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          Since the purpose of optional is to allow us to use objects with a formal
          uninitialized additional state, the interface could try to follow the interface
          of the underlying T type
          as much as possible. In order to choose the proper degree of adoption of
          the native T interface,
          the following must be noted: Even if all the operations supported by an
          instance of type T are
          defined for the entire range of values for such a type, an optional<T>
          extends such a set of values with a new value for which most (otherwise
          valid) operations are not defined in terms of T.
        
          Furthermore, since optional<T> itself is merely a T
          wrapper (modeling a T supertype),
          any attempt to define such operations upon uninitialized optionals will
          be totally artificial w.r.t. T.
        
          This library chooses an interface which follows from T's
          interface only for those operations which are well defined (w.r.t the type
          T) even if any of the operands
          are uninitialized. These operations include: construction, copy-construction,
          assignment, swap and relational operations.
        
          For the value access operations, which are undefined (w.r.t the type T) when the operand is uninitialized,
          a different interface is chosen (which will be explained next).
        
          Also, the presence of the possibly uninitialized state requires additional
          operations not provided by T
          itself which are supported by a special interface.
        
A relevant feature of a pointer is that it can have a null pointer value. This is a special value which is used to indicate that the pointer is not referring to any object at all. In other words, null pointer values convey the notion of nonexistent objects.
This meaning of the null pointer value allowed pointers to became a de facto standard for handling optional objects because all you have to do to refer to a value which you don't really have is to use a null pointer value of the appropriate type. Pointers have been used for decades—from the days of C APIs to modern C++ libraries—to refer to optional (that is, possibly nonexistent) objects; particularly as optional arguments to a function, but also quite often as optional data members.
          The possible presence of a null pointer value makes the operations that
          access the pointee's value possibly undefined, therefore, expressions which
          use dereference and access operators, such as: (
          *p
          = 2 ) and (
          p->foo() ), implicitly convey the notion of optionality,
          and this information is tied to the syntax of the
          expressions. That is, the presence of operators *
          and -> tell by themselves
          —without any additional context— that the expression will be undefined
          unless the implied pointee actually exist.
        
          Such a de facto idiom for referring to optional objects
          can be formalized in the form of a concept: the OptionalPointee concept. This
          concept captures the syntactic usage of operators *,
          -> and contextual conversion
          to bool to convey the notion
          of optionality.
        
However, pointers are good to refer to optional objects, but not particularly good to handle the optional objects in all other respects, such as initializing or moving/copying them. The problem resides in the shallow-copy of pointer semantics: if you need to effectively move or copy the object, pointers alone are not enough. The problem is that copies of pointers do not imply copies of pointees. For example, as was discussed in the motivation, pointers alone cannot be used to return optional objects from a function because the object must move outside from the function and into the caller's context.
          A solution to the shallow-copy problem that is often used is to resort
          to dynamic allocation and use a smart pointer to automatically handle the
          details of this. For example, if a function is to optionally return an
          object X, it can use shared_ptr<X>
          as the return value. However, this requires dynamic allocation of X. If X
          is a built-in or small POD, this technique is very poor in terms of required
          resources. Optional objects are essentially values so it is very convenient
          to be able to use automatic storage and deep-copy semantics to manipulate
          optional values just as we do with ordinary values. Pointers do not have
          this semantics, so are inappropriate for the initialization and transport
          of optional values, yet are quite convenient for handling the access to
          the possible undefined value because of the idiomatic aid present in the
          OptionalPointee
          concept incarnated by pointers.
        
          For value access operations optional<> uses operators *
          and -> to lexically warn
          about the possibly uninitialized state appealing to the familiar pointer
          semantics w.r.t. to null pointers.
        
| ![[Caution]](../../../../../../../doc/src/images/caution.png) | Caution | 
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            However, it is particularly important to note that  | 
          For instance, optional<> does not have shallow-copy so does
          not alias: two different optionals never refer to the same
          value unless T itself is
          a reference (but may have equivalent values). The
          difference between an optional<T> and a pointer must be kept in mind,
          particularly because the semantics of relational operators are different:
          since optional<T>
          is a value-wrapper, relational operators are deep: they compare optional
          values; but relational operators for pointers are shallow: they do not
          compare pointee values. As a result, you might be able to replace optional<T>
          by T*
          on some situations but not always. Specifically, on generic code written
          for both, you cannot use relational operators directly, and must use the
          template functions equal_pointees()
          and less_pointees()
          instead.