Question

void destroy()
{
    AList::const_iterator a;
    for(a = AList.begin(); a != AList.end();)
    {
        if(!a->second.BList.empty())
            a->second.BList.clear();//will give error if not mutable
    }
}
typedef std::map<unsigned int,int> bmap;
typedef std::map<unsigned int,someStruct> Alist;
typedef struct someStruct
{
    float x,y,z;
    bmap BList; //needs to be mutable for Blist.clear() above.
    //mutable bmap BList; //<---like this
} someStruct;

I only chanced across mutable as an option in a similar but not the same question. My question is am I doing the right thing, or if there are any pitfalls in doing so? Thank you for your help in advance.

//error given: (if otherwise not mutable)
// error: passing 'const AList' as 'this' argument of 'void std::map<_Key, _Tp, _Compare, _Alloc>::clear() [with _Key = unsigned int, _Tp = int, _Compare = std::less<unsigned int>, _Alloc = std::allocator<std::pair<const unsigned int, int> >]' discards qualifiers

Answer 1

You should use iterator instead of const_iterator, if your intent is to call clear. const_iterator is for cases where you are calling only const member functions.

Using mutable is not appropriate for this situation. Only mark member variables mutable if they are not part of the object’s visible state, e.g., cached data.