c++ - Where and how to define member variables? In header or implementation file?

Question

I am currently learning C++. I have practice (about 2 years) in Java (which I learned at my university).

I have problems understanding the concept of classes and member variables in C++. Given the following example:

File: Mems.h:

class Mems{

int n;
Mems();
};

File Mems.cpp:

class Mems{

Mems::Mems(){
    //Do something in constructor
}
};

I do not know, where I have to put my variables if I want them to stick to the object:

When i define them in the header-file I cant access them in the cpp File and vice versa.

Could you please give me a hint?

Answer 1

You don't need to re-declare the class in the .cpp file. You only need to implement its member functions:

#include "Mems.h"
#include <iostream> // only required for the std::cout, std::endl example

Mems::Mems() : n(42)  // n initialized to 42
{
  std::cout << "Mems default constructor, n = " << n << std::endl;    
}

Note that usually you want the default constructor to be public. Members are private by default in C++ classes, and public in structs.

class Mems
{
 public:
  Mems();
 private:
  int n;
};