Standard way to create a const set with the value as a union/intersection of two sets in C++?

Question

Suppose I have

constexpr std::set<int> a = {1, 2, 3};
constexpr std::set<int> b = {3, 4, 5};

And I want to create

constexpr std::set<int> c = union(a, b); // {1, 2, 3, 4, 5}

Is there a library function to do this without creating my own union/intersection function?

Answer 1

You can use the lambda trick to initialize a const variable:

// need to capture `a`, `b` if this is at block scope
const std::set<int> c = []() {
    std::set<int> result;
    std::set_union(a.begin(), a.end(), b.begin(), b.end(), std::inserter(result, result.end()));
    return result;  // compiler can probably NRVO this
}();