public e2 { public:     mix() : e1(1), e2(2)     {           // set nothing here         cerr << "this " << << " + " << sizeof(*this) << " in mix" << endl;         print(cerr);     }      void print(ostream& os)     {         os << "e1: " << e1::e1 << ", e2: " << e2::e2 << endl;         // os << "e1: " << e1 << ", e2: " << e2 << endl; won't compile     } };  class element1 { public:     element1(unsigned int e) : e1(e)     {         cerr << "this " << << " + " << sizeof(*this) << " in element1" << endl;     }      unsigned int e1; };  class element2 { public:     element2(unsigned int e) : e2(e)     {         cerr << "this " << << " + " << sizeof(*this) << " in element2" << endl;     }      unsigned int e2; };   int main(int argc, char** argv) {    mix<element1, element2> m;  } 

output:

this 0x7fffc9cad310 + 4 in element1 0x7fffc9cad314 + 4 in element2 0x7fffc9cad310 + 8 in mix e1: 1, e2: 2