Re: template

From:

"Jim Langston" <tazmaster@rocketmail.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 9 Jul 2007 05:38:00 -0700

Message-ID:

<8%pki.319$KH7.123@newsfe04.lga>

"yashwant pinge" <yashwantpinge@gmail.com> wrote in message
news:1183984239.177827.153620@o11g2000prd.googlegroups.com...

#include<iostream>

using namespace std;

template<class T, int size=100>
class Array
{
       T array[size];
public:
       T& operator [](int index)
       {
               return array[index];
       }

       int length() const
       {
               return size;
       }
};

class Number
{
       float f;
public:
       Number(float ff=0.0f):f(ff)
       {
       }
       Number& operator =(const Number& n)
       {
               f=n.f;
               return *this;
       }
       operator float const()
       {
               return f;
       }
       friend ostream& operator <<(ostream& os,const Number& x)
       {
               return os << x.f;
       }
};

template<class T,int size=2>
class Holder
{
       Array<T,size>*np;
public:
       Holder():np(0)
       {
       }

       T& operator [](int i)
       {
               if(!np) np = new Array<T,size>;
               return np ->operator[](i);
       }

       int length() const
       {
               return size;
       }

       ~Holder()
       {
               delete np;
       }
};

int main()
{
       Holder<Number> h;

       for(int i=0;i<2;i++)
               h[i]=i; // why the number constructor
called ...?

       for(int j=0;j<2;j++)
               cout<<h[j]<<endl;

       return 0;
}

Why the number constructor is called during the execution of line
" h[i]=i;"

Look at operator[] for Holder.

        T& operator [](int i)
        {
                if(!np) np = new Array<T,size>;
                return np ->operator[](i);
        }

It's sayign if np does not yet exist (the constructor initializes it to
null) then to call operator new on it. In this case it would be:
   np = new Array<Number, 2>
( the 2 comes from the template's 2nd parameter, if not passed is defaulted
to 2).
so the first time (when i == 0) 2 Numbers will be constructed and copied to
the Array, hence your number constructor being called.