Read/write all values of a std::multimap with a given key in C++20?

Question

Suppose I have some function:

void mutate(V& v);

that reads/writes v - and I want to write a function:

void mutate_map_values(std::multimap<K,V>& m, K k);

that applies mutate to all values of m that have key k.

What's the most succinct way to implement mutate_map_values in C++20?

What have you tried? What about your attempt(s) do you feel are not succinct enough? — Some programmer dude
– Some programmer dude, Commented Jan 9, 2020 at 7:40
@Someprogrammerdude: Best solution I have so far is .equal_range example here: en.cppreference.com/w/cpp/container/multimap/equal_range . I'd like to know if there is a shorter way I am missing before I propose one to the C++ standards commitee. — Andrew Tomazos
– Andrew Tomazos, Commented Jan 9, 2020 at 7:47
Well that currently seems like the best way to find all elements of a specific key. It's not very succinct to iterate over elements of the same key using it though. Some kind of for_each member function might have been nice. But is it a common enough use-case that it should be added to the standard? — Some programmer dude
– Some programmer dude, Commented Jan 9, 2020 at 7:57
@Kerndog73: I don't think your for_each works because mutate operates on V not std::pair<K,V>. mutate is just a proxy for some code that does something with a V. — Andrew Tomazos
– Andrew Tomazos, Commented Jan 9, 2020 at 8:03
In the linked example you could subsume the call to equal_range into the init expression of the for loop and use a structured binding such as for (auto [i, e] = m.equal_range(key); i != e; ++i). Other than that it's difficult to judge without seeing an example of the code you would like to be able to write. — G.M.
– G.M., Commented Jan 9, 2020 at 8:04

StoryTeller RIP 16.5.26 · Accepted Answer · 2020-01-09 08:16:19Z

5

std::ranges::subrange is a utility class that's constructible from anything that is like a pair of iterators. Which fits with what std::multimap::equal_range already returns. Combining the two, we may write the desired function as follows in C++20:

#include <ranges>

void mutate_map_values(std::multimap<K,V>& m, K k) {
    using namespace std::ranges;
    for (auto& [_, v] : subrange(m.equal_range(k))) {
        mutate(v);
    }
}

answered Jan 9, 2020 at 8:16

StoryTeller RIP 16.5.26

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