为什么 C++ boost 库中的累加器有类似函数的接口？

Question

In the boost library, we use accumulators like this:

acc(1); // push things into acc
cout << max( acc ) << endl; // get its result

Why can't we define its interface like this:

acc.push(1);
cout << acc.max() << endl;

So why do the accumulators from the boost library have a function-like interface? What is the advantage of it?

Answer 1

Here's my guess

The reason that operator () is used for pushing instead of push() is because acc(value) reads "accumulate another value" which sounds more natural than acc.push(value) which is "push a value to the accumulator"

Besides the accumulator can receive optional features like covariate or weight and that way probably the result looks and sounds better than acc.push(value, feature) . Some examples from the documentation :

acc( 1.2, covariate1 =  12 );
acc( 2.3, covariate1 = -23 );
acc( 3.4, covariate1 =  34 );
acc( 4.5, covariate1 = -45 );

acc(1, weight = 2); //   1 * 2
acc(2, weight = 4); //   2 * 4
acc(3, weight = 6); // + 3 * 6

acc(1, weight = 2, covariate1 = 3);
acc(0, weight = 4, covariate1 = 4);
acc(2, weight = 9, covariate1 = 8);

And then max(acc) is used instead of acc.max() because it allows extensibility while still maintaining consistency . By writing a new function that receives an accumulator you'll be able to do other things to the accumulated list

std::cout << "Mean:   " << mean(acc) << std::endl;
std::cout << "Moment: " << accumulators::moment<2>(acc) << std::endl;

If a member method was used then there'll only be a limited number of default operations like acc.mean() , acc.max() , the remaining things must be used like a function such as product(acc) , rolling_moment<2>(acc) , kurtosis(acc) , skewness(acc) ... which breaks consistency

Answer 2

Here are two reasons:

1. You can pass them to algorithms, such as for_each :

    acc = std::for_each(range.begin(), range.end(), acc);

2. This gives all accumulators a uniform interface. This is helpful to combine them eg. with accumulator_set .