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!

Solution 1

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());

Solution 2

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.

Comments

[Billy ONeal]

+1 for correct answer, but one should prefer member functions to algorithm calls.

[Stephen]

Well, if you're gonna avoid using std::copy, why not just use the copy c'tor? m2 = m1. edit: Ah, he wants the union of values.

[Billy ONeal]

@Stephen: Because using insert preserves the existing contents inside m2. Using the copy assignment operator destroys the existing contents inside m2.

[Dean Burge]

+1 @GMan, +1 @Billy, you guys both got it right. The OP should do what Billy suggests, but GMan in fact answered the question. So, with your powers combined ...

[GManNickG]

@WilhelmTell: I AM CAPTAIN PLANET!

[cheshirekow]

@Billy: Your answer is clearly the correct answer for the OP, but the answer to why would be "so I can use copy_if", and filter one map during the copy to another.

[Billy ONeal]

@Cheshire copy_if != copy.

[kalaxy]

This version works nicely when using the copy_if variant of c++11.

[Olshansk]

Stupid question, but why not just do Map m2 = m1;?