Error in operator function in multimap - c++

Question

I am trying to create a multimap using multikey structure as a key and I am getting a error described below:
code:
struct stCont
{
long long Tok;
char Reserved;
long long Asset;
}
struct MultiKey {
char InstrumentName[6];
char Symbol[10];
long long ExpiryDate;
}
struct myComp
{
bool operator() (const MultiKey& lhs, const MultiKey& rhs)
{
if((lhs.ExpiryDate==rhs.ExpiryDate)&&(memcmp(lhs.InstrumentName,rhs.InstrumentName,6))&&(memcmp(lhs.Symbol,rhs.Symbol,10)))
{
return 1;
}
return 0;
}
};
std::multimap<MultiKey, stCont,myComp> cont_map;
error:
expression having type 'const myComp' would lose some const-volatile qualifiers in order to call 'bool myComp::operator ()(const MultiKey &,const MultiKey &)'

Answer 1

you should rewrite the multimap code like this and remove the mycomp structure:
struct MultiKey {
char InstrumentName[6];
char Symbol[10];
long long ExpiryDate;
bool operator< (const MultiKey& lhs)const
{
if((lhs.ExpiryDate==ExpiryDate)&&(memcmp(lhs.InstrumentName,InstrumentName,6))&&(memcmp(lhs.Symbol,Symbol,10)))
{
return true;
}
return false;
}
};

Answer 2

Why don't you just write just write operator < for MultiKey? Or you'll have to change myComp because it isn't what multimap wants anyway (it wants a less-than comparison).

Answer 3

Look at the The C++11 Standard, §23.4.5.1 and the header:
template <class Key, class T, class Compare = less<Key>,
class Allocator = allocator<pair<const Key, T> > >
class multimap {
public:
// ...
class value_compare {
friend class multimap;
protected:
Compare comp;
value_compare(Compare c) : comp(c) { }
public:
typedef bool result_type;
typedef value_type first_argument_type;
typedef value_type second_argument_type;
bool operator()(const value_type& x, const value_type& y) const {
return comp(x.first, y.first);
}
};
// ...
};
The comparison function as defined by class value_compare is const. Now, I may be misinterpreting the standard, but this definition seems to be invalid if the operator() is non-const in class Compare.
As to why does it work for some people... Perhaps some finer points about instantiation rules prevent this from being an error, or the implementations are not requierd to adhere strictly to the type definitions in the standard; if so, I'd be glad if someone more versed with The Standard could clarify.

Answer 4

To fix the compile error, declare the comparison function to be a const member:
bool operator() (const MultiKey& lhs, const MultiKey& rhs) const
^^^^^
Then you have another problem: the comparator needs to perform a "less than" comparison, but yours does an equality comparison. You want something like
if (lhs.ExpiryDate < rhs.ExpiryDate) return true;
if (lhs.ExpiryDate > rhs.ExpiryDate) return false;
if (memcmp(lhs.InstrumentName,rhs.InstrumentName,6) < 0) return true;
if (memcmp(lhs.InstrumentName,rhs.InstrumentName,6) > 0) return false;
if (memcmp(lhs.SymbolName,rhs.SymbolName,10) < 0) return true;
return false;
You may find it more convenient to overload operator< rather than defining a named comparator type, so that you can use std::multimap<MultiKey, stCont> with the default comparator.