Sum of particular members of an object array - c++

Question

This is my first question here, so hopefully it's a good one (I have searched for an answer, but not found it yet).
I have an event based disease model written in C++, and I'm trying to convert it to OOP (partly for my own learning experience). The basic class (Herd) stores numbers of susceptible, infective, and resistant animals, and the rates at which possible events occur.
class Herd {
private:
int S,I,R,N;
float birth, death, infection, recovery, movement;
public:
void calc_birth(void) { birth = r*N*(1.0-N/c); }
void calc_infection(void) { infection = N>0 ? beta*S*I/N : 0.0; }
// etc.
};
I then have a vector of herds to keep track of. Throughout this model I will need to calculate the sum of each member across all herd, after an event changes the number or category of individuals in a herd (this happens a lot). I already have 4 categories and 5 events, and this model could easily be expanded and require considerably more.
In my old, procedural code, I simply had a separate vector for each member, and it was easy to create a sum() function to calculate the results, but I can't see how to do this with a class without writing a separate sum function for each member (which is possible, but I doubt it's a particularly good way of doing it). I could make a static member (e.g. sum_S) to track the total, and update it every time an event occurs but this might not be appropriate for all members, and I'm not sure if the total might not slowly wander away from the true value when it comes to the rates.
Is there a way to write a single sum() function, which takes the member I want as a parameter, and returns the sum of that particular member across all the herds?
Thank you!

Answer 1

The standard library header numeric contains the function accumulate. It does not do exactly what you want, but is easily adaptable:
#include <numeric>
#include <vector>
#include <iostream>
struct Foo {
int member;
};
int main()
{
std::vector<Foo> v = { {1}, {2}, {3} };
int sum
= std::accumulate(begin(v), end(v), 0,
// lambda that sums up
[](const int& x, const Foo& y)
{return x + y.member;});
std::cout << sum << std::endl;
return 0;
}
The code uses initializer_lists and lambdas. If those aren't supported by your compiler, use the corresponding C++03 code (push_back and functors).

Answer 2

Sure, using pointer-to-members.
float sum_for_member( std::vector< Herd > const &herds, float Herd::*member ) {
float acc = 0;
for ( std:vector< Herd >::const_iterator it = herds.begin();
it != herds.end(); ++ it ) {
acc += (*it).*member;
}
return acc;
}
to call it:
float total_recovery = sum_for_member( my_herds, & Herd::recovery );
If you want to use a member function getRecovery instead, the parameter declaration becomes float (Herd::*member)() and the summation becomes acc += ((*it).*member)().
If you're not afraid to get further from OO and into generic C++ style, you can use a function template and let the Standard Library take care of the loop:
template< float Herd::*member >
float plus( float lhs, Herd const &rhs )
{ return lhs + rhs.*member; }
vector< Herd > h;
std::accumulate( h.begin(), h.end(), 0., plus< & Herd::recovery > );

Answer 3

So this is what I ended up with (inside a class called State, which holds the simulation state including a vector<Herd>):
Class State {
...
template <typename T>
T sum(T (Herd::*getter)()) {
T acc = (T) 0.0;
for (int i=0; i<size; ++i) {
acc += (herds[i].*getter)();
}
return acc;
...
}
This works for both the numbers of individuals (ints) and the event rates (floats), and uses pointers to member functions to keep those values private. An example of how this is called in the code is:
cout << "total # of S = " << state.sum(&Herd::getS)
"total # of I = " << state.sum(&Herd::getI)
"total birth rate = " << state.sum(&Herd::get_birth)
"total infection rate = " << state.sum(&Herd::get_infection)
etc.
Thanks very much for your help, especially to Potatoswatter. This is much better than the tedious boilerplate code I had before!