Shift values in boost::bimap

Question

I have an unordered bimap like this:

using SymPressMap =
    boost::bimap<boost::bimaps::unordered_set_of<sym>,
                 boost::bimaps::unordered_set_of<Press>>;

Which is basically a bijection between "sym" and "Press". I want to cycle the subset of "Presses", like shown on the picture:bimap state before and after

Here is the algorithm which compiles with std::unordered_map but fails with bimap:

void Layout::cycle(SymVector syms) {
  assert(syms.size() >= 2);
  for (auto it = syms.rbegin(); it != syms.rend() - 1; it++) {
    std::swap(sympressmap.left.at(*it), sympressmap.left.at(*(it + 1)));
  }
}

The basic idea is to consecutively swap adjacent (in terms of "syms") elements. But I'm getting this error:

Error   C2678   binary '=': no operator found which takes a left-hand operand of type '_Ty' (or there is no acceptable conversion)  
KeyboardOptimizer   c:\program files (x86)\microsoft visual studio\2017\professional\vc\tools\msvc\14.16.27023\include\utility  68

So, the question is How to swap two elements in bimap?.

UPD: erase-insert version thanks to John Zwinck, which compiles

void Layout::cycle(SymVector syms) {
  assert(syms.size() >= 2);
  Press plast = pressmap.left.at(*syms.rbegin());
  pressmap.left.erase(*syms.rbegin());
  for (auto it = syms.rbegin() + 1; it != syms.rend(); it++) {
    auto p = pressmap.left.at(*it);
    pressmap.left.erase(*it);
    pressmap.left.insert(SymPressMap::left_value_type(*(it - 1), p));
  }
  pressmap.left.insert(SymPressMap::left_value_type(*syms.begin(), plast));
}

score 1 · Accepted Answer · answered Feb 09 '20 at 00:59

1

With a regular unordered_map, swapping the mapped_type values is no problem because the container structure does not depend on them. But modifying the key_type keys is a common area of difficulty and confusion which arises because the keys define the structure of the container (which values go in which buckets).

You have the same problem here, which is that you are trying to modify keys stored in the container (you're doing it in terms of swapping the values, but in a bimap of course keys and values are duals). You can't do that. What you can do is to copy the key-value pairs, swap their values, remove the originals from the container, and insert the modified pairs.

Ref: How to change the key in an unordered_map?

answered Feb 09 '20 at 00:59

John Zwinck

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Thanks. Though if undordered bimap was based on two maps, swapping could be done more efficient: just swapping values in both maps – Nikita Rudenko Feb 09 '20 at 12:26
The value_type of one would be the mapped_type of the other, so there'd be no way to swap them in general (nor in your example use case). – John Zwinck Feb 10 '20 at 07:50
When bimap is based on two maps, there is no need to shift keys: in the first map values are shifted clockwise, in the second map values are shifter counterclockwise. Thus, keys "stay" on their place. – Nikita Rudenko Feb 10 '20 at 12:13

Shift values in boost::bimap

1 Answers1