C++

12.8    Initialization via Constructor

You are already familiar with the simple struct Person:

struct Person { string name_; int age_; string city_; };  

You also know that you can initialize the aggregate Person with its three elements in four correct ways:[ 18 ]

Person p1 { name, age, city };Person p2 { name, age };Person p3 { name };Person p4 { };  

Now, if we add the designated initializers introduced in C++20, there are seven more possibilities:

Person p1 { .name_=name, .age_=age, .city_=city };Person p2 { .name_=name, .city_=city };          // and 2 morePerson p3 { .age_=age };                         // and 2 more  

All these variants are valid and initialize all fields. The ones not listed are value-initialized, meaning they are filled with a meaningful, usually null-like ...