Class Code And Header Files

Defining member functions outside the class definition

  • C++ provides a way to separate the definition portion of the class from the implementation portion.
  • This is done by defining the class member functions outside the class definition.

Example

class Date
{
private:
    int m_nMonth;
    int m_nDay;
    int m_nYear;
 
    Date() { } // private default constructor
 
public:
    Date(int nMonth, int nDay, int nYear)
    {
        SetDate(nMonth, nDay, nYear);
    }
 
    void SetDate(int nMonth, int nDay, int nYear)
    {
        m_nMonth = nMonth;
        m_nDay = nDay;
        m_nYear = nYear;
    }
 
    int GetMonth() { return m_nMonth; }
    int GetDay()  { return m_nDay; }
    int GetYear() { return m_nYear; }
};

Could be rewritten

class Date
{
private:
    int m_nMonth;
    int m_nDay;
    int m_nYear;
 
    Date() { } // private default constructor
 
public:
    Date(int nMonth, int nDay, int nYear);
 
    void SetDate(int nMonth, int nDay, int nYear);
 
    int GetMonth() { return m_nMonth; }
    int GetDay()  { return m_nDay; }
    int GetYear() { return m_nYear; }
};
 
// Date constructor
Date::Date(int nMonth, int nDay, int nYear)
{
    SetDate(nMonth, nDay, nYear);
}
 
// Date member function
void Date::SetDate(int nMonth, int nDay, int nYear)
{
    m_nMonth = nMonth;
    m_nDay = nDay;
    m_nYear = nYear;
}

Putting class definitions in a header file

  • Class definitions can be put in header files in order to facilitate reuse in multiple files.

Example
Date.h

#ifndef DATE_H
#define DATE_H
 
class Date
{
private:
    int m_nMonth;
    int m_nDay;
    int m_nYear;
 
    Date() { } // private default constructor
 
public:
    Date(int nMonth, int nDay, int nYear);
 
    void SetDate(int nMonth, int nDay, int nYear);
 
    int GetMonth() { return m_nMonth; }
    int GetDay()  { return m_nDay; }
    int GetYear() { return m_nYear; }
};
 
#endif

Date.cpp

#include "Date.h"
 
// Date constructor
Date::Date(int nMonth, int nDay, int nYear)
{
    SetDate(nMonth, nDay, nYear);
}
 
// Date member function
void Date::SetDate(int nMonth, int nDay, int nYear)
{
    m_nMonth = nMonth;
    m_nDay = nDay;
    m_nYear = nYear;
}

Now any other header or code file that wants to use the date class can simply #include "Date.h"

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