Separating class code into a header and cpp file

Question

I am confused on how to separate implementation and declarations code of a simple class into a new header and cpp file  For example  how would I separate the code for the following class   class A2DD     private    int gx    int gy     public    A2DD int x int y          gx   x      gy   y         int getSum           return gx   gy

User · Answer

Basically a modified syntax of function declaration/definitions:

a2dd.h

class A2DD
{
private:
  int gx;
  int gy;

public:
  A2DD(int x,int y);

  int getSum();
};

a2dd.cpp

A2DD::A2DD(int x,int y)
{
  gx = x;
  gy = y;
}

int A2DD::getSum()
{
  return gx + gy;
}

User · Answer

In general your  h contains the class defition  which is all your data and all your method declarations  Like this in your case   A2DD h   class A2DD     private    int gx    int gy     public    A2DD int x int y         int getSum         And then your  cpp contains the implementations of the methods like this   A2DD cpp   A2DD  A2DD int x int y      gx   x    gy   y     int A2DD  getSum       return gx   gy

User · Answer

The class declaration goes into the header file  It is important that you add the  ifndef include guards  or if you are on a MS platform you also can use  pragma once  Also I have omitted the private  by default C   class members are private      A2DD h  ifndef A2DD H  define A2DD H  class A2DD     int gx    int gy   public    A2DD int x int y     int getSum          endif   and the implementation goes in the CPP file      A2DD cpp  include  A2DD h   A2DD  A2DD int x int y      gx   x    gy   y     int A2DD  getSum       return gx   gy

User · Answer

A2DD h  class A2DD     private    int gx    int gy     public    A2DD int x int y      int getSum         A2DD cpp    A2DD  A2DD int x int y          gx   x      gy   y         int A2DD  getSum           return gx   gy        The idea is to keep all function signatures and members in the header file  This will allow other project files to see how the class looks like without having to know the implementation   And besides that  you can then include other header files in the implementation instead of the header  This is important because whichever headers are included in your header file will be included  inherited  in any other file that includes your header file

User · Answer

I won t refer too your example as it is quite simple for a general answer  for example it doesn t contain templated functions  which force you to implement them on the header    what I follow as a rule of thumb is the pimpl idiom  It has quite some benefits as you get faster compilation times and the syntactic sugar    class- gt member  instead of class member   The only drawback is the extra pointer you pay

User · Answer

It s important to point out to readers stumbling upon this question when researching the subject in a broader fashion that the accepted answer s procedure is not required in the case you just want to split your project into files  It s only needed when you need multiple implementations of single classes  If your implementation per class is one  just one header file for each is enough   Hence  from the accepted answer s example only this part is needed    ifndef MYHEADER H  define MYHEADER H    Class goes here  full declaration AND implementation   endif   The  ifndef etc  preprocessor definitions allow it to be used multiple times   PS  The topic becomes clearer once you realize C C   is  dumb  and  include is merely a way to say  dump this text at this spot

User · Answer

You leave the declarations in the header file   class A2DD     private    int gx    int gy     public      A2DD int x int y      leave the declarations here     int getSum         And put the definitions in the implementation file   A2DD  A2DD int x int y     prefix the definitions with the class name     gx   x    gy   y     int A2DD  getSum       return gx   gy      You could mix the two  leave getSum   definition in the header for instance   This is useful since it gives the compiler a better chance at inlining for example  But it also means that changing the implementation  if left in the header  could trigger a rebuild of all the other files that include the header   Note that for templates  you need to keep it all in the headers

User · Answer

Usually you put only declarations and really short inline functions in the header file   For instance   class A    public    A       only declaration in the  h unless only a short initialization list is used     inline int GetA   const       return a          void DoSomethingCoplex       only declaration   private     int a

[c++] Separating class code into a header and cpp file

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