map/unordered_map: Prefer find() and then at() or try at() catch out_of_range?

Question

When using std::map or std::unordered_map, what is the preferable way to access an element that might not be in the container? Should one ask permission or ask for forgiveness? Why?

That is,

if (mymap.find(key) != mymap.end()) {
    value = mymap.at(key);
    // do stuff
}
else {
    // do something else
}

vs.

try {
    value = mymap.at(key);
    // do stuff
}
catch(const std::out_of_range& oor) {
    // do something else
}

Answer 1

I suggest secret option C which avoids both non-exceptional exceptions AND doing the search twice:

MyMapType::const_iterator iter = mymap.find(key);

if(iter != mymap.end())
{
    // iter is item pair in the map. The value will be accessible as `iter->second`.
}

In C++ it is not idiomatic to use exception handling for flow control, which is what the second code example is doing. It will make it harder for maintainers to understand the code and, depending on the specific compiler, it may not perform as well.