here is an interview question that taught me a lot

[landonmkelsey]

Can a virtual function be static?

I added:

(1) yes/no why?
(2) how can one simulate a virtual function being static?

I collect interview question!

[landonmkelsey]

code a word histogram in C++

take a file and list words and the number of times they occur in the file

I did the answer in C and wrote a lot of code...this was before I learned STL.

g++ -o histo histo.cpp

answer:

#include <iostream>
#include <iterator>
#include <fstream>
#include <algorithm>
#include <map>
#include <string>
using namespace std;

map<string,int> histo;
void record(const string& s)
{
histo[s]++;
}
void print(const pair<const string, int>& r)
{
cout<<r.first<<" , "<<r.second<<endl;
}
int main()
{
ifstream fin("histo.cpp");
istream_iterator<string> is(fin);
istream_iterator<string> eos;
for_each(is,eos,record);
for_each(histo.begin(), histo.end(), print);
return 0;
}

[landonmkelsey]

forgot to add code tags!

sorry! Not happen again!

[landonmkelsey]

I am not
(1) trying to stump anybody
(2) impress anybody!

This is a learning machine to prepare for a C++ interview or test!

Look through the code and estimate what the value of count will be at the end!

Run the program and see if your estimate matches the value of count!

change fout to cout and see results while the program runs!

Insert cout or fout statements to trace the results!

#include <iostream>
#include <fstream>
using namespace std;
ofstream fout("result.txt");
struct Foo
{
        static int count;
        Foo()
        {
                fout<<" CTOR"<<endl;
                count++;
        }
        Foo(const Foo& f)                                  //g++ demands the const, VC++ does not
        {
                fout<<"Copy CTOR"<<endl;
                count++;
        }
        Foo operator+(Foo f)
        {
                fout<<"operator + Foo::count = "<<Foo::count<<endl;

                return f;
        }
};
int Foo::count = 0;

Foo add(Foo f1, Foo f2)
{
        return f1 + f2;
}
void f()
{
        fout<<"f 1st Foo::count = "<<Foo::count<<endl;

        Foo f1, f2;

        fout<<"f 2nd Foo::count = "<<Foo::count<<endl;


        add(f1,f2);

        fout<<"f after add() Foo::count = "<<Foo::count<<endl;

}
int main()
{
        f();
        fout<<"Main Foo::count = "<<Foo::count<<endl;
        return 0;
}

/* results in result.txt
 * f 1st Foo::count = 0
 *  CTOR
 *   CTOR
 *   f 2nd Foo::count = 2
 *   Copy CTOR
 *   Copy CTOR
 *   Copy CTOR
 *   operator + Foo::count = 5
 *   Copy CTOR
 *   Copy CTOR
 *   f after add() Foo::count = 6
 *   Main Foo::count = 6    several years ago under microsoft VC++, the answer was 7
 *   */

[landonmkelsey]

answer NO!

consider:
#include <iostream>
using namespace std;
class animal {
public:
animal(){cout<<"CTOR animal"<<endl;}
virtual void feed()=0; // no body pure vf
};
class tiger: public animal {
public:
tiger(){cout<<"CTOR tiger"<<endl;}
void feed(){cout<<"feed tiger"<<endl;}
};
class elephant: public animal {
public:
elephant(){cout<<"CTOR elephant"<<endl;}
void feed(){cout<<"feed elephant"<<endl;}
};
class giraffe: public animal {
public:
giraffe(){cout<<"CTOR giraffe"<<endl;}
void feed(){cout<<"feed giraffe"<<endl;}
};
int main() {
cout<<"build zoo"<<endl;
animal* p_zoo[3];
cout<<"assign animals to zoo"<<endl;
p_zoo[0] = new tiger;
p_zoo[1] = new elephant;
p_zoo[2] = new giraffe;
for (int i=0; i<3; i++) p_zoo[i]->feed();
// delete [] p_zoo; // uncomment this and make it work
}

a virtual method requires a pointer for access

a static method requires a class name and double colon (SRO)

a static method or variable can be accessed even if the class has never been instantiated

Simulate static access of virtual method:

hint: write a base class with a virtual method that calls static methods in base and derived classes!

