Question

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 besides Field1
        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?

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?

Answer 1

• Declare MoreFields as mutable, or

• const_cast the inchk.first expression to remove constness, or

• encapsulate MoreFields within a const-qualified accessor that returns a non-const reference.