How can I merge two STL maps?

C++17

As mentioned in John Perry's answer, since C++17 std::map provides a merge() member function. The merge() function produces the same result for the target map as jkerian's solution based on using insert(), as you can see from the following example, which I borrowed from jkerian. I just updated the code with some C++11 and C++17 features (such as using type alias, range-based for loop with structured binding, and list initialization):

using mymap = std::map<int, int>;

void printIt(const mymap& m) {
    for (auto const &[k, v] : m)
        std::cout << k << ":" << v << " ";
    std::cout << std::endl;
}

int main() {
    mymap foo{ {1, 11}, {2, 12}, {3, 13} };
    mymap bar{ {2, 20}, {3, 30}, {4, 40} };
    printIt(foo);
    printIt(bar);
    foo.merge(bar);
    printIt(foo);
    return 0;
}

Output:

1:11 2:12 3:13
2:20 3:30 4:40
1:11 2:12 3:13 4:40

As you can see, merge() also gives priority to the target map foo when keys overlap. If you want to have it the other way round, then you have to call bar.merge(foo);.

However, there is a difference between using insert() and merge() regarding what happens to the source map. The insert() functions adds new entries to the target map, while merge() moves entries over from the source map. This means for the example above, that insert() does not alter bar, but merge() removes 4:40 from bar, so that only 2:20 and 3:30 remain in bar.

Note: I reused the example from jkerian which uses map<int, int> for the sake of brevity, but merge() also works for your map<string, string>.

Code on Coliru