`foo() = default;` is an explicit way to generate a default constructor for `foo`. The default constructor works by recursively calling the default constructors

`foo() = default;` is an explicit way to generate a default constructor for `foo`. The default constructor works by recursively calling the default constructors for all class instance fields. In C++ there are a bunch of rules about when a class has a default constructor or not, but by explicitly declaring one you are guaranteed to have it so long as all your class instance fields have default constructors. Your example of having a field called `a` that is initialized to 0 is perfectly valid C++ as well but it's not the same as an explicitly declared default constructor.