Passing iterators to class functions - c++

Question

I'm trying to create a new templated class "CrazyBucket< T >" which has to hold multiple values of type T.
I want to create a constructor for this class that can accept any 2 forward iterators as its arguments and copy in those values (of type T) to the class instance.
Constructor:
CrazyBucket< T >::CrazyBucket( iterator< forward_iterator_tag, T > start, iterator< forward_iterator_tag, T > end )
{ ... }
But when I try to call it with,
vector< int > vec;
vec.push_back( 4 );
CrazyBucket< int > bucket( vec.begin(), vec.end() );
I get the following error,
candidate constructor not viable: no known conversion from 'iterator' (aka '__wrap_iter<pointer>') to 'std::iterator<std::forward_iterator_tag, int>' for 1st argument
Any help with how I should define my constructor is much appreciated.
Thanks in advance.

Answer 1

You can use SFINAE to exclude non-matching types in a fashion I think is close to what you want.
#include <iostream>
#include <iterator>
#include <vector>
template<class T>
class CrazyBucket
{
public:
template<class It, typename = typename std::enable_if<
std::is_same< typename std::iterator_traits<It>::value_type,T>::value>::type>
CrazyBucket(It beg, It end)
{
std::cout << __PRETTY_FUNCTION__ << '\n';
}
};
int main()
{
std::vector<int> vInt;
CrazyBucket<int> cbOK(vInt.begin(), vInt.end());
int ar[10];
CrazyBucket<int> cbAlsoOK(ar, ar+10);
// uncomment for failure test case.
//std::vector<double> vDbl;
//CrazyBucket<int> cbFail(vDbl.begin(), vDbl.end());
}
Also accomplished with a static assertion:
template<class It>
CrazyBucket(It beg, It end)
{
static_assert(std::is_same<T, typename std::iterator_traits<It>::value_type>::value,
"failed to match iterator value type");
std::cout << __PRETTY_FUNCTION__ << '\n';
}
Either is restrictive, and you should know that may not be the end goal you had in mind. For example, an iteration of short will naturally store as int without data loss, yet this kind of SFINAE will toss it. That too can be overcome with more expansion, but by that time I think you need to consider whether it is really worth it in the end.
Anyway, best of luck.

Answer 2

Does it really have to be the constructor? The problem as I see it is that you'd need to have a specialized constructor which isn't possible as per the standard. If you can defer your initialisation to a member function then the following approach will work:
template<class T>
struct CrazyContainer {
template<class U>
void init(U first,U last) {
for(auto it=first;it!=last;it++) {
// do stuff with 'it'
}
}
};
main() {
std::vector<int> vec;
CrazyContainer<int> f;
f.init(vec.begin(),vec.end());
}
I'm looking forward to seeing if there's someone else that can come up with a way that permits this via a constructor.
Edit:
Thanks to Sebastian for pointing out that a templated constructor will work just as well as the templated method:
template<class T>
struct CrazyContainer {
template<class U>
CrazyContainer(U first,U last) {
for(auto it=first;it!=last;it++) {
// do stuff
}
}
};
main() {
std::vector<int> v;
std::set<int> s;
CrazyContainer<int> cv(v.begin(),v.end());
CrazyContainer<int> cs(s.begin(),s.end());
}