Possible overflow with std::accumulate

Question

I have a function that generates some large numbers, puts them in a vector, sums them, an returns the sum. I have been using std::accumulate to calculate the sum, however, after some testing, I realized it was not returning the expected sum.

typedef unsigned long long ull;
ull sum(ull kLimit)
{
    ull testSum = 0;
    vector<ull> numbers;
    for (ull n = 0; n < kLimit; ++n) {
        if (/* number I want */) {
            numbers.push_back(n);
            // directly sum for testing
            testSum += n;
        }
    }
    ull sum = accumulate(begin(numbers), end(numbers), 0);
    return sum;
}

I would expect the value of sum and testSum to be equal. However, sum is equal to 470064632 and test sum is equal to 82074443256 , which is the expected value.

I tried reducing the limit to a much smaller number (500), and the values of sum and testSum were equal. This makes me think the error is overflow with accumulate , I'm not sure what the problem could be though. I am compiling with VS2012 for a x64 platform.

Answer 1

It does look like an overflow error: the numbers are 0x131C049DF8 and 0x1C049DF8.

Try casting your final parameter 0 to type ull , because the type returned by accumulate is the type of that final parameter:

T accumulate(InputIt first, InputIt last, T value) { ... }