Regarding error thrown by sizeof(class) usage in C++ -

when compile project in c++, following error thrown msvc :

error #94: size of array must greater zero

the error thrown in following line on doing sizeof :

if (sizeof (mynamespace::myclass) == 60) 

myclass defined :

class myclass: public parentclass {     public:         myclass( void *pcreate, int a, int b, bool c) :               parentclass( pcreate, a, b, c ) {}          virtual inline void myfunc ( )          {             //something         }     private:         virtual ~myclass(){};          /**         * copy assignment. intentionally made private , not implemented prohibit usage (noncopyable stereotype)         */         myclass& operator=(const myclass&); }; 

can tell me might wrong? if sizeof returns 0 size, why compiler error?

this error caused when take sizeof of class that's declared @ point. e.g. class myclass; const size_t error = sizeof(myclass);.

note doesn't matter whether class defined later: definition must precede sizeof.