The sentence in bold below in the book "Standard Library" by Nikolay Yosuttis, I do not understand

I am reading The Standard Library, Second Edition, by Nikolai Yosuttis. On page 183 we have:

Examples of using unordered maps and multiframes

The example provided for multiframes on page 179 also works for unordered multimap if you replace map

with unordered_map

in the include directive and multimap

with unordered_multimap

in the container declaration:

#include <unordered_map>
...
unordered_multimap<int,string> coll;
...

      

        
        
        
      

    

The only difference is that the order of the elements is undefined. However, on most platforms, items will still be sorted because the default hash function is the modulo operator.

I emphasized the bold part that I don't understand. My first impression when I read this was that the author says that both programs (the one on page 179, see below) and above) should print city names in the same order on most platforms. But that doesn't happen in clang and GCC. See Live Examples map

and unordered_map

at GCC. The same results were obtained in clang.

After thinking for a while, I interpreted the author saying that city names are printed in the same order for almost all platforms when processed with unordered_map

, and the output seems to confirm this. But even so, I find it difficult to accept this interpretation, since different implementations will probably use different hash functions!

Below you will find an example on page 179 mentioned above:

#include <map>
#include <string>
#include <iostream>
using namespace std;
int main()
{
    multimap<int,string> coll; // container for int/string values
    // insert some elements in arbitrary order
    // - a value with key 1 gets inserted twice
    coll = { {5,"tagged"},
             {2,"a"},
             {1,"this"},
             {4,"of"},
             {6,"strings"},
             {1,"is"},
             {3,"multimap"} };
    // print all element values
    // - element member second is the value
    for (auto elem : coll) {
        cout << elem.second << ’ ’;
    }
    cout << endl;
}