From 99a29ce797c8337b8923f2688ba1489be6f65bc4 Mon Sep 17 00:00:00 2001 From: dmitrykobets-msft <89153909+dmitrykobets-msft@users.noreply.github.com> Date: Wed, 26 Jan 2022 16:44:07 -0800 Subject: [PATCH] Document safe usage of undefined behavior in gsl::narrow (#1024) --- include/gsl/narrow | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/include/gsl/narrow b/include/gsl/narrow index 40016d1..bec30d1 100644 --- a/include/gsl/narrow +++ b/include/gsl/narrow @@ -36,7 +36,12 @@ GSL_SUPPRESS(f.6) // NO-FORMAT: attribute // TODO: MSVC /analyze does not recogn constexpr const bool is_different_signedness = (std::is_signed::value != std::is_signed::value); - const T t = narrow_cast(u); +GSL_SUPPRESS(es.103) // NO-FORMAT: attribute // don't overflow +GSL_SUPPRESS(es.104) // NO-FORMAT: attribute // don't underflow +GSL_SUPPRESS(p.2) // NO-FORMAT: attribute // don't rely on undefined behavior + const T t = narrow_cast(u); // While this is technically undefined behavior in some cases (i.e., if the source value is of floating-point type + // and cannot fit into the destination integral type), the resultant behavior is benign on the platforms + // that we target (i.e., no hardware trap representations are hit). if (static_cast(t) != u || (is_different_signedness && ((t < T{}) != (u < U{})))) {