diff --git a/.Rbuildignore b/.Rbuildignore
index a6dcd60..fd89c3a 100644
--- a/.Rbuildignore
+++ b/.Rbuildignore
@@ -7,4 +7,7 @@
 ^pkgdown$
 ^\.github$
 ^vignettes/articles$
+^compile_commands\.json$
+.clang-format
+^.cache$
 ^codecov\.yml$
diff --git a/.clang-format b/.clang-format
new file mode 100644
index 0000000..7786f44
--- /dev/null
+++ b/.clang-format
@@ -0,0 +1,12 @@
+BasedOnStyle: Google
+IndentWidth: 2
+DerivePointerAlignment: false
+PointerAlignment: Left
+ColumnLimit: 80
+AlignAfterOpenBracket: BlockIndent
+SpaceAfterCStyleCast: true
+IncludeBlocks: Regroup
+AllowShortFunctionsOnASingleLine: Empty
+BinPackArguments: false
+BinPackParameters: false
+AllowAllParametersOfDeclarationOnNextLine: false
\ No newline at end of file
diff --git a/.gitignore b/.gitignore
index 9168bf8..e2b322b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,3 +4,5 @@
 .httr-oauth
 .DS_Store
 docs
+compile_commands.json
+.cache
\ No newline at end of file
diff --git a/DESCRIPTION b/DESCRIPTION
index 40d25de..d561260 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -9,6 +9,8 @@ License: MIT + file LICENSE
 Encoding: UTF-8
 Roxygen: list(markdown = TRUE)
 RoxygenNote: 7.3.1
+Depends: 
+    R (>= 4.0.0)
 Imports: 
     Matrix,
     tibble,
@@ -17,5 +19,7 @@ URL: https://github.com/EmilHvitfeldt/sparsevctrs, https://emilhvitfeldt.github.
 BugReports: https://github.com/EmilHvitfeldt/sparsevctrs/issues
 Suggests: 
     rlang,
-    rsparse
+    rsparse,
+    testthat (>= 3.0.0)
 Config/Needs/website: rmarkdown
+Config/testthat/edition: 3
diff --git a/NAMESPACE b/NAMESPACE
index 51f82d5..25adadb 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -4,8 +4,10 @@ S3method(format,sparse_vector)
 S3method(vec_arith,sparse_vector)
 S3method(vec_math,sparse_vector)
 S3method(vec_ptype_abbr,sparse_vector)
+export(new_sparse_real)
 export(new_sparse_vector)
 export(sparse_to_tibble)
 export(sparse_vector)
 export(tibble_to_sparse)
 import(vctrs)
+useDynLib(sparsevctrs, .registration = TRUE)
diff --git a/R/altrep.R b/R/altrep.R
new file mode 100644
index 0000000..6a25353
--- /dev/null
+++ b/R/altrep.R
@@ -0,0 +1,12 @@
+#' Create sparse numeric vector
+#' 
+#' @param value Numeric vector, values of non-zero entries.
+#' @param position integer vector, indices of non-zero entries.
+#' @param length Integer, Length of vector.
+#'
+#' @export
+new_sparse_real <- function(value, position, length) {
+  x <- list(val = value, pos = as.integer(position), length = as.integer(length))
+
+  .Call(ffi_altrep_new_sparse_real, x)
+}
diff --git a/R/sparsevctrs-package.R b/R/sparsevctrs-package.R
index 9cf920c..2f0ec6e 100644
--- a/R/sparsevctrs-package.R
+++ b/R/sparsevctrs-package.R
@@ -9,5 +9,6 @@
 NULL
 
 ## usethis namespace: start
+#' @useDynLib sparsevctrs, .registration = TRUE
 ## usethis namespace: end
 NULL
diff --git a/man/new_sparse_real.Rd b/man/new_sparse_real.Rd
new file mode 100644
index 0000000..9a11932
--- /dev/null
+++ b/man/new_sparse_real.Rd
@@ -0,0 +1,18 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/altrep.R
+\name{new_sparse_real}
+\alias{new_sparse_real}
+\title{Create sparse numeric vector}
+\usage{
+new_sparse_real(value, position, length)
+}
+\arguments{
+\item{value}{Numeric vector, values of non-zero entries.}
+
+\item{position}{integer vector, indices of non-zero entries.}
+
+\item{length}{Integer, Length of vector.}
+}
+\description{
+Create sparse numeric vector
+}
diff --git a/src/.gitignore b/src/.gitignore
new file mode 100644
index 0000000..22034c4
--- /dev/null
+++ b/src/.gitignore
@@ -0,0 +1,3 @@
+*.o
+*.so
+*.dll
diff --git a/src/altrep-sparse-real.c b/src/altrep-sparse-real.c
new file mode 100644
index 0000000..3d55ab7
--- /dev/null
+++ b/src/altrep-sparse-real.c
@@ -0,0 +1,180 @@
+#define R_NO_REMAP
