This is a ‘type interface.’ See Type Interfaces for more info on what that is.

typedef .. Scalar: Type used to quantify the quantity which is binned into separate bins

typedef .. CountType: Type used to count the number of hits in each bin

static constexpr bool HasErrorBars = ..: Whether this Histogram type can provide error bars (e.g. obtained e.g. through binning analysis, or by averaging several histograms)

In the following, we use std::size_t as indexing type, but it can be replaced by any other integral type. You should use std::size_t if you store your histogram as a dense object (that's the type which can hold the size of the largest possible object which can be stored in memory).

std::size_t num_bins() const: The number of bins in this histogram.

CountType count(std::size_t i) const: Number of counts in the bin number i

CountType errorbar(std::size_t i) const: (Only if HasErrorBars = true) Error bar (standard deviation) associated to the bin number i.