CODEWITHSUNDEEP
c++dictionaryhashmap
MIKU_LINK
MIKU_LINK

Reputation: 192

Why should operator< be defined as non-member?

I do some test on hash_map, using struct as key. I define struct:

struct ST
{

    bool operator<(const ST &rfs)
    {
        return this->sk < rfs.sk;
    }

    int sk;
};

and:

size_t hash_value(const ST& _Keyval)
{   // hash _Keyval to size_t value one-to-one
    return ((size_t)_Keyval.sk ^ _HASH_SEED);
}

then:

stdext::hash_map<ST, int> map;
ST st;
map.insert(std::make_pair<ST, int>(st, 3));

It gives me a compiler error:binary '<' : no operator found which takes a left-hand operand of type 'const ST' (or there is no acceptable conversion)

So I change the operator to non-member:

bool operator<(const ST& lfs, const ST& rfs)
{
    return lfs.sk < rfs.sk;
}

It's OK.So I want to know why?

Upvotes: 0

Views: 96

Answers (3)

shengy
shengy

Reputation: 9749

Despite of above answers, I recommend you have a look at:

C++ Primer, Fourth Edition, Chapter 14, section 14.1

Ordinarily we define the arithmetic and relational operators as nonmember functions and we define assignment operators as members:

Upvotes: 0

Adrian Shum
Adrian Shum

Reputation: 40036

I believe the problem is

error:binary '<' : no operator found which takes a left-hand operand of type 'const ST' (or there is no acceptable conversion)

Your member function bool operator<(const ST &rfs) is declared non-const, so that it cannot be invoked against a const ST.

Just change it to bool operator<(const ST &rfs) const and it should work

Upvotes: 0

Jesse Good
Jesse Good

Reputation: 52365

You were missing a const:

bool operator<(const ST &rfs) const
{
    return this->sk < rfs.sk;
}

Upvotes: 4

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