Merging two unordered_maps with overlapping keys

Question

I would like to merge two std::unordered_map: mapA and mapB, while keeping priority for items from mapA if both maps contain the same key.

Is there an elegant way of doing this (rather then validating each key.. my maps contain a large number of elements)?

Example:

mapA = {{"sugar",0.1},{"salt",0.2}}
mapB = {{"sugar",0.3},{"pepper",0.4}}

The result I would like to have is:

result = {{"sugar",0.1},{"salt",0.2},{"pepper",0.4}}

Ignoring the key-value {"sugar",0.3} from mapB

Thanks.

Answer 1

Absolutely:

auto result = mapA;
result.insert(mapB.begin(), mapB.end());

The insert member functions of unordered_map do nothing if the key already exists within the container.

Demo.

Answer 2

I know this question is tagged C++11, but for completeness it seems worth mentioning that C++17 added the merge() function if you don't care about keeping mapB intact. merge() moves all of the elements whose keys are not in mapA from mapB to mapA. If a key are in mapA, mapA remains unchanged for that element and the element remains in mapB.

https://en.cppreference.com/w/cpp/container/unordered_map/merge