CodeGuru : Thinking in C++

Sensible uses for RTTI

Because it allows you to discover type information from an anonymous polymorphic pointer, RTTI is ripe for misuse by the novice because RTTI may make sense before virtual functions do. For many people coming from a procedural background, it’s very difficult not to organize their programs into sets of switch statements. They could accomplish this with RTTI and thus lose the very important value of polymorphism in code development and maintenance. The intent of C++ is that you use virtual functions throughout your code, and you only use RTTI when you must.

However, using virtual functions as they are intended requires that you have control of the base-class definition because at some point in the extension of your program you may discover the base class doesn’t include the virtual function you need. If the base class comes from a library or is otherwise controlled by someone else, a solution to the problem is RTTI: You can inherit a new type and add your extra member function. Elsewhere in the code you can detect your particular type and call that member function. This doesn’t destroy the polymorphism and extensibility of the program, because adding a new type will not require you to hunt for switch statements. However, when you add new code in your main body that requires your new feature, you’ll have to detect your particular type.

Putting a feature in a base class might mean that, for the benefit of one particular class, all the other classes derived from that base require some meaningless stub of a virtual function. This makes the interface less clear and annoys those who must redefine pure virtual functions when they derive from that base class. For example, suppose that in the Wind5.cpp program in Chapter XX you wanted to clear the spit valves of all the instruments in your orchestra that had them. One option is to put a virtual ClearSpitValve( ) function in the base class Instrument, but this is confusing because it implies that Percussion and electronic instruments also have spit valves. RTTI provides a much more reasonable solution in this case because you can place the function in the specific class ( Wind in this case) where it’s appropriate.

Finally, RTTI will sometimes solve efficiency problems. If your code uses polymorphism in a nice way, but it turns out that one of your objects reacts to this general-purpose code in a horribly inefficient way, you can pick that type out using RTTI and write case-specific code to improve the efficiency.

Revisiting the trash recycler

Here’s the trash recycling simulation from Chapter XX, rewritten to use RTTI instead of building the information into the class hierarchy:

//: C24:Recycle2.cpp
// Chapter XX example w/ RTTI
#include "../purge.h"
#include <fstream>
#include <vector>
#include <typeinfo>
#include <cstdlib>
#include <ctime>
using namespace std;
ofstream out("recycle2.out");

class Trash {
  float _weight;
public:
  Trash(float wt) : _weight(wt) {}
  virtual float value() const = 0;
  float weight() const { return _weight; }
  virtual ~Trash() { out << "~Trash()\n"; }
};

class Aluminum : public Trash {
  static float val;
public:
  Aluminum(float wt) : Trash(wt) {}
  float value() const { return val; }
  static void value(int newval) {
    val = newval;
  }
};

float Aluminum::val = 1.67;

class Paper : public Trash {
  static float val;
public:
  Paper(float wt) : Trash(wt) {}
  float value() const { return val; }
  static void value(int newval) {
    val = newval;
  }
};

float Paper::val = 0.10;

class Glass : public Trash {
  static float val;
public:
  Glass(float wt) : Trash(wt) {}
  float value() const { return val; }
  static void value(int newval) {
    val = newval;
  }
};

float Glass::val = 0.23;

// Sums up the value of the Trash in a bin:
template<class Container> void
sumValue(Container& bin, ostream& os) {
  typename Container::iterator tally = 
    bin.begin();
  float val = 0;
  while(tally != bin.end()) {
    val += (*tally)->weight() * (*tally)->value();
    os << "weight of "
        << typeid(*tally).name()
        << " = " << (*tally)->weight() << endl;
    tally++;
  }
  os << "Total value = " << val << endl;
}

int main() {
  srand(time(0)); // Seed random number generator
  vector<Trash*> bin;
  // Fill up the Trash bin:
  for(int i = 0; i < 30; i++)
    switch(rand() % 3) {
      case 0 :
        bin.push_back(new Aluminum(rand() % 100));
        break;
      case 1 :
        bin.push_back(new Paper(rand() % 100));
        break;
      case 2 :
        bin.push_back(new Glass(rand() % 100));
        break;
    }
  // Note difference w/ chapter 14: Bins hold
  // exact type of object, not base type:
  vector<Glass*> glassBin;
  vector<Paper*> paperBin;
  vector<Aluminum*> alBin;
  vector<Trash*>::iterator sorter = bin.begin();
  // Sort the Trash:
  while(sorter != bin.end()) {
    Aluminum* ap =
      dynamic_cast<Aluminum*>(*sorter);
    Paper* pp =
      dynamic_cast<Paper*>(*sorter);
    Glass* gp =
      dynamic_cast<Glass*>(*sorter);
    if(ap) alBin.push_back(ap);
    if(pp) paperBin.push_back(pp);
    if(gp) glassBin.push_back(gp);
    sorter++;
  }
  sumValue(alBin, out);
  sumValue(paperBin, out);
  sumValue(glassBin, out);
  sumValue(bin, out);
  purge(bin);

} ///:~

The nature of this problem is that the trash is thrown unclassified into a single bin, so the specific type information is lost. But later, the specific type information must be recovered to properly sort the trash, and so RTTI is used. In Chapter XX, an RTTI system was inserted into the class hierarchy, but as you can see here, it’s more convenient to use C++’s built-in RTTI.

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