Re: STL set lower_bound

From:

Paavo Helde <paavo@nospam.please.ee>

Newsgroups:

comp.lang.c++

Date:

Mon, 30 Mar 2009 02:32:26 -0500

Message-ID:

<Xns9BDE6B3A81201nobodyebiee@216.196.109.131>

mattg <gara.matt@gmail.com> kirjutas:

On Mar 29, 3:31?pm, Paavo Helde <pa...@nospam.please.ee> wrote:

mattg <gara.m...@gmail.com> kirjutas:

in my actual application I clean up. My original attempt at this was
using node as a struct instead of a class but I couldn't figure out
how to write the line

iter = s1.lower_bound(new node(5));

I would write this example as:

#include<iostream>
#include<set>

using namespace std;

class node {
private:
? ? ? ? int val;
public:
? ? ? ? node(int v){
? ? ? ? ? ? ? ? val = v;
? ? ? ? }
? ? ? ? int GetVal() const {
? ? ? ? ? ? ? ? return val;
? ? ? ? }
? ? ? ? bool operator<(const node& b) const {
? ? ? ? ? ? ? ? return val<b.val;
? ? ? ? }

};

set<node> s1;

void test(){
? ? ? ? s1.insert(1);
? ? ? ? s1.insert(2);
? ? ? ? s1.insert(8);
? ? ? ? s1.insert(9);
? ? ? ? s1.insert(12);

}

int main(){
? ? ? ? test();
? ? ? ? set<node>::iterator iter;

? ? ? ? iter = s1.lower_bound(5);
? ? ? ? cout << "iter =" << iter->GetVal() << endl;

}

Ahh ok, thats much cleaner, but I have 2 questions:

1) does this design not cause any memory leaks

No 'new', nothing to leak.

2) if I add to the node

class node {
private:
        int val;
        int val2;
public:
        node(int v){
                val = v;
                val2 = 0;
        }
        int GetVal() const {
                return val;
        }
        int Do() {
                val2++;
        }
        bool operator<(const node& b) const {
                return val<b.val;
        }

};

and I try calling iter->Do() I get error

test.cpp:80: error: passing 'const node' as 'this' argument of 'void
node::Do()' discards qualifiers

The things in std::set are considered immutable so by default they act as
const. If changing val2 is to be considered not to change the object
state logically (the set order is not affected, for example - this is the
case here), then you should be able to get this working as:

class node {
....
        mutable int val2;
....
        int Do() const { // note 'const' here
                val2++;
        }
};

Another way could be to use std::map<int,node> or something, instead of
std::set, depending on your goals.

Paavo