Copy a map in C++

This post will discuss how to copy a map in C++.

1. Using copy constructor

We can use a copy constructor to initialize a map from elements of another map. We can also use operator= to copy a map, that is functionally the same as above. This is demonstrated below:

#include <iostream>

#include <string>

#include <map>

int main()

{

std::map<int, char> map = {

{1, 'A'}, {2, 'B'}, {3, 'C'}

};

std::map<int, char> map_copy(map);

// or

// std::map<int, char> map_copy = map;

for (const auto &entry: map_copy) {

std::cout << "{" << entry.first << ", " << entry.second << "}" << std::endl;

}

return 0;

}

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Output:

{1, A}
{2, B}
{3, C}

2. Using `std::map::insert`

Another option is to use the std::map::insert function that extends the map container by inserting the elements from the specified range, as shown below. This function can be used if the map already contains some key-value pairs.

#include <iostream>

#include <string>

#include <map>

int main()

{

std::map<int, char> map = {

{1, 'A'}, {2, 'B'}, {3, 'C'}

};

std::map<int, char> map_copy;

map_copy.insert(map.begin(), map.end());

for (const auto &entry: map_copy) {

std::cout << "{" << entry.first << ", " << entry.second << "}" << std::endl;

}

return 0;

}

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Output:

{1, A}
{2, B}
{3, C}

3. Using `std::copy`

Finally, we can use the std::copy standard algorithm with an insert iterator to copy the contents of the original map to another map. The following solution demonstrates this using std::inserter, defined in header <iterator>:

#include <iostream>

#include <string>

#include <map>

#include <iterator>

int main()

{

std::map<int, char> map = {

{1, 'A'}, {2, 'B'}, {3, 'C'}

};

std::map<int, char> map_copy;

std::copy(map.begin(), map.end(), std::inserter(map_copy, map_copy.end()));

for (const auto &entry: map_copy) {

std::cout << "{" << entry.first << ", " << entry.second << "}" << std::endl;

}

return 0;

}

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Output:

{1, A}
{2, B}
{3, C}

That’s all about copying a map in C++.

1. Using copy constructor

2. Using std::map::insert

3. Using std::copy

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2. Using `std::map::insert`

3. Using `std::copy`