CODEWITHSUNDEEP
c++templates
user1652319
user1652319

Reputation: 103

When creating a custom map class in c++ how to define and use a comparator in the template

I'm creating a container that works like the STL using an array. One thing I am stuck on is how to implement a comparator object to the template 

template <typename  K, typename V, typename KeyEqual = myEqualObject<K> >
class Map {
......
}

would that template header need to be repeated on all method bodies outside of the class ? i.e

template <typename K, typename V >
V& Map<K,V>::operator[] (K key) {
...
}

should be 

template <typename  K, typename V, typename KeyEqual = myEqualObject<K> >
V& Map<K,V>::operator[] (K key) {
...
}

Also I am struggling with how to use/implement this extra object in my class to check if two key values are the same. Can anyone shed any light ?

I have looked around this subject but cannot find any examples in this context

Upvotes: 0

Views: 2467

Answers (1)

Mike Seymour
Mike Seymour

Reputation: 254431

would that template header need to be repeated on all method bodies outside of the class ?

You need all the template parameters, but there's no need to repeat the default arguments:

template <typename K, typename V, typename KeyEqual>
V& Map<K,V,KeyEqual>::operator[] (K key) {
...
}

Also I am struggling with how to use/implement this extra object in my class to check if two key values are the same.

Following the example of the standard containers, store a comparator object in a (probably private) member variable:

KeyEqual equal;

initialise it in the constructor(s), allowing the user to supply one:

Map(/* arguments, */ KeyEqual equal = KeyEqual()) : 
    // initialisers,
    equal(equal)
{
    // anything else
}

and use it as a binary predicate:

if (equal(key1, key2)) {
    // treat them as equal
}

It can be implemented as a binary function class:

template <typename K> struct myEqualObject {
    bool operator()(K const & k1, K const & k2) {return k1 == k2;}
};

although I'd use std::equal_to<K> as the default, rather than rewriting my own version.

Upvotes: 4

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