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set & lt; T & gt; vs set & lt; T, comparator & gt; (C ++ Polymor...

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set & lt; T & gt; vs set & lt; T, comparator & gt; (C ++ Polymorphism)

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Why does this code

struct ThingComparator
{
    ...
}

static void Blah (set <CString> &things)
{
    ...
}

...

set<CString, ThingComparator>things;
Blah (things);

fail to compile with the following error (Visual Studio 2010):

error C2664: 'Blah' : cannot convert parameter 1 from 'std::set<_Kty,_Pr>' to 'std::set<_Kty> &'

My C++ knowledge is obviously limited, but I expected to hear a trumpet announcing the polymorphism knight on his trusty steed, but instead all I can hear is a horse fart and a sad trombone :-(

std::set is declared as follows:

template<
    class Key,
    class Compare = std::less<Key>,
    class Allocator = std::allocator<Key>
> class set;

Hence std::set<CString> really means std::set<CString, std::less<CString>, std::allocator<CString>>, and std::less<CString> isn't ThingComparator. Write the following instead:

struct ThingComparator {
    ...
};

template<typename Comparator>
static void Blah(std::set<CString, Comparator>& things) {
    ...
}

...

std::set<CString, ThingComparator> things;
Blah(things);

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