c how to construct variant efficiently?-CodePudding

may I know if there is more efficient way of constructing below variant?

#include <variant>
#include <string>
class Person{
public:
    Person(std::string name, int age, std::string school)
    :name_(name), age_(age){}

private:
    std::string name_;
    int age_;

};
class Dummy1{};
class Dummy2{};
using PersonVariant = std::variant<Person, Dummy2, Dummy1>;
int main() {
    Person person("jack", 10, "school");
    PersonVariant pv(person); // does this involve an uneccessary copy?
}

The construct seems involve an unnecessary copy.

CodePudding user response：

Most efficient way is to use in-place constructor:

PersonVariant pv{std::in_place_type<Person>, "jack", 10, "school"};

CodePudding user response：

PersonVariant pv(person); // does this involve an uneccessary copy?

Yes. person is an lvalue so a copy will be made to construct the Person object in the variant. You have a couple of options to make this better. The simplest fix is to use

PersonVariant pv(Person("jack", 10, "school"));

Which will construct a temporary object that will get moved into pv.

You can also use variants in place construction to pass the values to construct the Person object directly inside the variant like

PersonVariant pv(std::in_place_type<Person>, "jack", 10, "school");

There is no moving or copy of a Person in this case and at most you would have a move or copy of the parameters to construct the Person object but you have that cost no matter what so this can be considered the most efficient way to do this.

For completeness sake you could also use

Person person("jack", 10, "school");
PersonVariant pv(std::move(person));

which will force person to be moved into the variant like PersonVariant pv(Person("jack", 10, "school")); does but now person is a moved from object and you can only reuse it as it is essentially an empty object (in the context of the values of the members).