Structured binding for std::views::enumerate of std::map [duplicate]

Question

I'm trying to do something like this:

const std::map<char, int> histogram{
    {'A', 2},
    {'D', 1},
    {'M', 1},
};

for (const auto& [index, key, value] : std::views::enumerate(histogram))
{
    // ...
}

but it doesn't compile (Compiler Explorer) reporting error:

<source>:13:21: error: 3 names provided for structured binding
   13 |     for (const auto& [index, key, value] : std::views::enumerate(histogram))
      |                     ^~~~~~~~~~~~~~~~~~~
<source>:13:21: note: while 'const std::tuple<long int, const std::pair<const char, int>&>' decomposes into 2 elements

Naively using [index, [key, value]] of course didn't work either...

Is there some other trick I'm missing or in this case I just cannot "unpack" the map element and instead have to keep it like [index, entry] and then use entry.first and entry.second?

Or maybe <ranges> offers something else than enumerate for this scenario?

Answer 1

Structured binding can "unpack" only a single level - in your case it would be to the index and entry in the map.

You can solve it by adding a second structured binding to "unpack" the entry in the map:

for (const auto& [index, entry] : std::views::enumerate(histogram)) 
{
    // Second binding:
    const auto& [key, value] = entry;

    std::cout << "index: " << index << ", key: " << key << ", value: " << value << '\n';
}

Output:

index: 0, key: A, value: 2
index: 1, key: D, value: 1
index: 2, key: M, value: 1

Live demo