How can I merge two STL maps?

Assuming you want to preserve the elements in mapA, and merge elements in mapB for which there is no key in mapA:

mapA.insert(mapB.begin(), mapB.end())

will do what you want, I think.

(EDIT: If you are using C++17 or newer, consider this answer: https://stackoverflow.com/a/56594603/118150)

Working example:

#include <iostream>
#include <map>

void printIt(std::map<int,int> m) {
    for(std::map<int,int>::iterator it=m.begin();it!=m.end();++it)
        std::cout << it->first<<":"<<it->second<<" ";
    std::cout << "\n";
}

int main() {
    std::map<int,int> foo,bar;
    foo[1] = 11; foo[2] = 12; foo[3] = 13;
    bar[2] = 20; bar[3] = 30; bar[4] = 40;
    printIt(foo);
    printIt(bar);
    foo.insert(bar.begin(),bar.end());
    printIt(foo);
    return 0;
}

output:

:!./insert
1:11 2:12 3:13
2:20 3:30 4:40
1:11 2:12 3:13 4:40