How can I implement the behavior of unique in matlab using C++?

In Matlab, the [C, ia, ic] = unique(A) function returns an sorted array C in which the duplicates in A are removed, and the ia and ic arrays contains the indices such that C = A(ia) and A = C(ic). For example:

A = [1, 2, 3, -1, -2, 0, 0, 0, 4]; 
[C, ia, ic] = unique(A); 
C = [-2, -1, 0, 1, 2, 3, 4]; 
% The indices are incremented by 1 to accomodate the C++ convection.
ia = [4, 3, 7, 0, 1, 2, 8]; % size(ia) = size(C) 
ic = [3, 4, 5, 1, 0, 2, 2, 2, 6]; % size(ic) = size(A) 

Here is my implementation:

  std::vector<int> A = {1, 2, 3, -1, -2, 0, 0, 0, 4};
  std::set<int> C(A.begin(), A.end());

  std::vector<int> ic;
  ic.reserve(A.size());

  std::transform(A.begin(), A.end(), std::back_inserter(ic),
                 [&](int x) { return (std::distance(C.begin(), C.find(x))); });

I now can correctly get the array C and ic, but I don't know how to get the array ia.

Can someone help me with this?