C++ iterate over members of struct

Question

Say I have a struct:

struct Boundary {
  int top;
  int left;
  int bottom;
  int right;
}

and a vector

std::vector<Boundary> boundaries;

What would be the most C++ style way to access the structs to get the sum of top, left, bottom and right separately?

I could write a loop like

for (auto boundary: boundaries) {
  sum_top+=boundary.top;
  sum_bottom+=boundary.bottom;
  ...
}

This seems like a lot of repetition. Of course I could do this instead:

std::vector<std::vector<int>> boundaries;

for (auto boundary: boundaries) {
  for(size_t i=0; i<boundary.size();i++) {
    sums.at(i)+=boundary.at(i)
  }
}

But then I'd loose all the meaningful struct member names. Is there a way so that I can write a something like the following function:

sum_top=make_sum(boundaries,"top");

Reflection does not seem to be an option in C++. I am open to use C++ up to Version 14.

Answer 1

std::accumulate(boundaries.begin(), boundaries.end(), 0, 
 [](Boundary const & a, Boundary const & b) { return a.top + b.top); });

(IIRC the Boundary const &'s can be auto'd in C++17)

This doesn't make it generic for the particular element, which - indeed, due to the lack of reflection - isn't easy to generalize.

There are a few ways to ease your pain, though;

You could use a pointer-to-member, which is fine for your szenario but not very c-plusplus-y:

int Sum(vector<Boundary>const & v, int Boundary::*pMember)
{
   return std::accumulate( /*...*/, 
     [&](Boundary const & a, Boundary const & b)
     {
        return a.*pMember + b.*pMember;
     });
}

int topSum = Sum(boundaries, &Boundary::top);

(For pointer-to-member, see e.g. here: Pointer to class data member "::*")

You could also make this generic (any container, any member type), and you could also replace the pointer-to-member with a lambda (also allowing member functions)