Как инициализировать переменные?

An object whose initializer is an empty set of parentheses, i.e., (), shall be value-initialized.

To value-initialize an object of type T means:
if T is a (possibly cv-qualified) class type (Clause 9) with either no default constructor (12.1) or a
default constructor that is user-provided or deleted, then the object is default-initialized;
if T is a (possibly cv-qualified) class type without a user-provided or deleted default constructor, then
the object is zero-initialized and the semantic constraints for default-initialization are checked, and if
T has a non-trivial default constructor, the object is default-initialized;
if T is an array type, then each element is value-initialized;
otherwise, the object is zero-initialized.

To zero-initialize an object or reference of type T means:
(6.1) — if T is a scalar type (3.9), the object is initialized to the value obtained by converting the integer literal
0 (zero) to T; 104
(6.2) — if T is a (possibly cv-qualified) non-union class type, each non-static data member and each base-class
subobject is zero-initialized and padding is initialized to zero bits;
(6.3) — if T is a (possibly cv-qualified) union type, the object’s first non-static named data member is zero-
initialized and padding is initialized to zero bits;
(6.4) — if T is an array type, each element is zero-initialized;
(6.5) — if T is a reference type, no initialization is performed.

To default-initialize an object of type T means:
(7.1) — If T is a (possibly cv-qualified) class type (Clause 9), constructors are considered. The applicable
constructors are enumerated (13.3.1.3), and the best one for the initializer () is chosen through
overload resolution (13.3). The constructor thus selected is called, with an empty argument list, to
initialize the object.
(7.2) — If T is an array type, each element is default-initialized.
(7.3) — Otherwise, no initialization is performed.

всегда инициализирует переменную