gas@cs.nott.ac.uk (Alan Shepherd) (01/09/90)
In the code for the NIH library, there are several declarations of the
following form:
const Object* Object::method() const;
and also:
friend const Object* Object::method() const;
I've tried to compile this with g++-1.36.2 and discovered that friends
aren't allowed to have function modifiers in g++ i.e the terminating const.
Looking at the gnu manual, it seems that the use of the terminating const
(see 6.12) supposedly has a meaning unique to g++, yet must be legal syntax
for AT&T 2.0 since the library is written for that compiler.
Removing the terminating const causes lots of other related error messages
to do with const member functions calling non-const member functions etc.
Unfortunately, we don't have AT&T C++ or a reference manual, so I'm unsure
of the precise effect the use of const.
Could someone clarify this for me please and perhaps clear up the differences
between AT&T and G++ ?
Alan Shepherdcomeau@utoday.UUCP (Greg Comeau) (01/11/90)
In article <14940@robin.cs.nott.ac.uk> gas@cs.nott.ac.uk (Alan Shepherd) writes: >In the code for the NIH library, there are several declarations ...: > const Object* Object::method() const; > friend const Object* Object::method() const; > >I've tried to compile this with g++-1.36.2 and discovered that friends >aren't allowed to have function modifiers in g++ i.e the terminating const. >Removing the terminating const causes lots of other related error messages >to do with const member functions calling non-const member functions etc. > >Alan Shepherd This is correct. The thing is a const member function can be used with both const data and non-const data, however a non-const member function can only be used with non-const data so removing the const means that any const object that was being manipulated by the previous const function can no longer be manipulated (where manipulated is really == modified) since it must maintain the semantics of operations performed on const data whether that data be member data or non-member data (say an external global const instance of a class). So since NIH is working code though, removing the const means that when a const MF calls a non-const MF it has no way to keep track of what the non-const function is doing. In cfront 2.0 this produces a warning though and not an error. -- Greg, Comeau Computing, 91-34 120th Street, Richmond Hill, NY, 11418 Also, mag writer for UNIX Today! (SysAdm columnist), Microsoft Systems Journal (C programming), + others. Also, BIX c.language & c.plus.plus conf. moderator. Here:attmail!csanta!greg / BIX:comeau / CIS:72331, 3421 / voice:718-849-2355
jeffa@hpmwtd.HP.COM (Jeff Aguilera) (01/11/90)
> In the code for the NIH library, there are several declarations of the > following form: > > const Object* Object::method() const; > > and also: > > friend const Object* Object::method() const; > > I've tried to compile this with g++-1.36.2 and discovered that friends > aren't allowed to have function modifiers in g++ i.e the terminating const. > Looking at the gnu manual, it seems that the use of the terminating const > (see 6.12) supposedly has a meaning unique to g++, yet must be legal syntax > for AT&T 2.0 since the library is written for that compiler. > > Removing the terminating const causes lots of other related error messages > to do with const member functions calling non-const member functions etc. > Unfortunately, we don't have AT&T C++ or a reference manual, so I'm unsure > of the precise effect the use of const. > > Could someone clarify this for me please and perhaps clear up the differences > between AT&T and G++ ? > > Alan Shepherd > ---------- Within a non-static member function of a class X, this implicitly has one of four types: X *const const X *const volatile X *const volatile const X *const //like boiling ice That is, this cannot be modified (it's a *const), and *this is an X, const X, volatile X, or volatile const X. An X can be promoted to a const X, but not the conversely. Your problem with const friends can probably be circumvented by delegating broader access rights: friend class Object; If gnu doesn't compile the const member function, you're up a creek. ----- jeffa
keith@nih-csl.UUCP (keith gorlen) (01/14/90)
In article <14940@robin.cs.nott.ac.uk>, gas@cs.nott.ac.uk (Alan Shepherd) writes: > > In the code for the NIH library, there are several declarations of the > following form: > > const Object* Object::method() const; > > and also: > > friend const Object* Object::method() const; > > I've tried to compile this with g++-1.36.2 and discovered that friends > aren't allowed to have function modifiers in g++ i.e the terminating const. In C++ 2.0, you can overload member functions based on their const-ness; i.e., you can declare a const member function that has the same name and arguments as a non-const member function. This is very useful. Consider: class String { // ... char& operator[](int); const char& operator[](int) const; }; // ... const String s = "A constant string"; String t = "A variable string"; t[1] = 'x'; // OK -- t[1] calls String::operator[](int), which returns a char& char z = s[1]; // OK -- s[1] calls String::operator[](int) const, which returns a const char& s[1] = 'x'; // Error -- can't assign to a const If you want to make a const member function a friend, you must be able to specify the const modifier in the friend declaration to uniquely identify it, since it may be overloaded in this fashion. -- Keith Gorlen phone: (301) 496-1111 Building 12A, Room 2033 uucp: uunet!nih-csl!keith National Institutes of Health Internet: keith@alw.nih.gov Bethesda, MD 20892