Renaming first and second of a map iterator

Question

Is there any way to rename the first and second accessor functions of a map iterator. I understand they have these names because of the underlying pair which represents the key and value, but I'd like the iterators to be a little more readable. I think this might be possible using an iterator adaptor, but I'm not sure how to implement it.

Please note that I can't use boost.

Example of what I mean:

map<Vertex, Edge> adjacency_list;
for(map<Vertex, Edge>::iterator it = adjacency_list.begin();
    it != adjacency_list.end();
    ++it)
{
    Vertex v = it->first;
    //instead I would like to have it->vertex
}

Solution 1

If you're just concerned about readability you could do something like this:

typedef map<Vertex, Edge> AdjacencyList;
struct adjacency
{
    adjacency(AdjacencyList::iterator& it) 
      : vertex(it->first), edge(it->second) {}
    Vertex& vertex;
    Edge& edge;
};

And then:

Vertex v = adjacency(it).vertex;

Solution 2

You can't rename the members, but you can have some functions to help.

inline Vertex& vertex(map<Vertex, Edge>::iterator& it) {return it->first;}
inline Edge& edge(map<Vertex, Edge>::iterator& it) {return it->second;}

Then, instead of it->vertex like you want, you can do vertex(it)

Solution 3

Unfortunately, no. What I usually do is this:

typedef map<Vertex, Edge> AdjacencyList;
typedef AdjacencyList::value_type Vertex_Edge_Pair;

For readability. Inside your loop you can also say

Vertex& current_vertex = it->first;
Edge& current_edge = it->second;

Solution 4

Sure, reimplement or wrap iterator, but is it worth the effort? Wouldn't

Vertex& v = it->first;

be easier?

Solution 5

If you don't need the iterator (e.g., a range-based for loop suits your purpose), then as of c++17 you can use structured bindings:

map<Vertex, Edge> adjacency_list;
for( auto & [ vertex, edge ] : adjacency_list )
{
    // do stuff with vertex
}

View more solutions

Comments

[phoku]

Also remember that using first and second of the std::map iterator is such a common idiom that you'd confuse anyone reading your code - a bit.

[Michael Krelin - hacker]

That true, but from the sanity perspective, first and second are confusing enough ;-)

[user1353535]

Nobody else suggested this because it's a terrible idea.

[Himanshuman]

Why no one suggests this?

[Jarek C]

It is requested to use it->vertex (not just v). So the answer IHMO should not be "sure", but rather "somewhat".

[Michael Krelin - hacker]

@JarekC, sure, you have a point, but I still think "sure" sounds better :)