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enable the sugar length checks in development versions only
Twelve CRAN packages rely on a longer sugar operand being cut short, so an unconditional error would block the release until all of them update. Derive RCPP_SUGAR_LENGTH_CHECKS from the version in config.h: on when the fourth component is non-zero (development snapshots, which is what reverse-dependency checks run against) and off in releases. Defining the macro before including Rcpp.h overrides the default. The tests check for the errors only when enabled, and for the truncation otherwise.
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Lines changed: 94 additions & 26 deletions

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‎ChangeLog‎

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2026-10-09 Kevin Ushey <kevinushey@gmail.com>
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* inst/include/Rcpp/config.h: Define RCPP_SUGAR_LENGTH_CHECKS, on for
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development versions and off for releases unless overridden
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* inst/include/Rcpp/sugar/tools/check_sizes.h: Compile the length checks
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out when RCPP_SUGAR_LENGTH_CHECKS is 0
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* inst/tinytest/cpp/sugar_expressions.cpp: Report whether the checks are
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enabled
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* inst/tinytest/test_sugar.R: Test the length errors only when enabled,
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and the truncation otherwise
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* vignettes/rmd/Rcpp-sugar.Rmd: Document when the lengths are checked
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* inst/NEWS.Rd: Idem
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2026-10-09 Kevin Ushey <kevinushey@gmail.com>
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* inst/include/Rcpp/sugar/functions/lapply.h: Index the operand through

‎inst/NEWS.Rd‎

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\item The \code{col} constructor now uses \code{nrow} in an
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initialization (Iñaki in \ghpr{1509})
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\item Sugar expressions hold nested expressions by value so they can be
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stored, evaluate expressions that may read the target before assigning
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them in place, and signal an error for operands of different lengths
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(Kevin in \ghpr{1513} closing \ghit{1512}). Same-length assignment
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of an expression that is not elementwise, such as \code{x = rev(x)},
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now goes through a temporary vector.
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stored, and evaluate expressions that may read the target before
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assigning them in place (Kevin in \ghpr{1513} closing \ghit{1512}).
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Same-length assignment of an expression that is not elementwise, such
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as \code{x = rev(x)}, now goes through a temporary vector.
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\item Development versions check that sugar operands, and vectors
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assigned into a range, row or column, have matching lengths, and signal
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an error otherwise; releases do not check, and the define
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\code{RCPP_SUGAR_LENGTH_CHECKS} overrides the default (Kevin in
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\ghpr{1513})
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\item Assignment between overlapping subsets of the same vector, as in
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\code{x[i] = x[j]}, reads the source elements before writing (Kevin in
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\ghpr{1513})

‎inst/include/Rcpp/config.h‎

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#define RCPP_DEV_VERSION RcppDevVersion(1,1,2,4)
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#define RCPP_DEV_VERSION_STRING "1.1.2.4"
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// Whether sugar checks that the operands of an expression, and a vector
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// assigned into a range, row or column, have matching lengths. On by default
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// in development versions (a non-zero fourth version component), so that
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// reverse-dependency checks catch mismatches; off in releases, so that
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// packages on CRAN are not affected. Define RCPP_SUGAR_LENGTH_CHECKS as 1 or
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// 0 before including Rcpp.h to override.
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#ifndef RCPP_SUGAR_LENGTH_CHECKS
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# if (RCPP_DEV_VERSION % 100) != 0
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# define RCPP_SUGAR_LENGTH_CHECKS 1
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# else
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# define RCPP_SUGAR_LENGTH_CHECKS 0
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# endif
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#endif
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#endif

‎inst/include/Rcpp/sugar/tools/check_sizes.h‎

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#ifndef Rcpp__sugar__tools__check_sizes_h
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#define Rcpp__sugar__tools__check_sizes_h
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#include <Rcpp/config.h>
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namespace Rcpp{
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namespace sugar{
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#if RCPP_SUGAR_LENGTH_CHECKS
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// Kept out of line: inlining the error path keeps the compiler from
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// optimizing the loops that follow these checks.
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#if defined(__GNUC__)
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stop_sizes("sugar operands have different lengths (%d and %d)", n1, n2);
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}
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inline void check_sizes(R_xlen_t n1, R_xlen_t n2, R_xlen_t n3) {
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check_sizes(n1, n2);
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check_sizes(n1, n3);
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}
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// Likewise, a vector assigned into a range, row or column must have the
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// same length as that target.
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inline void check_assign_size(R_xlen_t target, R_xlen_t value) {
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if (target != value)
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stop_sizes("cannot assign a vector of length %d to a target of length %d", value, target);
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}
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#else
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// Checks disabled (the default in released versions, see config.h): the
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// result is sized by the first operand, or by the target, as it always
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// was, and a longer operand is cut short.
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inline void check_sizes(R_xlen_t, R_xlen_t) {}
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inline void check_assign_size(R_xlen_t, R_xlen_t) {}
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#endif
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inline void check_sizes(R_xlen_t n1, R_xlen_t n2, R_xlen_t n3) {
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check_sizes(n1, n2);
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check_sizes(n1, n3);
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}
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} // sugar
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} // Rcpp
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‎inst/tinytest/cpp/sugar_expressions.cpp‎

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);
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}
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// [[Rcpp::export]]
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bool sugar_length_checks_enabled() {
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return RCPP_SUGAR_LENGTH_CHECKS != 0;
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}
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// [[Rcpp::export]]
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NumericVector length_plus(NumericVector x, NumericVector y) {
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return x + y;

