C how to declare a struct in a header file

Question

I ve been trying to include a structure called  student  in a student h file  but I m not quite sure how to do it   My student h file code consists of entirely    include lt string gt  using namespace std   struct Student    while the student cpp file consists of entirely    include lt string gt  using namespace std   struct Student       string lastName  firstName        long list of other strings    just strings though      Unfortunately  files that use  include  student h  come up with numerous errors like   error C2027  use of undefined type  Student   error C2079   newStudent  uses undefined struct  Student    where newStudent is a function with a  Student  parameter   error C2228  left of   lastName  must have class struct union    It appears the compiler  VC    does not recognize struct Student from  student h    How can I declare struct Student in  student h  so that I can just  include  student h  and start using the struct

User · Accepted Answer

You should not place an using directive in an header file  it creates unnecessary headaches   Also you need an include guard in your header   EDIT  of course  after having fixed the include guard issue  you also need a complete declaration of student in the header file  As pointed out by others the forward declaration is not sufficient in your case

User · Answer

You can t    In order to  use  the struct  i e  to be able to declare objects of that type and to access its internals you need the full definition of the struct  So  it you want to do any of that  and you do  judging by your error messages   you have to place the full definition of the struct type into the header file

User · Answer

Okay so three big things I noticed   You need to include the header file in your class file Never  EVER place a using directive inside of a header or class  rather do something like std  cout  lt  lt   say stuff   Structs are completely defined within a header  structs are essentially classes that default to public   Hope this helps

User · Answer

Try this new source    student h   include  lt iostream gt   struct Student       std  string lastName      std  string firstName       student cpp   include  student h   struct Student student

User · Answer

Your student h file only forward declares a struct named  Student   it does not define one   This is sufficient if you only refer to it through reference or pointer   However  as soon as you try to use it  including creating one  you will need the full definition of the structure   In short  move your struct Student          into the  h file and use the  cpp file for implementation of member functions  which it has none so you don t need a  cpp file

User · Answer

C    how to declare a struct in a header file   Put this in a file called main cpp    include  lt cstdlib gt   include  lt iostream gt   include  student h   using namespace std       Watchout for clashes between std and other libraries  int main int argc  char   argv        struct Student s1      s1 firstName    fred   s1 lastName    flintstone       cout  lt  lt  s1 firstName  lt  lt       lt  lt  s1 lastName  lt  lt  endl      return 0      put this in a file named student h   ifndef STUDENT H  define STUDENT H   include lt string gt  struct Student       std  string lastName  firstName       endif   Compile it and run it  it should produce this output   s1 firstName    fred     Protip   You should not place a using namespace std  directive in the C   header file because you may cause silent name clashes between different libraries   To remedy this  use the fully qualified name   std  string foobarstring  instead of including the std namespace with string foobarstring

User · Answer

You ve only got a forward declaration for student in the header file  you need to place the struct declaration in the header file  not the  cpp   The method definitions will be in the  cpp  assuming you have any

[c++] C++, how to declare a struct in a header file

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