Lookup objects by name on std::set

Question

I need to store some objects in a std:set (or any other kind of lookup table) and search those by name.

For example, suppose I have a class like (pseudo code):

class Person
{
    std::string mName;
    int mAge;
    ... //etc
};

I would like to store this on a container and search for objects by name. I cannot insert those on a std::set, because as far I know, I must construct an entire object for searching.

My second though was to use a std::map like std::map, but, I will need to duplicate the name for this and I do not want to duplicate the key.

Is there a way to store this kind of object in a std::set (or any other container) and search by a key (not the object) ?

Thank you

This probably is a duplicate question, but I don't think the one selected is a good fit. — Mark Ransom, Jan 09 '15 at 23:34
P.S. In C++14 you'll be able to specify a parameter to `find` which is *not* the exact type of the set item. See http://en.cppreference.com/w/cpp/container/set/find for details. — Mark Ransom, Jan 09 '15 at 23:43
@Praetorian not relevant for the question. The main concern is: how to store a object in a set and search it based on a property. Assume that the property is unique for each element. — bcsanches, Jan 10 '15 at 04:30
As Mark mentioned earlier, there is a very elegant c++14 solution to this problem depending on what the default ordering looks like, but I guess it's not relevant. — Praetorian, Jan 10 '15 at 04:43

score 1 · Answer 1 · answered Jan 09 '15 at 23:24

Please take a look at boost::intrusive containers, in particular boost::intrusive::set.

In exchange for some uglification of the Item class comes the great flexibility. Uglification happens because Item must derive from a certain class or it must declare a special member variable to store tree links. This is all nicely wrapped though.

Concerning the particular requests, boost::intrusive::set allows to

reuse mName for the key (not unlike std::set)
lookup Person by a string (unlike std::set there is no need to construct a Person to do a lookup)

Yes, i am aware of the intrusive, but I do not want to make it intrusive in this case :) — bcsanches, Jan 10 '15 at 04:31

Cory Kramer · Answer 2 · 2015-01-09T23:33:48.937

0

You can use std::find_if, for example if you have a std::set<Person> people

auto match = std::find_if(people.begin(), people.end(), [](const Person& foo){return foo.mName == "Bob";});
if (match != people.end())
{
    std::cout << "Found Bob!"
}

So you don't need to create a full Person object, just a std::string to match their name, for example.

edited Jan 09 '15 at 23:33

answered Jan 09 '15 at 23:26

Cory Kramer

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That's the really slow way to do it. – Mark Ransom Jan 09 '15 at 23:30
@MarkRansom: in fact it's the slowest way that doesn't perversely go out of its way to be slow. That is to say, the ideal Agile solution v1.0. If it passes the tests, ship it! If it doesn't, ask another question ;-) – Steve Jessop Jan 09 '15 at 23:31
Thats my concern, it will be a linear search right? – bcsanches Jan 10 '15 at 04:30

score 0 · Answer 3 · answered Jan 09 '15 at 23:26

So you can use find_if to achieve this:

struct find_by_name {
    find_by_name(const std::string & name) : name(name) {}
    bool operator()(const Person & person) {
        return person.name == name;
    }
private:
    std::string name;
};

// in your code

std::set<Person>::iterator result = std::find_if(people.begin(), people.end(), 
                                              find_by_name("Ben"));
if(result != people.end()) {
    // we found something
}
else {
    // no match
}

This way you search based only on a generated string (in a struct) and its very adaptable, you can initialize it with any string, you can also use it to search for person in any container that implements an iterator.

Does find_if figure out is is a set and do a "fast" search? – bcsanches Jan 10 '15 at 04:32 — bcsanches, Jan 10 '15 at 04:32

Lookup objects by name on std::set

3 Answers3