crochet is an R package that provides functions to help implement the extraction / subsetting / indexing function [ and replacement function [<- of custom matrix-like types (based on S3, S4, etc.), modeled as closely to the base matrix class as possible (with tests to prove it).
Check out the Example of a Custom Type that Implements 'extract' and 'replace' vignette on the crochet page on CRAN.
Install the stable version from CRAN:
Alternatively, install the development version from GitHub:
Further documentation can be found on RDocumentation.