An Inheritance Example with Dynamic Memory Allocation and Friends
To illustrate these ideas of inheritance and dynamic memory allocation, let’s integrate the baseDMA, lacksDMA, and hasDMA classes just discussed into a single example. Listing 13.14 is a header file for these classes. To what we’ve already discussed, it adds a friend function that illustrates how derived classes can access friends to a base class.
// dma.h -- inheritance and dynamic memory allocation
#ifndef DMA_H_
#define DMA_H_
#include <iostream>
// Base Class Using DMA
class baseDMA
{
private:
char * label;
int rating;
public:
baseDMA(const char * l = "null", int r = 0);
baseDMA(const baseDMA & rs);
virtual ~baseDMA();
baseDMA & operator=(const baseDMA & rs);
friend std::ostream & operator<<(std::ostream & os,
const baseDMA & rs);
};
// derived class without DMA
// no destructor needed
// uses implicit copy constructor
// uses implicit assignment operator
class lacksDMA :public baseDMA
{
private:
enum { COL_LEN = 40};
char color[COL_LEN];
public:
lacksDMA(const char * c = "blank", const char * l = "null",
int r = 0);
lacksDMA(const char * c, const baseDMA & rs);
friend std::ostream & operator<<(std::ostream & os,
const lacksDMA & rs);
};
// derived class with DMA
class hasDMA :public baseDMA
{
private:
char * style;
public:
hasDMA(const char * s = "none", const char * l = "null",
int r = 0);
hasDMA(const char * s, const baseDMA & rs);
hasDMA(const hasDMA & hs);
~hasDMA();
hasDMA & operator=(const hasDMA & rs);
friend std::ostream & operator<<(std::ostream & os,
const hasDMA & rs);
};
#endif
Listing 13.15 provides the method definitions for the baseDMA, lacksDMA, and hasDMA classes.
// dma.cpp --dma class methods
#include "dma.h"
#include <cstring>
// baseDMA methods
baseDMA::baseDMA(const char * l, int r)
{
label = new char[std::strlen(l) + 1];
std::strcpy(label, l);
rating = r;
}
baseDMA::baseDMA(const baseDMA & rs)
{
label = new char[std::strlen(rs.label) + 1];
std::strcpy(label, rs.label);
rating = rs.rating;
}
baseDMA::~baseDMA()
{
delete [] label;
}
baseDMA & baseDMA::operator=(const baseDMA & rs)
{
if (this == &rs)
return *this;
delete [] label;
label = new char[std::strlen(rs.label) + 1];
std::strcpy(label, rs.label);
rating = rs.rating;
return *this;
}
std::ostream & operator<<(std::ostream & os, const baseDMA & rs)
{
os << "Label: " << rs.label << std::endl;
os << "Rating: " << rs.rating << std::endl;
return os;
}
// lacksDMA methods
lacksDMA::lacksDMA(const char * c, const char * l, int r)
: baseDMA(l, r)
{
std::strncpy(color, c, 39);
color[39] = ' ';
}
lacksDMA::lacksDMA(const char * c, const baseDMA & rs)
: baseDMA(rs)
{
std::strncpy(color, c, COL_LEN - 1);
color[COL_LEN - 1] = ' ';
}
std::ostream & operator<<(std::ostream & os, const lacksDMA & ls)
{
os << (const baseDMA &) ls;
os << "Color: " << ls.color << std::endl;
return os;
}
// hasDMA methods
hasDMA::hasDMA(const char * s, const char * l, int r)
: baseDMA(l, r)
{
style = new char[std::strlen(s) + 1];
std::strcpy(style, s);
}
hasDMA::hasDMA(const char * s, const baseDMA & rs)
: baseDMA(rs)
{
style = new char[std::strlen(s) + 1];
std::strcpy(style, s);
}
hasDMA::hasDMA(const hasDMA & hs)
: baseDMA(hs) // invoke base class copy constructor
{
style = new char[std::strlen(hs.style) + 1];
std::strcpy(style, hs.style);
}
hasDMA::~hasDMA()
{
delete [] style;
}
hasDMA & hasDMA::operator=(const hasDMA & hs)
{
if (this == &hs)
return *this;
baseDMA::operator=(hs); // copy base portion
delete [] style; // prepare for new style
style = new char[std::strlen(hs.style) + 1];
std::strcpy(style, hs.style);
return *this;
}
std::ostream & operator<<(std::ostream & os, const hasDMA & hs)
{
os << (const baseDMA &) hs;
os << "Style: " << hs.style << std::endl;
return os;
}
The new feature to note in Listings 13.14 and 13.15 is how derived classes can make use of a friend to a base class. Consider, for example, the following friend to the hasDMA class:
friend std::ostream & operator<<(std::ostream & os,
const hasDMA & rs);
Being a friend to the hasDMA class gives this function access to the style member. But there’s a problem: This function is not a friend to the baseDMA class, so how can it access the label and rating members? The solution is to use the operator<<() function that is a friend to the baseDMA class. The next problem is that because friends are not member functions, you can’t use the scope-resolution operator to indicate which function to use. The solution to this problem is to use a type cast so that prototype matching will select the correct function. Thus, the code type casts the type const hasDMA & parameter to a type const baseDMA & argument:
std::ostream & operator<<(std::ostream & os, const hasDMA & hs)
{
// type cast to match operator<<(ostream & , const baseDMA &)
os << (const baseDMA &) hs;
os << "Style: " << hs.style << endl;
return os;
}
Listing 13.16 tests the baseDMA, lacksDMA, and hasDMA classes in a short program.
// usedma.cpp -- inheritance, friends, and DMA
// compile with dma.cpp
#include <iostream>
#include "dma.h"
int main()
{
using std::cout;
using std::endl;
baseDMA shirt("Portabelly", 8);
lacksDMA balloon("red", "Blimpo", 4);
hasDMA map("Mercator", "Buffalo Keys", 5);
cout << "Displaying baseDMA object:
";
cout << shirt << endl;
cout << "Displaying lacksDMA object:
";
cout << balloon << endl;
cout << "Displaying hasDMA object:
";
cout << map << endl;
lacksDMA balloon2(balloon);
cout << "Result of lacksDMA copy:
";
cout << balloon2 << endl;
hasDMA map2;
map2 = map;
cout << "Result of hasDMA assignment:
";
cout << map2 << endl;
return 0;
}
Here’s the output of the program in Listings 13.14, 13.15, and 13.16:
Displaying baseDMA object:
Label: Portabelly
Rating: 8
Displaying lacksDMA object:
Label: Blimpo
Rating: 4
Color: red
Displaying hasDMA object:
Label: Buffalo Keys
Rating: 5
Style: Mercator
Result of lacksDMA copy:
Label: Blimpo
Rating: 4
Color: red
Result of hasDMA assignment:
Label: Buffalo Keys
Rating: 5
Style: Mercator
Class Design Review
C++ can be applied to a wide variety of programming problems, and you can’t reduce class design to some paint-by-numbers routine. However, there are some guidelines that often apply, and this is as good a time as any to go over them by reviewing and amplifying earlier discussions.