How to Correctly Compile a Set with a Custom Comparator in C ?

Compiling Set with Custom Comparator

When attempting to create a set of integers ordered by lexicographic comparison instead of numeric, issues can arise when compiling with g .

The error encountered is due to an incorrect template parameter type for the set. In the provided code:

set<int64_t, lex_compare> s;

lex_compare is not a type but a function. To fix this, we need to create a comparator object as per C 's template requirements.

Modern C Solutions

1. C 20 Solution:

auto cmp = [](int64_t a, int64_t b) { return ... };
std::set<int64_t, decltype(cmp)> s;

Lambdas can be used as comparators in C 20. The lambda should return a boolean value indicating the order of the elements.

2. C 11 Solution:

auto cmp = [](int64_t a, int64_t b) { return ... };
std::set<int64_t, decltype(cmp)> s(cmp);

In C 11, lambdas must be passed as an argument to the set constructor.

Traditional Solutions

3. Function Pointer:

bool cmp(int64_t a, int64_t b) {
    return ...;
}
std::set<int64_t, decltype(cmp)*> s(cmp);

4. Struct with Operator():

struct cmp {
    bool operator() (int64_t a, int64_t b) const {
        return ...
    }
};

std::set<int64_t, cmp> s;

5. Struct from Boolean Function:

bool cmp(int64_t a, int64_t b) {
    return ...;
}

#include <type_traits>
using Cmp = std::integral_constant<decltype(&cmp), &cmp>;
std::set<int64_t, Cmp> set;

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