How to initialize an intializer_list?

Question

Some cpp class accept initializer_list as input, for example:

std::map<std::string, int> m{{"CPU", 10}, {"GPU", 15}, {"RAM", 20}};

or

std::map<std::string, int> m={{"CPU", 10}, {"GPU", 15}, {"RAM", 20}};

Is it possible, to define an initializer_list variable, then use it to call the constructor? i.e. something like:

auto il = {{"CPU", 10}, {"GPU", 15}, {"RAM", 20}}; std::map<std::string, int> m = il;

Note that initializer_list is a proxy object, it does not actually hold values. So using it as a universal storage to pass stuff around is not a good idea. — user7860670
– user7860670, Commented May 4, 2024 at 19:48

Ted Lyngmo · Accepted Answer · 2024-05-04 19:46:40Z

Yes. Example:

#include <initializer_list> #include <map> #include <string> int main() { std::initializer_list<std::pair<const std::string, int>> il = { {"CPU", 10}, {"GPU", 15}, {"RAM", 20}}; std::map<std::string, int> m = il; }

3CEZVQ · Accepted Answer · 2024-05-04 20:11:39Z

Yes, it's possible. You only need to specify the value type of the initializer list explicitly.

#include <map> #include <string> int main() { using map = std::map<std::string, int>; auto il = {map::value_type{"CPU", 10}, {"GPU", 15}, {"RAM", 20}}; std::map<std::string, int> m = il; }

or

#include <map> #include <string> #include <utility> int main() { auto il = { std::pair<const std::string, int>{"CPU", 10}, {"GPU", 15}, {"RAM", 20}}; std::map<std::string, int> m = il; }

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