+#include "R.h"
+#include <Rinternals.h>
+#include <R_ext/Altrep.h>
+
+// Initialised at load time
+R_altrep_class_t altrep_sparse_real_class;
+
+SEXP ffi_altrep_new_sparse_real(SEXP x) {
+  return R_new_altrep(altrep_sparse_real_class, x, R_NilValue);
+}
+
+SEXP alrep_sparse_real_Materialize(SEXP vec) {
+  SEXP out = R_altrep_data2(vec);
+
+  if (out != R_NilValue) {
+    return out;
+  }
+
+  SEXP data1 = R_altrep_data1(vec);
+  SEXP val = VECTOR_ELT(data1, 0);
+  SEXP pos = VECTOR_ELT(data1, 1);
+  SEXP len = VECTOR_ELT(data1, 2);
+
+  R_xlen_t c_len = (R_xlen_t) INTEGER_ELT(len, 0);
+  
+  out = PROTECT(Rf_allocVector(REALSXP, c_len));
+  
+  // Reminder about performance
+  for (R_xlen_t i = 0; i < c_len; ++i) {
+    SET_REAL_ELT(out, i, 0);
+  }
+
+  R_xlen_t n_positions = Rf_xlength(pos);
+
+  // Reminder about performance
+  for (R_xlen_t i = 0; i < n_positions; ++i) {
+    SET_REAL_ELT(out, INTEGER_ELT(pos, i) - 1, REAL_ELT(val, i));
+  }
+
+  R_set_altrep_data2(vec, out);
+
+  UNPROTECT(1);
+  return out;
+}
+
+// -----------------------------------------------------------------------------
+// ALTVEC
+
+void* altrep_sparse_real_Dataptr(SEXP x, Rboolean writeable) {
+  return STDVEC_DATAPTR(alrep_sparse_real_Materialize(x));
+}
+
+const void* altrep_sparse_real_Dataptr_or_null(SEXP vec) {
+    SEXP out = R_altrep_data2(vec);
+
+  if (out == R_NilValue) {
+    return NULL;
+  } else {
+    return out;
+  }
+}
+
+static SEXP altrep_sparse_real_Extract_subset(SEXP x, SEXP indx, SEXP call) {
+
+  SEXP data1 = R_altrep_data1(x);
+  SEXP val_old = VECTOR_ELT(data1, 0);
+  SEXP pos_old = VECTOR_ELT(data1, 1);
+  SEXP matches = PROTECT(Rf_match(pos_old, indx, R_NaInt));
+
+  int n = 0;
+
+  for (int i = 0; i < Rf_length(matches); ++i) { 
+    if (INTEGER_ELT(matches, i) != R_NaInt) {
+      n++;
+    }
+  }
+
+  SEXP val_new = PROTECT(Rf_allocVector(REALSXP, n));
+  SEXP pos_new = PROTECT(Rf_allocVector(INTSXP, n));
+ 
+  int step = 0;
+  int what_pos = 1;
+
+  for (int i = 0; i < Rf_length(matches); ++i) {
+
+    int match = INTEGER_ELT(matches, i);
+    if (match != R_NaInt) {
+      SET_REAL_ELT(val_new, step, REAL_ELT(val_old, match - 1));
+
+      for (int j = 0; j < Rf_length(matches); ++j) {
+        if (INTEGER_ELT(indx, j) == INTEGER_ELT(pos_old, match - 1)) {
+          break;
+        } else {
+          what_pos++;
+        }
+      }
+      SET_INTEGER_ELT(pos_new, step, what_pos);
+      what_pos = 1;
+      step++;
+    }
+  }
+
+  const char *names[] = {"val", "pos", "length", ""};
+  SEXP res = PROTECT(Rf_mkNamed(VECSXP, names)); 
+  SET_VECTOR_ELT(res, 0, val_new);
+  SET_VECTOR_ELT(res, 1, pos_new);
+  SET_VECTOR_ELT(res, 2, Rf_ScalarInteger(Rf_length(matches)));
+
+  UNPROTECT(4);
+
+  return ffi_altrep_new_sparse_real(res);
+}
+
+// -----------------------------------------------------------------------------
+// ALTREP
+
+R_xlen_t altrep_sparse_real_Length(SEXP x) {
+  SEXP data = R_altrep_data1(x);
+  SEXP len = VECTOR_ELT(data, 2);
+  R_xlen_t out = (R_xlen_t) INTEGER_ELT(len, 0);
+  return out;
+}
+
+// What gets printed when .Internal(inspect()) is used
+Rboolean altrep_sparse_real_Inspect(SEXP x,
+                                    int pre,
+                                    int deep,
+                                    int pvec,
+                                    void (*inspect_subtree)(SEXP, int, int, int)) {
+  Rprintf("sparsevctrs_altrep_sparse_real (materialized=%s)\n",
+          R_altrep_data2(x) != R_NilValue ? "T" : "F");
+  return TRUE;
+}
+
+// -----------------------------------------------------------------------------
+// ALTREAL
+
+static double altrep_sparse_real_Elt(SEXP x, R_xlen_t i) {
+  SEXP data1 = R_altrep_data1(x);
+  SEXP val = VECTOR_ELT(data1, 0);
