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As with the ti(dynamic_cast) operator, ti(typeid) is usually applied to
references to base class objects that refer to derived class
objects. tt(Typeid) should only be used with base classes offering virtual
members.
Before using tt(typeid) the tthi(typeinfo) header file must be included.
The tt(typeid) operator returns an object of type tt(type_info).
Different compilers may offer different implementations of the class
tt(type_info), but at the very least tt(typeid) must offer the following
interface:
verb( class type_info
{
public:
virtual ~type_info();
int operator==(type_info const &other) const;
int operator!=(type_info const &other) const;
bool before(type_info const &rhs) const;
char const *name() const;
private:
type_info(type_info const &other);
type_info &operator=(type_info const &other);
};)
Note that this class has a private copy constructor and a private
overloaded assignment operator. This prevents code from constructing
tt(type_info) objects and prevents code from assigning tt(type_info) objects
to each other. Instead, tt(type_info) objects are
constructed and returned by the tt(typeid) operator.
If the tt(typeid) operator is passed a base class reference it is able to
return the actual name of the type the reference refers to. Example:
verb( class Base;
class Derived: public Base;
Derived d;
Base &br = d;
cout << typeid(br).name() << '\n';)
In this example the tt(typeid) operator is given a base class reference.
It prints the text ``tt(Derived)'', being the i(class name) of the class
tt(br) actually refers to. If tt(Base) does not contain virtual functions, the
text ``tt(Base)'' is printed.
The tt(typeid) operator can be used to determine the name of the actual
type of hi(expression: type of) expressions, not just of class type
objects. For example:
verb( cout << typeid(12).name() << '\n'; // prints: int
cout << typeid(12.23).name() << '\n'; // prints: double)
Note, however, that the above example is suggestive at most. It em(may)
print tt(int) and tt(double), but this is not necessarily the case. If
portability is required, make sure no tests against these static, built-in
text-strings are required. Check out what your compiler produces in case of
doubt.
In situations where the tt(typeid) operator is applied to
determine the type of a derived class, a base class em(reference)
hi(typeid: argument)
should be used as the argument of the tt(typeid) operator. Consider
the following example:
verb( class Base; // contains at least one virtual function
class Derived: public Base;
Base *bp = new Derived; // base class pointer to derived object
if (typeid(bp) == typeid(Derived *)) // 1: false
...
if (typeid(bp) == typeid(Base *)) // 2: true
...
if (typeid(bp) == typeid(Derived)) // 3: false
...
if (typeid(bp) == typeid(Base)) // 4: false
...
if (typeid(*bp) == typeid(Derived)) // 5: true
...
if (typeid(*bp) == typeid(Base)) // 6: false
...
Base &br = *bp;
if (typeid(br) == typeid(Derived)) // 7: true
...
if (typeid(br) == typeid(Base)) // 8: false
...)
Here, tt((1)) returns tt(false) as a tt(Base *) is not a tt(Derived
*). tt((2)) returns tt(true), as the two pointer types are the same, tt((3))
and tt((4)) return tt(false) as pointers to objects are not the objects
themselves.
On the other hand, if tt(*bp) is used in the above expressions, then
tt((1)) and tt((2)) return tt(false) as an object (or reference to an object)
is not a pointer to an object, whereas tt((5)) now returns tt(true): tt(*bp)
actually refers to a tt(Derived) class object, and tt(typeid(*bp)) returns
tt(typeid(Derived)). A similar result is obtained if a base class reference
is used: tt(7) returning tt(true) and tt(8) returning tt(false).
The tt(type_info::before(type_info const &rhs)) member is used to
determine the emi(collating order) of classes. This is useful when comparing
two em(types) for equality. The function returns a nonzero value if tt(*this)
precedes tt(rhs) in the hierarchy or collating order of the used types. When a
derived class is compared to its base class the comparison returns 0,
otherwise a non-zero value. E.g.:
verb( cout << typeid(ifstream).before(typeid(istream)) << '\n' << // not 0
typeid(istream).before(typeid(ifstream)) << '\n'; // 0)
With built-in types the implementor may implement that non-0 is returned
when a `wider' type is compared to a `smaller' type and 0 otherwise:
verb( cout << typeid(double).before(typeid(int)) << '\n' << // not 0
typeid(int).before(typeid(double)) << '\n'; // 0)
When two equal types are compared, 0 is returned:
verb( cout << typeid(ifstream).before(typeid(ifstream)) << '\n'; // 0)
When a tt(0)-pointer is passed to the tt(operator typeid) a ti(bad_typeid)
exception is thrown.
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