Constructors and runtime typing

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2 total messages Started by John Ellis Tue, 26 Jan 1993 04:33

#1368

Author: John Ellis
Date: Tue, 26 Jan 1993 04:33

15 lines
772 bytes

Regarding Stroustrup's proposal for runtime typing:

Suppose class B is derived from A and A's constructor calls typeof(*this):

    class A    {A() {...typeof(*this)...}; ...};
    class B: A {...};

(Excuse any mistakes in notation.)

When "new B" is evaluated, will typeof(*this) yield typeof(A) or
typeof(B)? 

The recommended implementation technique seems to suggest typeof(A),
and that's consistent with the handling of virtual functions within
constructors.

Re: Constructors and runtime typing

#1373

Author: Bjarne Stroustrup
Date: Tue, 26 Jan 1993 18:38

35 lines
1532 bytes


ellis@parc.xerox.com (John Ellis @ Xerox PARC) writes

 > Regarding Stroustrup's proposal for runtime typing:
 > 
 > Suppose class B is derived from A and A's constructor calls typeof(*this):
 > 
 >     class A    {A() {...typeof(*this)...}; ...};
 >     class B: A {...};
 > 
 > (Excuse any mistakes in notation.)

It is typeid() not typeof().

 > When "new B" is evaluated, will typeof(*this) yield typeof(A) or
 > typeof(B)? 
 > 
 > The recommended implementation technique seems to suggest typeof(A),
 > and that's consistent with the handling of virtual functions within
 > constructors.  

typeid(*this)==typeid(A)

As you point out anything else would be inconsistent with the behavior
of virtual functions.

References: 

	Run-Time Type Identification for \*C (Revised). Proc USENIX 1992 C++ Conference

	x3j16/92-0121, WG21/N0198: Run-Time Type Identification for \*C (Revised yet again)

Both by me and Dmitry Lenkov. The latter was revised to reflect discussions at the USENIX conference and
elsewhere.

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