c++ - "Undefined struct" error when creating an object -

i'm working on c++ project links c library. c++ class follows:

extern "c" {     struct __struct1__;     struct __struct2__;     struct __struct3__;     struct __struct4__; }  namespace mynamespace {    class myclass : public parent {      public:           ....       private:           ....            __struct1__* s1;           __struct2__* s2;           struct __struct3__ s3;           struct __struct4__ s4;    }; } 

when creating pointers s1 , s2, ok. objects don't work well. following error generated:

error using undefined struct

how can create objects s3 , s4?

variable:

__struct1__ *s1 

is of type pointer (to structure, still pointer), compiler knows how memory on stack variable needs. in case of:

__struct3__ s3 

this variable of type __struct3__, without whole definition compiler not know size.