How do you use std::accumulate to sum up a class member of elements?

Question

I always try to incorporate STL algorithms wherever I can, rather than writing manual loops. However, I'm having difficulty understanding how std::accumulate is generally useful. Whenever I need to calculate sums or averages, I almost always end up resorting to manual loops, because I have difficulty getting std::accumulate to do what I need.

The problem is that I rarely ever have a simple vector of integers that need to be summed. Usually, I want to sum an array of objects using a particular member variable. Yes, I know there is a version of std::accumulate that takes a BinaryFunction, but the problem I see is that this function needs to take two values of type T, where T is the type of the sum, rather than the type of the operands. I'm having trouble understanding how this is useful.

Consider a case which I assume is pretty common. I have the following class:

struct Foo
{
    Foo(int cost_, int id_) : cost(cost_), id(id_)
    { }

    int cost;
    int id;
};

Now, say I want to calculate the sum of an array of Foo objects, using Foo::cost.

I want to say:

std::vector<Foo> vec;
// fill vector with values
int total_cost = std::accumulate(vec.begin(), vec.end(), 0, sum_cost);

And sum_cost is defined as:

int sum_cost(const Foo& f1, const Foo& f2)
{
    return f1.cost + f2.cost;
}

The problem is, this doesn't work because std::accumulate expects a BinaryFunction which takes in two instances of the resulting sum type - which in this case is just int. But how is that useful to me? If my BinaryFunction takes in two ints, I can't specify that I want to sum the cost field.

So, why is std::accumulate designed this way? Am I just not seeing something obvious here?

Answer 1

You're wrong about accumulate operator taking two of the same type. It does that only if you want to. The use the operator is specifically sum = op(sum, *iter). Thus your code:

int count = std::accumulate(stuff.begin(), stuff.end(), 0,
                            [](int current_sum, stuff_value_t const& value) {
    return current_sum + value.member;
});

If you can't use a lambda, then of course you use the standard binders or boost::bind.