C++ 中的静态映射

Question

I am working on optimizing the code that creates tens of thousands of maps of type

std::unordered_map<std::wstring, std::wstring>

The number of possible keys is about 20 and they all are known at compile time.

My idea is to probably have some specialized container with the minimal memory footprint, but ideally with C++ standard-like interface.

The other idea is to have something like std::tuple but with the index of type char * . Probably it is a strange idea and I did not think yet on its implementation and I am not sure it can be implemented, but theoretically it can be

std::static_map<"key1", "key2", ... > m;
m<"key1"> = "value1";
std::string val = m<"key1">;

theoretically there can be an implementation that uses consexpr hash function.

EDIT1: It is a data processing software that collects logs (or messages) over UDP protocol. Once a message is received an MSG structure containing this map along with other data is created. Then MSG is added to the processing queue and after further processing is added to some other queue and so on.... If the processing is slow there can be tens of thousands of MSG's in the queues.

Answer 1

Your code creates and populates lots of maps and lots of wstrings, which means lots of allocations.

A first reduction of the allocation effort could be to create the unordered_map with already a fixed number of buckets (20 ?).

But given the small number of keys known at compile time, I'd suggest to have a mapping between the key string to an integer between 0 and the known maximum number of keys. You may then replace the unordered_map with a vector (also created with a fixed size), and avoid hashing for direct access to any given field.

Depending on the length of the key strings, you could perhaps use a trie to parse the key strings in the input message.