use constexpr const char* for encoding

Author: jagerman

include/pybind11/cast.h

@@ -620,6 +620,8 @@ struct type_caster<std::basic_string<CharT, Traits, Allocator>, enable_if_t<is_s
         std::is_same<CharT, char32_t>::value ? 32 :
         (sizeof(CharT) == 2 ? 16 : 32); /* std::wstring is UTF-16 on Windows, UTF-32 everywhere else */
 
+    static constexpr const char *encoding = UTF_N == 8 ? "utf8" : UTF_N == 16 ? "utf16" : "utf32";
+
     // C++ only requires char/char16_t/char32_t to be at least 8/16/32 bits, but Python's encoding
     // assumes exactly 1/2/4 bytes:
     static_assert(sizeof(CharT) == UTF_N / 8,
@@ -639,9 +641,7 @@ struct type_caster<std::basic_string<CharT, Traits, Allocator>, enable_if_t<is_s
         }
 
         object utfNbytes = reinterpret_steal<object>(PyUnicode_AsEncodedString(
-            load_src.ptr(),
-            UTF_N == 8 ? "utf8" : UTF_N == 16 ? "utf16" : "utf32",
-            nullptr));
+            load_src.ptr(), encoding, nullptr));
         if (!utfNbytes) { PyErr_Clear(); return false; }
 
         const CharT *buffer = reinterpret_cast<const CharT *>(PYBIND11_BYTES_AS_STRING(utfNbytes.ptr()));
@@ -655,7 +655,7 @@ struct type_caster<std::basic_string<CharT, Traits, Allocator>, enable_if_t<is_s
     static handle cast(const StringType &src, return_value_policy /* policy */, handle /* parent */) {
         const char *buffer = reinterpret_cast<const char *>(src.c_str());
         ssize_t nbytes = ssize_t(src.size() * sizeof(CharT));
-        handle s = PyUnicode_Decode(buffer, nbytes, UTF_N == 8 ? "utf8" : UTF_N == 16 ? "utf16" : "utf32", nullptr);
+        handle s = PyUnicode_Decode(buffer, nbytes, encoding, nullptr);
         if (!s) throw error_already_set();
         return s;
     }