overwriting map with map possible with <algorithm>?

Question

I would like to do the following but it seems it is not possible. I am no expert on STL.

typedef std::map<int,int> CMap;

CMap m1;
m1[0] = 10;
m1[1] = 11;
m1[2] = 12;

CMap m2;
m2[20] = 30;
m2[21] = 31;
m2[22] = 32;

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

Is there a way to do this using an algorithm (C++98)? Could it be done with transform() or replace()? If yes, how?

Thanks!

@Morb: Thanks. insert() is of course possible. I wondered if it is also possible doing likewise with an algorithm. — Lorenz
– Lorenz, Commented Apr 29, 2015 at 6:49
@Joachim: Thanks. That is one way that I did not consider. Actually I did not specify what I want to do properly. I would like to have m2 overwritten by m1. Would that also be possible? — Lorenz
– Lorenz, Commented Apr 29, 2015 at 6:59

Galik · Accepted Answer · 2015-04-29 07:02:01Z

4

You can do this:

m2 = m1;

Or even this if you like:

m2.swap(m1);

And there is this too:

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

answered Apr 29, 2015 at 7:02

Galik

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3 Comments

Lorenz Over a year ago

Thanks for the answer. The first two overwrite which is good but the third will just append as I understand. Is there a way to overwrite using copy()?

Galik Over a year ago

@Lorenz You could overwrite using std::copy() if you created your own iterator as std::insert_iterator will only insert if there is not a value already present. TBH you might as well just write a loop as any method is going to at least require you to write a functor. If you have C++11 a simple range based for loop would do it: for(auto&& p: m1) m2[p.first] = p.second;

Lorenz Over a year ago

Thanks for the answer. Did not think of the possibility to write a specific iterator. Now clear.

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