Effective Analysis Programming Part 2

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Contents

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  • Code for hands-on exercises:

Download this tarball: Gks2013cpp.tar.gz

wget http://wiki.scc.kit.edu/gridkaschool/upload/a/ac/Gks2013cpp.tar.gz
tar xvfz Gks2013cpp.tar.gz


Polymorphism

exercise 1: virtual functions

#include<iostream>
#include<cmath>
#include<vector>
 
using namespace std;
 
class Shape {
 
public:
  virtual double getArea() const =0;
 
};
 
class Circle: public Shape {
public:
  Circle() : radius(1.) {
  }
 
  explicit Circle(double r) : radius(r) {
  }
 
  virtual double getArea() const {
    cout<<"Circle::getArea"<<endl;    
    return M_PI*radius*radius; 
  }
 
private:
  double radius;
};
 
class Square: public Shape {
public:
  Square() : length(1.) {
  }
 
  explicit Square(double l) : length(l) {
  }
 
  virtual double getArea() const {
    cout<<"Square::getArea"<<endl;    
    return length*length; 
  }
 
private:
  double length;
};
 
 
int main() {
  vector<Shape*> myShapes;
  myShapes.push_back(new Circle(2.));  
  myShapes.push_back(new Square(3.));  
 
  cout<<myShapes[0]->getArea()<<endl;
  cout<<myShapes[1]->getArea()<<endl;
 
  delete myShapes[0];
  delete myShapes[1];
}
  • Run the code to see which versions of getArea() get called. Remove the keyword virtual in the base class and run the code again.
  • Recall why non-virtal destructors are dangerous?
  • Why are not all destructors virtual by default? The next execise will help you to answer this question.

exercise 2

#include<iostream>
 
struct A {
  void foo() {
  }
};
 
struct B {
  virtual void foo() {
  }
};
 
int main() {
  A a;
  B b;
  std::cout<<sizeof(a)<<std::endl;
  std::cout<<sizeof(b)<<std::endl;
}
  • Can you explain the output?

C++ Templates

exercise 0: shapes

#include<iostream>
#include<cmath>
 
using namespace std;
 
class Circle {
public:
  Circle() : radius(1.) {
  }
 
  explicit Circle(double r) : radius(r) {
  }
 
  double getArea() const {
    cout<<"Circle::getArea"<<endl;
    return M_PI*radius*radius;
  }
 
private:
  double radius;
};
 
 
class Square {
public:
  Square() : length(1.) {
  }
 
  explicit Square(double l) : length(l) {
  }
 
  double getArea() const {
    cout<<"Square::getArea"<<endl;
    return length*length;
  }
 
private:
  double length;
};
 
template <typename Shape>
void PrintArea(Shape s) {
  cout<<s.getArea()<<endl;
}
 
int main() {
  Circle c(2.);
  Square s(3.);
  PrintArea(c);
  PrintArea(s);
}
  • Compare the code with version using virtal functions?
  • What are the differences/pros/cons of templates vs. dynamic polymorphism?

exercise 1: pair

std::pair is a simple template class to store a pair of values.

exercise 2: read paramerters

The library readparamertes can be used to read values from a text file like:

# GKS 2013
answer 42
parser_test "\\\"" "#" #should result in \" and #
useful yes

The following code is an example how to use readparameters:

#define MAIN_CPP
 
#include <iostream>
#include <exception>
#include "readparameters.h"
 
using namespace std;
 
int main(int argc,char **argv){ 
  //check arguments
  if (argc!=2) {
    cerr<<"Usage: ./params <paramfile>"<<endl;
    return 1;
  } 
 
  //get input filename from parameter file
  int answer;
  vector<string> v;
  try { 
      readparameters rp(argv[1]);
      answer = rp.get<int>("answer");
      v=rp.get_vector<string>("parser_test");
      cout<<answer<<endl;
      rp.print_tabular(); //debug output
      rp.get<long>("Atlantis"); //exception
 
  }
  catch (exception& e) {
      cerr<<e.what()<<endl;
      return 1;
  }
 
  return 0;
}
  • Run the code, add more parameters to the text file, read a boolean parameter.
  • Read the code of the class readparamerters (you may skip the parsing part in the constructor).
    • get and get_vector are template functions. Why are there specializations for bool and string?
    • Why is the map tabular mutual?
    • What are the requirements for types to work with readparameters?
    • Why is the default constructor privat?

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