Why Does My Custom `std::set` Comparator Cause a Type Mismatch Error?

Error: Mismatched Type in Custom std::set Comparator Definition

In your code, you're attempting to specify a custom comparator function for sorting elements in a std::set using the lexicographic order instead of numeric order. However, the error message indicates that there's a type mismatch error in the declaration of your custom comparator.

The issue arises because you're declaring lex_compare as a function with two int64_t parameters that returns a boolean value. However, the template for std::set expects a comparator that implements the std::less<> template. std::less<> is a function object with the following signature:

template <typename T>
struct less {
  bool operator()(const T& a, const T& b) const;
};

To fix this issue, you need to implement lex_compare to conform to the std::less<> signature. Here's a modification to your code:

struct lex_compare {
  bool operator()(const int64_t& a, const int64_t& b) const {
    stringstream s1, s2;
    s1 << a;
    s2 << b;
    return s1.str() < s2.str();
  }
};

std::set<int64_t, lex_compare> s;

Now, the lex_compare struct implements the operator() function that takes two int64_t arguments and returns a boolean value, which matches the signature of std::less<>. By specifying this custom comparator in the definition of your std::set, you can modify the ordering behavior of the set to sort elements lexicographically.

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