How can I copy one map into another using std::copy?

Question

I would like to copy the content of one std::map into another. Can I use std::copy for that? Obviously, the following code won't work:

int main() {
  typedef std::map<int,double> Map;
  Map m1;
  m1[3] = 0.3;
  m1[5] = 0.5;
  Map m2;
  m2[1] = 0.1;
  std::copy(m1.begin(), m1.end(), m2.begin());
  return 0;
}

This won't work because copy will call operator* on m2.begin() to "dereference" it and assign a value (all values are of type std::pair<const int, double>). Then it will call operator++ to move to the next space in m2. Both of these operations don't work because of the const in const int and there is no space reserved for any new elements.

Is there any way to make it work with std::copy?

Thanks!

Answer 1

You need a variant of an insert iterator:

std::copy(m1.begin(), m1.end(), std::inserter(m2, m2.end()) );

inserter is defined in <iterator>. It requires a place to insert into (hence the m2.end()), and returns an insert_iterator.

Answer 2

You can use GMan's answer --- but the question is, why do you want to use std::copy? You should use the member function std::map<k, v>::insert instead.

m2.insert(m1.begin(), m1.end());

Answer 3

What if you simply reverse the order of events? First create a copy of the first map, then add a special element.

Map m1;
.... // populate m1;

Map m2( m1 );
m2[1] = 0.1;

... If you need a copy, just use the copy constructor.