Vector object initialization

If the vector stores built-in elements, the standard library uses 0 to create the element initialization type.

If the vector stores the class type containing the default constructor, the standard library uses the default constructor of this type to initialize the element.

If the vector stores class types with custom constructor and no default constructor, it not only provides the number of elements, but also the initial element values.

If the vector stores classes without any constructor, the standard library still produces an object with an initial value.