Regarding C Include another class

Question

I have two files   File1 cpp File2 cpp   File1 is my main class which has the main method  File2 cpp has a class call ClassTwo and I want to create an object of ClassTwo in my File1 cpp  I compile them together by  g   -o myfile File1 cpp File2 cpp   but when I try to create by    create class two object  ClassTwo ctwo    It doesn t work   Error was  ClassTwo was not declared in this scope   This is my main cpp   include  lt iostream gt   include  lt string gt  using namespace std   int main       here compile error - undeclare ClassTwo in scope  ClassTwo ctwo     some codes     Here is my File2 cpp   include  lt iostream gt   include  lt string gt   using namespace std   class ClassTwo   private  string myType  public  void setType string   string getType         void ClassTwo  setType string sType    myType   sType     void ClassTwo  getType float fVal    return myType      Got respond of splitting my File2 cpp into another  h file but if i am declaring a class  how do i split it into another  h file as i need to maintain the public and private of the variable private  and functions public  and how do i get ClassTwo ctwo to my File1 cpp at main method

User · Accepted Answer

What is the basic problem in your code    Your code needs to be separated out in to interfaces  h  and Implementations  cpp   The compiler needs to see the composition of a type when you write something like  ClassTwo obj    This is because the compiler needs to reserve enough memory for object of type ClassTwo to do so it needs to see the definition of ClassTwo  The most common way to do this in C   is to split your code in to header files and source files  The class definitions go in the header file while the implementation of the class goes in to source files  This way one can easily include header files in to other source files which need to see the definition of class who s object they create             Why can t I simply put all code in cpp files and include them in other files       You cannot simple put all the code in source file and then include that source file in other files C   standard mandates that you can declare a entity as many times as you need but you can define it only once One Definition Rule ODR    Including the source file would violate the ODR because a copy of the entity is created in every translation unit where the file is included      How to solve this particular problem    Your code should be organized as follows     File1 h  Define ClassOne      File2 h         include  lt iostream gt   include  lt string gt    class ClassTwo   private     string myType  public     void setType string      std  string getType            File1 cpp   include File1 h   Implementation of ClassOne      File2 cpp   include File2 h   void ClassTwo  setType std  string sType        myType   sType     void ClassTwo  getType float fVal        return myType         main cpp   include  lt iostream gt   include  lt string gt   include  file1 h   include  file2 h  using namespace std   int main          ClassOne cone      ClassTwo ctwo         some codes        Is there any alternative means rather than including header files    If your code only needs to create pointers and not actual objects you might as well use Forward Declarations but note that using forward declarations adds some restrictions on how that type can be used because compiler sees that type as an Incomplete type

User · Answer

When you want to convert your code to result  executable  library or whatever    there is 2 steps  1  compile 2  link In first step compiler should now about some things like sizeof objects that used by you  prototype of functions and maybe inheritance  on the other hand linker want to find implementation of functions and global variables in your code   Now when you use ClassTwo in File1 cpp compiler know nothing about it and don t know how much memory should allocate for it or for example witch members it have or is it a class and enum or even a typedef of int  so compilation will be failed by the compiler  adding File2 cpp solve the problem of linker that look for implementation but the compiler is still unhappy  because it know nothing about your type   So remember  in compile phase you always work with just one file  and of course files that included by that one file   and in link phase you need multiple files that contain implementations  and since C C   are statically typed and they allow their identifier to work for many purposes  definition  typedef  enum class        so you should always identify you identifier to the compiler and then use it and as a rule compiler should always know size of your variable

User · Answer

C    and C for that matter  split the  declaration  and the  implementation  of types  functions and classes  You should  declare  the classes you need in a header-file   h or  hpp   and put the corresponding implementation in a  cpp-file  Then  when you wish to use  access  a class somewhere  you  include the corresponding headerfile   Example  ClassOne hpp   class ClassOne   public    ClassOne       note  no function body           int method       no body here either private    int member       ClassOne cpp    include  ClassOne hpp      implementation of constructor ClassOne  ClassOne     member 0         implementation of  method  int ClassOne  method       return member        main cpp    include  ClassOne hpp     Bring the ClassOne declaration into  view  of the compiler  int main int argc  char  argv        ClassOne c1    c1 method       return 0

User · Answer

The thing with compiling two  cpp files at the same time  it doesnt t mean they  know  about eachother  You will have to create a file  the  tells  your File1 cpp  there actually are functions and classes like ClassTwo  This file is called header-file and often doesn t include any executable code   There are exception  e g  for inline functions  but forget them at first  They serve a declarative need  just for telling  which functions are available   When you have your File2 cpp and include it into your File1 cpp  you see a small problem  There is the same code twice  One in the File1 cpp and one in it s origin  File2 cpp   Therefore you should create a header file  like File1 hpp or File1 h  other names are possible  but this is simply standard   It works like the following     File1 cpp  void SomeFunc char c    Definition aka Implementation     do some stuff       File1 hpp  void SomeFunc char c     Declaration aka Prototype     And for a matter of clean code you might add the following to the top of File1 cpp    include  File1 hpp    And the following  surrounding File1 hpp s code    ifndef FILE1 HPP INCLUDED  define FILE1 HPP INCLUDED      All your declarative code     endif   This makes your header-file cleaner  regarding to duplicate code

User · Answer

you need to forward declare the name of the class if you don t want a header   class ClassTwo    Important  This only works in some cases  see Als s answer for more information

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