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Thinking in C++
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Nested structures

The convenience of taking data and function names out of the global name space extends to structures. You can nest a structure within another structure, and therefore keep associated elements together. The declaration syntax is what you would expect, as you can see in the following structure, which implements a push-down stack as a simple linked list so it “never” runs out of memory:

//: C04:Stack.h
// Nested struct in linked list
#ifndef STACK_H
#define STACK_H

struct Stack {
  struct Link {
    void* data;
    Link* next;
    void initialize(void* dat, Link* nxt);
  }* head;
  void initialize();
  void push(void* dat);
  void* peek();
  void* pop();
  void cleanup();
#endif // STACK_H ///:~

The nested struct is called Link, and it contains a pointer to the next Link in the list and a pointer to the data stored in the Link. If the next pointer is zero, it means you’re at the end of the list.

Notice that the head pointer is defined right after the declaration for struct Link, instead of a separate definition Link* head. This is a syntax that came from C, but it emphasizes the importance of the semicolon after the structure declaration; the semicolon indicates the end of the comma-separated list of definitions of that structure type. (Usually the list is empty.)

The nested structure has its own initialize( ) function, like all the structures presented so far, to ensure proper initialization. Stack has both an initialize( ) and cleanup( ) function, as well as push( ), which takes a pointer to the data you wish to store (it assumes this has been allocated on the heap), and pop( ), which returns the data pointer from the top of the Stack and removes the top element. (When you pop( ) an element, you are responsible for destroying the object pointed to by the data.) The peek( ) function also returns the data pointer from the top element, but it leaves the top element on the Stack.

Here are the definitions for the member functions:

//: C04:Stack.cpp {O}
// Linked list with nesting
#include "Stack.h"
#include "../require.h"
using namespace std;

Stack::Link::initialize(void* dat, Link* nxt) {
  data = dat;
  next = nxt;

void Stack::initialize() { head = 0; }

void Stack::push(void* dat) {
  Link* newLink = new Link;
  newLink->initialize(dat, head);
  head = newLink;

void* Stack::peek() { 
  require(head != 0, "Stack empty");
  return head->data; 

void* Stack::pop() {
  if(head == 0) return 0;
  void* result = head->data;
  Link* oldHead = head;
  head = head->next;
  delete oldHead;
  return result;

void Stack::cleanup() {
  require(head == 0, "Stack not empty");
} ///:~

The first definition is particularly interesting because it shows you how to define a member of a nested structure. You simply use an additional level of scope resolution to specify the name of the enclosing struct. Stack::Link::initialize( ) takes the arguments and assigns them to its members.

Stack::initialize( ) sets head to zero, so the object knows it has an empty list.

Stack::push( ) takes the argument, which is a pointer to the variable you want to keep track of, and pushes it on the Stack. First, it uses new to allocate storage for the Link it will insert at the top. Then it calls Link’s initialize( ) function to assign the appropriate values to the members of the Link. Notice that the next pointer is assigned to the current head; then head is assigned to the new Link pointer. This effectively pushes the Link in at the top of the list.

Stack::pop( ) captures the data pointer at the current top of the Stack; then it moves the head pointer down and deletes the old top of the Stack, finally returning the captured pointer. When pop( ) removes the last element, then head again becomes zero, meaning the Stack is empty.

Stack::cleanup( ) doesn’t actually do any cleanup. Instead, it establishes a firm policy that “you (the client programmer using this Stack object) are responsible for popping all the elements off this Stack and deleting them.” The require( ) is used to indicate that a programming error has occurred if the Stack is not empty.

Why couldn’t the Stack destructor be responsible for all the objects that the client programmer didn’t pop( )? The problem is that the Stack is holding void pointers, and you’ll learn in Chapter 13 that calling delete for a void* doesn’t clean things up properly. The subject of “who’s responsible for the memory” is not even that simple, as we’ll see in later chapters.

Here’s an example to test the Stack:

//: C04:StackTest.cpp
//{L} Stack
//{T} StackTest.cpp
// Test of nested linked list
#include "Stack.h"
#include "../require.h"
#include <fstream>
#include <iostream>
#include <string>
using namespace std;

int main(int argc, char* argv[]) {
  requireArgs(argc, 1); // File name is argument
  ifstream in(argv[1]);
  assure(in, argv[1]);
  Stack textlines;
  string line;
  // Read file and store lines in the Stack:
  while(getline(in, line))
    textlines.push(new string(line));
  // Pop the lines from the Stack and print them:
  string* s;
  while((s = (string*)textlines.pop()) != 0) {
    cout << *s << endl;
    delete s; 
} ///:~

This is similar to the earlier example, but it pushes lines from a file (as string pointers) on the Stack and then pops them off, which results in the file being printed out in reverse order. Note that the pop( ) member function returns a void* and this must be cast back to a string* before it can be used. To print the string, the pointer is dereferenced.

As textlines is being filled, the contents of line is “cloned” for each push( ) by making a new string(line). The value returned from the new-expression is a pointer to the new string that was created and that copied the information from line. If you had simply passed the address of line to push( ), you would end up with a Stack filled with identical addresses, all pointing to line. You’ll learn more about this “cloning” process later in the book.

The file name is taken from the command line. To guarantee that there are enough arguments on the command line, you see a second function used from the require.h header file: requireArgs( ), which compares argc to the desired number of arguments and prints an appropriate error message and exits the program if there aren’t enough arguments.

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