Add bit_cast

include/fmt/format.h

@@ -211,6 +211,17 @@ inline uint32_t clzll(uint64_t x) {
 namespace fmt {
 namespace internal {
 
+// An equivalent of `*reinterpret_cast<Dest*>(&source)` that doesn't produce
+// undefined behavior (e.g. due to type aliasing).
+// Example: uint64_t d = bit_cast<uint64_t>(2.718);
+template <typename Dest, typename Source>
+inline Dest bit_cast(const Source& source) {
+  static_assert(sizeof(Dest) == sizeof(Source), "size mismatch");
+  Dest dest;
+  std::memcpy(&dest, &source, sizeof(dest));
+  return dest;
+}
+
 // An implementation of begin and end for pre-C++11 compilers such as gcc 4.
 template <typename C>
 FMT_CONSTEXPR auto begin(const C &c) -> decltype(c.begin()) {

test/util-test.cc

@@ -397,6 +397,16 @@ TEST(FixedBufferTest, BufferOverflow) {
   EXPECT_THROW_MSG(buffer.resize(11), std::runtime_error, "buffer overflow");
 }
 
+TEST(UtilTest, BitCast) {
+  struct S {
+    uint32_t u[2];
+  };
+  auto s = fmt::internal::bit_cast<S>(uint64_t(42));
+  EXPECT_EQ(fmt::internal::bit_cast<uint64_t>(s), 42);
+  s = fmt::internal::bit_cast<S>(uint64_t(~0ull));
+  EXPECT_EQ(fmt::internal::bit_cast<uint64_t>(s), ~0ull);
+}
+
 TEST(UtilTest, Increment) {
   char s[10] = "123";
   increment(s);