Re: Accessing member via set's NON-const iterator that doesn't affect invariants

From:

Anand Hariharan <mailto.anand.hariharan@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Thu, 30 Aug 2012 10:41:50 -0700 (PDT)

Message-ID:

<2323d02f-57be-42e3-9256-c5f99cce003f@z17g2000yqk.googlegroups.com>

On Aug 30, 12:25 pm, Victor Bazarov <v.baza...@comcast.invalid> wrote:

On 8/30/2012 1:14 PM, Anand Hariharan wrote:

Let's say I have code like so:

#include <set>
#include <math.h>

typedef int OtherTypes;

struct MyType
{
     double Field1;
     OtherTypes MoreFields;

     MyType(double blah) :
         Field1(blah)
     {
     }

     bool operator < (const MyType &That) const
     {
         // Does not use any other member
         return ( fabs(Field1 - That.Field1) > 1e-6 &&
                  Field1 < That.Field1 );
     }
};

int main()
{
     std::set<MyType> foo;
     std::pair< std::set<MyType>::iterator,
                bool > inchk = foo.insert(MyType(1.0)=

);

     OtherTypes SomeVal = 1;
     if ( inchk.second )
         inchk.first->MoreFields = SomeVal; // error
}

How do I reassure the compiler that writing MoreFields will not affect
any invariants or will not do anything to invalidate the order of
elements in the set?

Perhaps you should declare 'MoreFields' "mutable"?

Thank you, Victor! I'd forgotten all about that keyword despite that
I'd argued a case for that keyword with a colleague many years ago.

If the only recourse is to use another container such as vector, how
do I insert a new value in the sorted position while checking if one
exists already?

If your vector is sorted, use 'lower_bound' (or is it 'upper_bound'?) to
find the place where the sorting would place your element, then compare
what you want to insert to what's already there (the next element or the
previous, can't divine now) thus /checking/ if one exists.

That is more tedious than the alternative of simply declaring other
fields as mutable, IMHO.

thank you again,
- Anand