+  SEXP pos = VECTOR_ELT(data1, 1);
+  SEXP len = VECTOR_ELT(data1, 2);
+  R_xlen_t c_len = (R_xlen_t) INTEGER_ELT(len, 0);
+
+  if (i > c_len) {
+    return NA_REAL;
+  }
+  
+  const R_xlen_t n = Rf_xlength(val);
+
+  double out = 0;
+
+  for (int j = 0; j < n; ++j) {
+    if (INTEGER_ELT(pos, j) == i + 1) {
+      out = REAL_ELT(val, j);
+      break;
+    }
+  }
+  
+  return out;
+}
+
+// -----------------------------------------------------------------------------
+
+void sparsevctrs_init_altrep_sparse_real(DllInfo* dll) {
+  altrep_sparse_real_class =  R_make_altreal_class("altrep_sparse_real", "sparsevctrs", dll);
+
+  // ALTVEC
+  R_set_altvec_Dataptr_method(altrep_sparse_real_class, altrep_sparse_real_Dataptr);
+  R_set_altvec_Dataptr_or_null_method(altrep_sparse_real_class, altrep_sparse_real_Dataptr_or_null);
+  R_set_altvec_Extract_subset_method(altrep_sparse_real_class, altrep_sparse_real_Extract_subset);
+
+  // ALTREP
+  R_set_altrep_Length_method(altrep_sparse_real_class, altrep_sparse_real_Length);
+  R_set_altrep_Inspect_method(altrep_sparse_real_class, altrep_sparse_real_Inspect);
+
+  // ALTREAL
+  R_set_altreal_Elt_method(altrep_sparse_real_class, altrep_sparse_real_Elt);
+}
diff --git a/src/init.c b/src/init.c
new file mode 100644
index 0000000..12a160d
--- /dev/null
+++ b/src/init.c
@@ -0,0 +1,18 @@
+#include <Rinternals.h>
+
+// Defined in altrep-sparse-real.c
+extern SEXP ffi_altrep_new_sparse_real(SEXP);
+extern void sparsevctrs_init_altrep_sparse_real(DllInfo*);
+
+static const R_CallMethodDef CallEntries[] = {
+    {"ffi_altrep_new_sparse_real", (DL_FUNC) &ffi_altrep_new_sparse_real, 1},
+    {NULL, NULL, 0}
+};
+
+void R_init_sparsevctrs(DllInfo* dll) {
+  R_registerRoutines(dll, NULL, CallEntries, NULL, NULL);
+  R_useDynamicSymbols(dll, FALSE);
+
+  // altrep classes
+  sparsevctrs_init_altrep_sparse_real(dll);
+}
diff --git a/tests/testthat.R b/tests/testthat.R
new file mode 100644
index 0000000..93d5488
--- /dev/null
+++ b/tests/testthat.R
@@ -0,0 +1,12 @@
+# This file is part of the standard setup for testthat.
+# It is recommended that you do not modify it.
+#
+# Where should you do additional test configuration?
+# Learn more about the roles of various files in:
+# * https://r-pkgs.org/testing-design.html#sec-tests-files-overview
+# * https://testthat.r-lib.org/articles/special-files.html
+
+library(testthat)
+library(sparsevctrs)
+
+test_check("sparsevctrs")
diff --git a/tests/testthat/test-altrep.R b/tests/testthat/test-altrep.R
new file mode 100644
index 0000000..4ab24f3
--- /dev/null
+++ b/tests/testthat/test-altrep.R
@@ -0,0 +1,43 @@
+test_that("length() works with new_sparse_real()", {
+  expect_identical(
+    length(new_sparse_real(numeric(), integer(), 0)),
+    0L
+  )
+
+  expect_identical(
+    length(new_sparse_real(1, 1, 10)),
+    10L
+  )
+
+  expect_identical(
+    length(new_sparse_real(1, 1, 100)),
+    100L
+  )
+})
+
+test_that("subsetting works with new_sparse_real()", {
+  x_sparse <- new_sparse_real(value = c(10, 13, 20), position = c(1, 5, 8), 10)
+  x_dense <- c(10, 0, 0, 0, 13, 0, 0, 20, 0, 0)
+
+  for (i in seq_len(10)) {
+    expect_identical(x_sparse[i], x_dense[i])
+  }
+
+  expect_identical(x_sparse[1:2], x_dense[1:2])
+
+  expect_identical(x_sparse[3:7], x_dense[3:7])
+
+  expect_identical(x_sparse[-5], x_dense[-5])
+
+  expect_identical(x_sparse[-c(5:7)], x_dense[-c(5:7)])
+
+  # testing outside range returns NA
+  # expect_identical(x_sparse[c(1, 11)], x_dense[c(1, 11)])
+})
+
+test_that("materialization works with new_sparse_real()", {
+  x_sparse <- new_sparse_real(value = c(10, 13, 20), position = c(1, 5, 8), 10)
+  x_dense <- c(10, 0, 0, 0, 13, 0, 0, 20, 0, 0)
+
+  expect_identical(x_sparse[], x_dense)
+})
\ No newline at end of file