How to update an existing element of std::set?

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Since val is not involved in comparison, it could be declared mutable

struct Foo {
  Foo(int i, int j) : id(i), val(j) {}
  int id;
  mutable int val;
  bool operator<(const Foo& other) const {
    return id < other.id;
  }
};

This implies that the value of val may change in a logically-const Foo, which means that it shouldn’t affect other comparison operators etc.

Or you could just remove and insert, that takes O(1) additional time (compared to accessing and modifying) if insertion uses the position just before just after the old one as the hint.

Something like:

bool alreadyThere = !p.second; // you forgot the !
if (alreadyThere)
{
    Set::iterator hint = p.first;
    hint++;
    s.erase(p.first);
    s.insert(hint, f);
}

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