std::unordered_map<TLeft, const TRight *> _leftToRight;  // pointer into _rightToLeft
std::unordered_map<TRight, const TLeft *> _rightToLeft;  // pointer into _leftToRight

auto aitr = this->_leftToRight.insert_or_assign(left, nullptr);
auto bp = &((*aitr.first).first);           // raw pointer to key — invalidated on rehash
auto bitr = this->_rightToLeft.insert_or_assign(right, bp).first;
auto ap = &((*bitr).first);
(*aitr.first).second = ap;

std::unordered_map<TLeft, TRight> _leftToRight;  // value copy
std::unordered_map<TRight, TLeft> _rightToLeft;  // value copy

this->_leftToRight.insert_or_assign(left, right);
this->_rightToLeft.insert_or_assign(right, left);