Why does the first element in a map get displayed last?

Question

I am attempting to write a general utility code file that I can include whenever I want to for debugging purposes, instead of having to write the same code over and over again. Among those utility functions is an overloaded operator to output maps in C++.

template<typename T, typename U>
ostream &operator << (ostream &o, map<T, U> m) {
    for (typename map<T, U>::iterator it = m.begin(); it != m.end(); it++) {
        o << "{" << it->first << "} = [" << it->second << "]" << endl;
    }
    return o;
}

This function works, but for some reason the first element in the map always gets displayed last. For example, if I initialize a map with the following code:

map<string, int> test;
test["one"] = 1;
test["two"] = 2;
test["three"] = 3;

and then try to output it using the above function, it gets printed as:

{two} = [2]
{three} = [3]
{one} = [1]

It's not a huge deal since it's just output, but I'm wondering why this happens. Does this affect operations that are actually dependent on order, or is it just funky output?

The ordering of the keys is a template parameter. Insertion order is irrelevant. — Ulrich Eckhardt, Oct 24 '22 at 20:32
Can't reproduce, it prints the correct order `one` -> `three` -> `two` for me: https://godbolt.org/z/Gx5cxjfWz — Yksisarvinen, Oct 24 '22 at 20:33
When I run this, I get `{one}, {three}, {two}`. Which is expected, because by default `std::map` is ordered lexicographically by its keys (i.e. it is *not* insertion ordered, which seems to be what you are expecting). — 0x5453, Oct 24 '22 at 20:33
That is funky that it is printing backwards for you. Should be printing one three two, not two three one. — Eljay, Oct 24 '22 at 20:40
A very minimal effort would be to provide the actual output you get. The above code won't produce the output in shown order. Also, you should read documentation of a class/function when you don't knows what it does. C++ is not a language you should use for guesswork. — Phil1970, Oct 24 '22 at 21:20

score 2 · Answer 1 · answered Oct 24 '22 at 20:34

2

This is because a std::map in C++ is an ordered container which is sorted by it's keys values ascending by default.

answered Oct 24 '22 at 20:34

Bemwa Malak

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The order presented by OP is descending tho – Yksisarvinen Oct 24 '22 at 20:36
@Yksisarvinen maybe the map has a comparison function that orders things backwards? – Mark Ransom Oct 24 '22 at 20:38
1

@MarkRansom Well, it's not in the presented example. – Yksisarvinen Oct 24 '22 at 20:40
We know OP is using default comparator for the map in the function overload which is `std::less` – User 10482 Oct 24 '22 at 20:42
2

@User10482 I'm willing to bet that the code in the question is not exactly the same as the code where they're seeing this effect. In fact if the key were `char*` instead of `std::string` it might explain everything. – Mark Ransom Oct 24 '22 at 20:57

Why does the first element in a map get displayed last?

1 Answers1