c++ - Class declarations in header files -

ive been using c++ months ive come across error when use header , source code files. create source code file contains class gun(example not actual program):

class gun { private: int stuff; public: void dostuff(); };   void gun::dostuff() { cout << stuff << endl; } 

and created header file , declared class this:

class gun; 

then in main source file this:

int main() { gun *mygun = new gun; mygun->dostuff(); return 0; } 

however when try execute error: invalid use of incomplete type 'class gun' , think problem how declared in header, did wrong? how meant it? thanks.

thanks helped! understand now, thought forward declaration went header file, answers!

you seem going seperating implementation , header file wrong way. forward declarations should not go in header file. entire declaration should! how code should structured

gun.hpp

#pragma once  class gun { private:     int stuff; public:     void dostuff(); }; 

gun.cpp

#include "gun.hpp" #include <iostream> using std::cout; using std::endl;  void gun::dostuff() {     cout << stuff << endl; } 

main.cpp

int main() {     gun *mygun = new gun;     mygun->dostuff();      delete mygun; // <-- remember this!     return 0; } 

the separation of header , implementation crucial in c++ , other languages! should declare class in header along full interface (as above) , include implementation details in .cpp file (as above :)