Is it possible to define a variable that can be set only once?

I know of const, that can't be changed after creation. But I was wondering if there is a way to declare a variable that you set only once and after that, can't overwrite. In my code, I would like to avoid the bool variable by having an nFirst that, once set to nIdx, can't be set to the new value of nIdx.

My code:

    int nFirst = 0;
    int nIdx = 0;
    bool bFound = false;
    BOOST_FOREACH(Foo* pFoo, aArray)
    {
        if (pFoo!= NULL)
        {
            pFoo->DoSmth();
            if (!bFound)
            {
                nFirst= nIdx;
                bFound = true;
            }
        }
        nIdx++;
    }