如何根据已排序索引的向量对 std::set 索引进行排序？

Question

I have a class MyClass , which operates with some double values beta , stored as a class member, in it's member function g . It sorts them and stores the permutation in the class member std::vector<int> sorted_beta_ind :

double MyClass::g() {
  // ...
  sorted_beta_ind.resize(n);
  for(unsigned int i=0; i<n; ++i) {
    sorted_beta_ind[i] = i;
  }
  std::sort(sorted_beta_ind.begin(), sorted_beta_ind.end(),
            [this] (const int &a, const int &b) {++op_cmp; return beta[a] > beta[b];});
  // ...
}

Next I want to have several ordered sets of indices in another member function f , which will store the indices in the same order as in sorted_beta_ind . I'm trying to use std::set objects, and as such, I need a comparator. The best solution I figured out is a lambda function

double MyClass::f() {
  auto ind_comp = [&order = sorted_beta_ind] (const int &a, const int &b) {
    int pos_a = ~0, pos_b = ~0;
    for(unsigned int i=0; i<order.size(); ++i) {
      if(order[i] == a) {
        pos_a = i;
      }
      if(order[i] == b) {
        pos_b = i;
      }
    }
    return pos_a < pos_b;
  };
  std::set<int, decltype(ind_comp)> d0, d1;
  // the rest of the function which uses std::union and std::instersection
}

but on building the project I get

error: use of deleted function ‘MyClass::f()::<lambda(const int&, const int&)>& MyClass::f(int**, int)::<lambda(const int&, const int&)>::operator=(const MyClass::f()::<lambda(const int&, const int&)>&)’

Can this approach work or I should try something entirely else?

Answer 1

Capturing lambda expressions, like yours, are not DefaultConstructible . And that's exactly what std::set tries to do unless it receives a comparator object that can be copied from as a constructor call argument. That is:

std::set<int, decltype(ind_comp)> d0, d1;

Here std::set knows only the type of the comparator, and will attempt to constuct one using its default constructor. Instead, it should be:

std::set<int, decltype(ind_comp)> d0(ind_comp), d1(ind_comp);
//                                   ~~~~~~~^      ~~~~~~~^