Vrftjkry

Consider the following data structures and code.

struct Sentence {

    std::string words;

    int frequency;

    Sentence(std::string words, int frequency) : words(words), frequency(frequency) {}

};

struct SentencePCompare {

    bool operator() (const Sentence* lhs, const Sentence* rhs) const {

        if (lhs->frequency != rhs->frequency) {

            return lhs->frequency > rhs->frequency;

        }

        return lhs->words.compare(rhs->words) < 0;

    }

};

std::set<Sentence*, SentencePCompare> sentencesByFrequency;



int main(){

    Sentence* foo = new Sentence("foo", 1);

    Sentence* bar = new Sentence("bar", 2);

    sentencesByFrequency.insert(foo);

    sentencesByFrequency.insert(bar);

    for (Sentence* sp : sentencesByFrequency) {

        std::cout << sp->words << std::endl;

    }

    foo->frequency = 5;

    for (Sentence* sp : sentencesByFrequency) {

        std::cout << sp->words << std::endl;

    }

}

The output of the above code is the following.

bar

foo

bar

foo

As we might expect, when an object pointed to by the pointer in the set is updated, the set does not automatically re-evaluate the predicate, even though the predicate orders the pointers based on the objects they point at.

Is there a way to force the std::set to re-evaluate the predicates, so that the order is correct again?

No.

There's a reason why set only allows const access to its elements. If you sneak past that by using shallow-const pointers and custom predicates and then destroy the invariant by modifying the pointee in a way that affects ordering, you'll pay the price in the form of nasal demons.

Before C++17, you need to erase and insert again, which incurs a key copy plus node deallocation and allocation. After, you can extract the node, modify it, and reinsert it, which is free.