iterator_traits::value_type on back_insert_iterator - returns void

[antoanish]

Hello,

My Requirement is
1. Copy data from iterator to an array ( array should be created dynamically within the function based on data_type of the data held by the iterator /container. )

2. Copy data from array into iterator ( array should be created dynamically within the function based on data_type of the data held by the iterator /container. )

Implementation
-----------------------
//copies data from begin to end of container to array.
template <class T>
void writeiter( T& iter1, T& iter2)
{
int t2= iter2 -iter1;
cout <<t2<<endl;
iterator_traits<T>::value_type *twrite= new iterator_traits<T>::value_type[t2];
std::copy(iter1,iter2,twrite);
}

//copies data from array to iter (back_insert_iterator)
template <class T> void readiter( T& iter, int len)
{
//This line does not compile since return is a void.
iterator_traits<T>::value_type *tread= new iterator_traits<T>::value_type[len]; for (int i=0; i<len;++i)
tread[i] = i;
std::copy(tread, tread+ len,iter );
}

int main()
{
vector <int> v1;
int len=5;
back_insert_iterator< vector<int> > v1back(v1);
readiter(v1back,len); //Does not work - the function does not compile

vector<int>::iterator vbdir1=v1.begin();
vector<int>::iterator vbdir2=v1.end();
writeiter(vbdir1,vbdir2); //works good

}

The readiter function does not compile since a iterator_trait::value_type of back_insert_iterator's returns void.
what i would need is to get the datatype of the container which is INT held by the back_insert_iterator. My assumption was that i would get the iterator_traits of back_insert_iterator from function readiter , which would later be used to create an array or a array of pointers.

Any help appreciated?

Thanks
Anish

[antoanish]

Hello,

My Requirement is
1. Copy data from iterator to an array ( array should be created dynamically within the function based on data_type of the data held by the iterator /container. )

2. Copy data from array into iterator ( array should be created dynamically within the function based on data_type of the data held by the iterator /container. )

Implementation
-----------------------
//copies data from begin to end of container to array.
template <class T>
void writeiter( T& iter1, T& iter2)
{
int t2= iter2 -iter1;
cout <<t2<<endl;
iterator_traits<T>::value_type *twrite= new iterator_traits<T>::value_type[t2];
std::copy(iter1,iter2,twrite);
}

//copies data from array to iter (back_insert_iterator)
template <class T> void readiter( T& iter, int len)
{
//This line does not compile since return is a void.
iterator_traits<T>::value_type *tread= new iterator_traits<T>::value_type[len]; for (int i=0; i<len;++i)
tread[i] = i;
std::copy(tread, tread+ len,iter );
}

int main()
{
vector <int> v1;
int len=5;
back_insert_iterator< vector<int> > v1back(v1);
readiter(v1back,len); //Does not work - the function does not compile

vector<int>::iterator vbdir1=v1.begin();
vector<int>::iterator vbdir2=v1.end();
writeiter(vbdir1,vbdir2); //works good

}

The readiter function does not compile since a iterator_trait::value_type of back_insert_iterator's returns void.
what i would need is to get the datatype of the container which is INT held by the back_insert_iterator. My assumption was that i would get the iterator_traits of back_insert_iterator from function readiter , which would later be used to create an array or a array of pointers.


Thanks
Anish

Something more i should addup.
i understand that back_insert_iterator does not
have a value_type coz its an output iterator.
My whole question is - is there a way to get information abt the
back_insert_iterators traits ? I undertand that its not possible. But,
is there alternate approach, rather than getting datatype as a parameter as i have given below.

Basically, my intention is to create an array which is of the same
data type as the container.

if i change the function readiter as below, It works how i want. But i
would be passing a type parameter.
template <class T, class T1> void readiter( T& iter, int len, T1& a)
{
T1 *tread= new T1[len];
for (int i=0;i<len;++i)
t3[i]=i; //originally this array is populated from X
std::copy(t3, t3+ len,iter );

}

called from main as
//for int
int type1=1;
readiter(v1back,len,type1)

//for float
float type2=(float)1.0;
readiter(v1back,len,type2)