Using try_emplace with a shared_ptr

Question

So I have a std::unordered_map<std::string, std::shared_ptr<T>> and I'd like to use try_emplace to add/reference elements. Previously, it was simply a std::unordered_map<std::string, T> and T has a constructor which just takes a std::string, so I was using myMap.try_emplace(myString, myString).first->second to reference the T and create it if necessary.

However, if I simply change it to myMap.try_emplace(myString, std::make_shared<T>(myString)).first->second, it, of course, constructs the T every time.

What's the most idiomatic way to get around this? To clarify, I'd like to construct the object on the heap with a shared pointer and insert the shared pointer into the unordered map if and only if there is not already a matching element in the map.

Answer 1

You can use operator[] instead:

auto &ptr = map[ key ];
if( !ptr ) ptr = std::make_shared<T>( myString );

Note: this solution came with assumption that you do not want to keep default constructed std::shared_ptr in the map. If that is the case, then you would need to use little more verbose code with try_emplace:

auto p = map.try_emplace( key, nullptr );
if( p.second ) p.first->second = std::make_shared<T>( myString );
else {
    if( !p.first->second ) { // handle nullptr }
}
auto &ptr = p.first->second;