MFC Programmer's SourceBook : Thinking in C++

stack

The stack, along with the queue and priority_queue, are classified as adapters, which means they are implemented using one of the basic sequence containers: vector, list or deque. This, in my opinion, is an unfortunate case of confusing what something does with the details of its underlying implementation – the fact that these are called “adapters” is of primary value only to the creator of the library. When you use them, you generally don’t care that they’re adapters, but instead that they solve your problem. Admittedly there are times when it’s useful to know that you can choose an alternate implementation or build an adapter from an existing container object, but that’s generally one level removed from the adapter’s behavior. So, while you may see it emphasized elsewhere that a particular container is an adapter, I shall only point out that fact when it’s useful. Note that each type of adapter has a default container that it’s built upon, and this default is the most sensible implementation, so in most cases you won’t need to concern yourself with the underlying implementation.

The following example shows stack<string> implemented in the three possible ways: the default (which uses deque), with a vector and with a list:

//: C20:Stack1.cpp
// Demonstrates the STL stack
#include <iostream>
#include <fstream>
#include <stack>
#include <list>
#include <vector>
#include <string>
#include "../require.h"
using namespace std;

// Default: deque<string>:
typedef stack<string> Stack1;
// Use a vector<string>:
typedef stack<string, vector<string> > Stack2;
// Use a list<string>:
typedef stack<string, list<string> > Stack3;

int main(int argc, char* argv[]) {
  requireArgs(argc, 1); // File name is argument
  ifstream in(argv[1]);
  assure(in, argv[1]);
  Stack1 textlines; // Try the different versions
  // Read file and store lines in the stack:
  string line;
  while(getline(in, line)) 
    textlines.push(line + "\n");
  // Print lines from the stack and pop them:
  while(!textlines.empty()) {
    cout << textlines.top();
    textlines.pop();
  }
} ///:~

The top( ) and pop( ) operations will probably seem non-intuitive if you’ve used other stack classes. When you call pop( ) it returns void rather than the top element that you might have expected. If you want the top element, you get a reference to it with top( ). It turns out this is more efficient, since a traditional pop( ) would have to return a value rather than a reference, and thus invoke the copy-constructor. When you’re using a stack (or a priority_queue, described later) you can efficiently refer to top( ) as many times as you want, then discard the top element explicitly using pop( ) (perhaps if some other term than the familiar “pop” had been used, this would have been a bit clearer).

The stack template has a very simple interface, essentially the member functions you see above. It doesn’t have sophisticated forms of initialization or access, but if you need that you can use the underlying container that the stack is implemented upon. For example, suppose you have a function that expects a stack interface but in the rest of your program you need the objects stored in a list. The following program stores each line of a file along with the leading number of spaces in that line (you might imagine it as a starting point for performing some kinds of source-code reformatting):

//: C20:Stack2.cpp
// Converting a list to a stack
#include <iostream>
#include <fstream>
#include <stack>
#include <list>
#include <string>
#include "../require.h"
using namespace std;

// Expects a stack:
template<class Stk>
void stackOut(Stk& s, ostream& os = cout) {
  while(!s.empty()) {
    os << s.top() << "\n";
    s.pop();
  }
}

class Line {
  string line; // Without leading spaces
  int lspaces; // Number of leading spaces
public:
  Line(string s) : line(s) {
    lspaces = line.find_first_not_of(' ');
    if(lspaces == string::npos)
      lspaces = 0;
    line = line.substr(lspaces);
  }
  friend ostream& 
  operator<<(ostream& os, const Line& l) {
    for(int i = 0; i < l.lspaces; i++)
      os << ' ';
    return os << l.line;
  }
  // Other functions here...
};

int main(int argc, char* argv[]) {
  requireArgs(argc, 1); // File name is argument
  ifstream in(argv[1]);
  assure(in, argv[1]);
  list<Line> lines;
  // Read file and store lines in the list:
  string s;
  while(getline(in, s)) 
    lines.push_front(s);
  // Turn the list into a stack for printing:
  stack<Line, list<Line> > stk(lines);
  stackOut(stk);
} ///:~

The function that requires the stack interface just sends each top( ) object to an ostream and then removes it by calling pop( ). The Line class determines the number of leading spaces, then stores the contents of the line without the leading spaces. The ostream operator<< re-inserts the leading spaces so the line prints properly, but you can easily change the number of spaces by changing the value of lspaces (the member functions to do this are not shown here).

In main( ), the input file is read into a list<Line>, then a stack is wrapped around this list so it can be sent to stackOut( ).

You cannot iterate through a stack; this emphasizes that you only want to perform stack operations when you create a stack. You can get equivalent “stack” functionality using a vector and its back( ), push_back( ) and pop_back( ) methods, and then you have all the additional functionality of the vector. Stack1.cpp can be rewritten to show this:

//: C20:Stack3.cpp
// Using a vector as a stack; modified Stack1.cpp
#include <iostream>
#include <fstream>
#include <vector>
#include <string>
#include "../require.h"
using namespace std;

int main(int argc, char* argv[]) {
  requireArgs(argc, 1);
  ifstream in(argv[1]);
  assure(in, argv[1]);
  vector<string> textlines;
  string line;
  while(getline(in, line)) 
    textlines.push_back(line + "\n");
  while(!textlines.empty()) {
    cout << textlines.back();
    textlines.pop_back();
  }
} ///:~

You’ll see this produces the same output as Stack1.cpp, but you can now perform vector operations as well. Of course, list has the additional ability to push things at the front, but it’s generally less efficient than using push_back( ) with vector. (In addition, deque is usually more efficient than list for pushing things at the front).

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