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Chapter 6

Runtime Type Identification

This chapter explains the use of Runtime Type Identification (RTTI). Use this feature while a program is running to find out type information that you could not determine at compile time.

6.1 Static and Dynamic Types

In C++, pointers to classes have a static type, the type written in the pointer declaration, and a dynamic type, which is determined by the actual type referenced. The dynamic type of the object could be any class type derived from the static type. In the following example, ap has the static type A* and a dynamic type B*.

class A {};
class B: public A {};
extern B bv;
extern A* ap = &bv;

RTTI allows the programmer to determine the dynamic type of the pointer.

6.2 RTTI Options

In compatibility mode (-compat[=4]), RTTI support requires significant resources to implement. RTTI is disabled by default in that mode. To enable RTTI implementation and recognition of the associated typeid keyword, use the option -features=rtti. To disable RTTI implementation and recognition of the associated typeid keyword, use the option --features=no%rtti (the default).

In standard mode (the default mode), RTTI does not have a significant impact on program compilation or execution. RTTI is always enabled in standard mode.

6.3 typeid Operator

The typeid operator produces a reference to an object of class type_info, which describes the most-derived type of the object. To make use of the typeid() function, the source code must #include the <typeinfo> header file. The primary value of this operator and class combination is in comparisons. In such comparisons, the top-level const and volatile qualifiers are ignored, as in the following example. Note that, in this example, A and B are types which have default constructors.

#include <typeinfo>
#include <assert.h>
void use_of_typeinfo( )
      A a1;
      const A a2;
      assert( typeid(a1) == typeid(a2) );
      assert( typeid(A)  == typeid(const A) );
      assert( typeid(A)  == typeid(a2) );
      assert( typeid(A)  == typeid(const A&) );
      B b1;
      assert( typeid(a1) != typeid(b1) );
      assert( typeid(A)  != typeid(B) ); 

The typeid operator throws a bad_typeid exception when given a null pointer.

6.4 type_info Class

The class type_info describes type information generated by the typeid operator. The primary functions provided by type_info are equality, inequality, before and name. From <typeinfo.h>, the definition is:

    class type_info {
            virtual ~type_info( );
            bool operator==( const type_info &rhs ) const;
            bool operator!=( const type_info &rhs ) const;
            bool before( const type_info &rhs ) const;
            const char *name( ) const;
            type_info( const type_info &rhs );
            type_info &operator=( const type_info &rhs );

The before function compares two types relative to their implementation-dependent collation order. The name function returns an implementation-defined, null-terminated, multibyte string, suitable for conversion and display.

The constructor is a private member function, so you cannot create a variable of type type_info. The only source of type_info objects is in the typeid operator.

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