‎inst/tinytest/test_sugar.R‎

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@@ -1775,19 +1775,29 @@ expect_equal(
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expect_equal(length_plus(c(1, 2), c(10, 20)), c(11, 22), info = "x + y, same length")
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expect_equal(length_plus(numeric(), numeric()), numeric(), info = "x + y, both empty")
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expect_equal(length_plus_scalar(c(1, 2)), c(2, 3), info = "x + scalar")
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expect_error(length_plus(c(1, 2, 3, 4, 5), c(10, 20)), "different lengths", info = "x + <shorter y>")
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expect_error(length_plus(c(1, 2), c(10, 20, 30, 40)), "different lengths", info = "x + <longer y>")
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expect_error(length_compare(c(1, 2, 3), c(1, 2)), "different lengths", info = "x < y")
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expect_error(length_and(c(TRUE, FALSE), TRUE), "different lengths", info = "x & y")
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expect_error(length_pmax(c(1, 2, 3), c(1, 2)), "different lengths", info = "pmax(x, y)")
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expect_error(length_ifelse(c(TRUE, FALSE), c(1, 2), c(1, 2, 3)), "different lengths", info = "ifelse(c, x, y)")
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expect_equal(length_range(c(1, 2, 3, 4, 5), c(7, 8, 9)), c(7, 8, 9, 4, 5), info = "x[0:2] = y")
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expect_error(length_range(c(1, 2, 3, 4, 5), c(7, 8)), "cannot assign", info = "x[0:2] = <shorter y>")
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expect_error(length_range(c(1, 2, 3, 4, 5), c(6, 7, 8, 9)), "cannot assign", info = "x[0:2] = <longer y>")
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expect_error(length_range_range(c(1, 2, 3, 4, 5)), "cannot assign", info = "x[0:1] = x[0:3]")
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expect_error(length_column(matrix(1, 3, 2), c(7, 8)), "cannot assign", info = "m(_, 0) = <shorter y>")
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expect_error(length_row(matrix(1, 3, 2), c(7, 8, 9)), "cannot assign", info = "m(0, _) = <longer y>")
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## the length checks are on in development versions and off in releases
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## (RCPP_SUGAR_LENGTH_CHECKS, see config.h)
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if (sugar_length_checks_enabled()) {
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expect_error(length_plus(c(1, 2, 3, 4, 5), c(10, 20)), "different lengths", info = "x + <shorter y>")
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expect_error(length_plus(c(1, 2), c(10, 20, 30, 40)), "different lengths", info = "x + <longer y>")
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expect_error(length_compare(c(1, 2, 3), c(1, 2)), "different lengths", info = "x < y")
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expect_error(length_and(c(TRUE, FALSE), TRUE), "different lengths", info = "x & y")
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expect_error(length_pmax(c(1, 2, 3), c(1, 2)), "different lengths", info = "pmax(x, y)")
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expect_error(length_ifelse(c(TRUE, FALSE), c(1, 2), c(1, 2, 3)), "different lengths", info = "ifelse(c, x, y)")
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expect_error(length_range(c(1, 2, 3, 4, 5), c(7, 8)), "cannot assign", info = "x[0:2] = <shorter y>")
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expect_error(length_range(c(1, 2, 3, 4, 5), c(6, 7, 8, 9)), "cannot assign", info = "x[0:2] = <longer y>")
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expect_error(length_range_range(c(1, 2, 3, 4, 5)), "cannot assign", info = "x[0:1] = x[0:3]")
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expect_error(length_column(matrix(1, 3, 2), c(7, 8)), "cannot assign", info = "m(_, 0) = <shorter y>")
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expect_error(length_row(matrix(1, 3, 2), c(7, 8, 9)), "cannot assign", info = "m(0, _) = <longer y>")
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} else {
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## with the checks off, a longer operand is cut short, as before
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expect_equal(length_plus(c(1, 2), c(10, 20, 30, 40)), c(11, 22), info = "x + <longer y>, checks off")
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expect_equal(length_range(c(1, 2, 3, 4, 5), c(6, 7, 8, 9)), c(6, 7, 8, 4, 5), info = "x[0:2] = <longer y>, checks off")
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expect_equal(length_row(matrix(1, 3, 2), c(7, 8, 9)), matrix(c(7, 1, 1, 8, 1, 1), 3, 2), info = "m(0, _) = <longer y>, checks off")
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}
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## &&, || and ! on single logical results (e.g. all(), any()) follow R's
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## three-valued logic

‎vignettes/rmd/Rcpp-sugar.Rmd‎

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@@ -528,10 +528,18 @@ assigned to a vector or otherwise consumed, for example by `sum()`.
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## Operand lengths
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Unlike \proglang{R}, \sugar does not recycle: the vectors combined by an
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expression must all have the same length, and an error is signalled
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otherwise. Likewise, a vector assigned into a range, row or column (as in
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`m(_, 0) = x`) must have the length of that target. Single values, like the
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`2.0` in `x + 2.0`, can be combined with vectors of any length.
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expression must all have the same length. Likewise, a vector assigned into a
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range, row or column (as in `m(_, 0) = x`) must have the length of that
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target. Single values, like the `2.0` in `x + 2.0`, can be combined with
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vectors of any length.
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Development versions of \pkg{Rcpp} (those with a fourth version component,
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such as 1.1.2.5) check these lengths and signal an error on a mismatch, so
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that the problem is caught when a package is checked against them. Released
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versions do not check: the result takes the length of the first operand, or
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of the target, and a longer operand is silently cut short. Defining
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`RCPP_SUGAR_LENGTH_CHECKS` as `1` or `0` before including `Rcpp.h` turns the
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checks on or off regardless of the version.
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## Storing expressions
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