C++ :: Retrieving Class Types From File Efficiently

Suppose your program has many concrete subtypes of Person, and each person will have their own file saved, with their type stored in that file. When reading the files to create the people again (of their proper types), what is the best method to maximize performance (and elegance by shortening the code)? Here is what I have so far.

First I used if statements, which is terrible, and now I've improved the performance logarithmically using std::map. I still suspect there is a better way, especially if there are going to be hundreds of different classes. If you want to test it, you can change the PATH constant to whatever path you want, or just leave it as an empty string, and the files will be created in the same directory as your cpp file. The part I'm trying to improve is pointed out in the comments.

struct Person {
std::string name;
Person (const std::string& newName) : name (newName) {}
virtual ~Person() = default;
};
struct Guy : Person {using Person::Person;};
struct Girl : Person {using Person::Person;};

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