The virtual method should exercise the static method in each base and derived!

try it!

Great question for interview:

Can a derived class access a static method in the base?

Verify with cout!

Consider the issue of hiding!

[landonmkelsey]

drat! I forgot code tags on previous post!

g++ -o staticv staticv.cpp (Linux Fedora 9)

  #include <iostream>
using namespace std;
class Base
{
public:
        virtual int CallStatic()
        {
                BaseStatic();
        }
        static int BaseStatic()
        {
                cout<<"BaseStatic()"<<endl;
        }
};

class Derived:public Base
{

public:
        int CallStatic()
        {
                DerivedStatic();
        }
        static int DerivedStatic()
        {
                cout<<"DerivedStatic()"<<endl;
        }
};
int main()
{
        Base* base = new Base;
        Base* baseDerived = new Derived;
        Derived* derived = new Derived;
        base->CallStatic();
// here the virtual mechanism simulates a virtual static mechanism
        baseDerived->CallStatic();
        derived->CallStatic();
//      Base::BaseStatic();
//      Derived::DerivedStatic();

}

[landonmkelsey]

Not all methods exercised!

I threw this together so there could be errors!

compare results with other string classes

// string1.cpp -- String class methods
#include <iostream>
#include "string1.h"
#include <fstream>
using namespace std;
ofstream fout(".\\results.txt");

// init static class members

int String::num_strings = 0;
const int String::CINLIM = 80;
// static method
int String::HowMany()
{
	return num_strings;
}


String::String(const char * s)     // construct String from C string
{
	fout<<"CTOR = "<<s<<endl;
    len = strlen(s);               // set size
    str = new char[len + 1];       // allot storage
    strcpy(str, s);                // initialize pointer
    num_strings++;                 // set object count
}

String::String()                   // default constructor
{
	fout<<"default CTOR = "<<endl;
    len = 4;
    str = new char[1];
    str[0] = '\0';                 // default string
    num_strings++;
}

String::String(const String & st)
{
	fout<<"copy CTOR = "<<st<<endl;
    num_strings++;             // handle static member update
    len = st.len;              // same length
    str = new char [len + 1];  // allot space
    strcpy(str, st.str);       // copy string to new location
}

String::~String()                     // necessary destructor
{
	fout<<"DTOR = "<<str<<endl;
    --num_strings;                    // required
    delete [] str;                    // required
}

// overloaded operator methods    

// assign a String to a String
String & String::operator=(const String & st)
{
	fout<<"operator = (const String & st)"<<st<<endl;
    if (this == &st)
        return *this;
    delete [] str;
    len = st.len;
    str = new char[len + 1];
    strcpy(str, st.str);
    return *this;
}

// assign a C string to a String
String & String::operator=(const char * s)
{
	fout<<"operator =(const char * s) "<<s<<endl;
    delete [] str;
    len = strlen(s);
    str = new char[len + 1];
    strcpy(str, s);
    return *this;
}


// read-write char access for non-const String
char & String::operator[](int i)
{
    return str[i];
}

// read-only char access for const String
const char & String::operator[](int i) const
{
    return str[i];
}

// overloaded operator friends

bool operator<(const String &st1, const String &st2)
{
    return (strcmp(st1.str, st2.str) < 0);
}

bool operator>(const String &st1, const String &st2)
{
    return st2.str < st1.str;
}

bool operator==(const String &st1, const String &st2)
{
    return (strcmp(st1.str, st2.str) == 0);
}

String operator+(String& strl, const String& strr)
{
	fout<<"operator +(const char * s) "<<strl.str<<" "<<strr.str<<endl;

