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Thinking in C++
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Composition vs. inheritance (revisited)

One of the clearest ways to determine whether you should be using composition or inheritance is by asking whether you’ll ever need to upcast from your new class. Earlier in this chapter, the Stack class was specialized using inheritance. However, chances are the StringStack objects will be used only as string containers and never upcast, so a more appropriate alternative is composition:

//: C14:InheritStack2.cpp
// Composition vs. inheritance
#include "../C09/Stack4.h"
#include "../require.h"
#include <iostream>
#include <fstream>
#include <string>
using namespace std;

class StringStack {
  Stack stack; // Embed instead of inherit
  void push(string* str) {
  string* peek() const {
    return (string*)stack.peek();
  string* pop() {
    return (string*)stack.pop();

int main() {
  ifstream in("InheritStack2.cpp");
  assure(in, "InheritStack2.cpp");
  string line;
  StringStack textlines;
  while(getline(in, line))
    textlines.push(new string(line));
  string* s;
  while((s = textlines.pop()) != 0) // No cast!
    cout << *s << endl;
} ///:~

The file is identical to InheritStack.cpp, except that a Stack object is embedded in StringStack, and member functions are called for the embedded object. There’s still no time or space overhead because the subobject takes up the same amount of space, and all the additional type checking happens at compile time.

Although it tends to be more confusing, you could also use private inheritance to express “implemented in terms of.” This would also solve the problem adequately. One place it becomes important, however, is when multiple inheritance might be warranted. In that case, if you see a design in which composition can be used instead of inheritance, you may be able to eliminate the need for multiple inheritance.

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire