Re: How are objects inserted into a set?

From:

"Victor Bazarov" <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Fri, 8 Jun 2007 10:35:42 -0400

Message-ID:

<f4bph1$265$1@news.datemas.de>

desktop wrote:

Victor Bazarov wrote:

Johs wrote:

[..]
I can see operators that must be defined for std::set in 23.3.3,
but I can't find any requirements for the objects that I would like
to insert. This is my object that I would like to insert into a
std::set: class test {
public:
int getpp(){return pp;}
void setpp(int i){pp = i;}

int operator<(int a) const
{
return 22;
}

private:
int pp;
};

But I still get the error:

error: no match for ?operator<? in ?__x < __y

where can I find an interface for the objects to insert?

You are supposed to implement a comparison between two objects of the
type you're going to store, not between an object and an int:

    ...
    bool operator <(test const& t) const {
        ...
    }

and the actual implementation is supposed to adhere to "strict weak
ordering" rules: if two objects ('a', 'b') are equivalent (for the
purporses of storing in that set), then 'a < b' and 'b < a' should
both return false.

V

Where are such rules defined. Could not find them in the C++ Standard.

The 'set' and 'map' templates have a template argument: the "Compare"
relation (see 23.1.2), which is by default 'std::less'. Now, look up
'std::less' and keep reading until you don't understand (and cannot
figure it out) by reading it again. Then ask more specific questions.

Again: what book are you reading that doesn't explain those things?

V
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