	String ts;
	delete [] ts.str;
	ts.len = strl.len + 1 + strr.len + 1;
	ts.str = new char[ts.len];
	strcpy(ts.str, strl.str);
	strcat(ts.str, strr.str);
	fout<<"operator+ str = "<<ts.str<<endl;
	fout<<"exit operator +(const char * s) "<<strl.str<<" "<<strr.str<<endl;

	return ts;
}
// simple String output
ostream & operator<<(ostream & os, const String & st)
{
    os << st.str;
    return os; 
}

// quick and dirty String input
istream & operator>>(istream & is, String & st)
{
    char temp[String::CINLIM];
    is.get(temp, String::CINLIM);
    if (is)
        st = temp;
    while (is && is.get() != '\n')
        continue;
    return is; 
}
int main() {
	fout<<"go"<<endl;
	String B = "789";
	fout<<"_________________________"<<endl;
	String A = B + "lmk";
	fout<<"stop"<<endl;
	int i = 9;
	return 0;
}

// string1.h -- fixed and augmented string class definition
#include <iostream>
using namespace std;
#ifndef STRING1_H_
#define STRING1_H_
class String
{
private:
    char* str;             // pointer to string
	char* temp;
    int len;                // length of string
    static int num_strings; // number of objects
	//    static const int CINLIM = 80;  // cin input limit
    static const int CINLIM;  // cin input limit
	
public:
	// constructors and other methods
    String(const char * s); // constructor
    String();               // default constructor
    String(const String &); // copy constructor
    ~String();              // destructor
    int length () const { return len; }
	// overloaded operator methods    
    String & operator=(const String &);
    String & operator=(const char *);
    char & operator[](int i);
    const char & operator[](int i) const;
	// overloaded operator friends
    friend bool operator<(const String &st, const String &st2);
    friend bool operator>(const String &st1, const String &st2);
    friend bool operator==(const String &st, const String &st2);
    friend ostream & operator<<(ostream & os, const String & st);
    friend istream & operator>>(istream & is, String & st);
	friend String operator+(String &strl, const String& strr);

	// static function
    static int HowMany();
};
#endif

[landonmkelsey]

C++ interview questions contain pure "C" questions!

Buy the book "C Puzzlebook" by Feuer and work through it!

I've gotten interview questions out of this little book!

The drill is to be careful and understand why your answer is right or wrong!

[landonmkelsey]

I missed C++ too until I started using Qt4.4 under Redhat Linux Fedora 9.

A lot of fun since Qt compiles and runs nearly everywhere!

I am reviewing since C# seems to overrun!

I study C++ interviews and have a 50 page C++ Review

Best to go into a C++ interview with confidence.

I've learned from years of experience/failure!

C++ and now C# are very very big subjects!

[DJMaze]

C# is not for me. I like C++ and D
D is a new language that overcomes the issues and uses the ideas of Pascal, C++ and Java, and put them in one language... D

Intro - D Programming Language - Digital Mars

[DJMaze]

As fot Qt... Did you see GTK, FLTK or wxWidgets?

[landonmkelsey]

Qt is far from perfect but it does port to just about any platform.

Comes free with Redhat Fedora 9

There are those who think Smalltalk is #1 but it would be tough finding a job doing Smalltalk... practicalities of life

C# will cause C++ VB and Java to die slowly of attrition. I occasionally see job requiring FORTRAN that one can learn in 5 minutes!

.NET has really gotten complex...up to .NET 3.5

DJMaze: What OS(s) do you use?

[DJMaze]

Windows:
- 2000
- XP (occasionally, i hate it)
- Vista (at times when someone has issues with his again)

GNU/Linux:
- Fedora 8
- Kubuntu 8.04
- RedHat 9
- CentOS 4

And the odd one: Syllable

I still don't get the .NET hype. Why built an easy 20kb app in .NET that needs:
- 30MB of .NET memory
- massive .NET download/install
- a CPU flops overhead

[landonmkelsey]

thanks! that is interesting!

I just think the C#.NET is interesting...subjective

I'll never buy another MS product!

[landonmkelsey]

Qt4.4 with eclipse is pretty good

Has a built in window designer