src: do not use non-static class member for constant value (#1134)

Non-static class members need to be set for each instantiated
object and take up extra space, which is why constant data
is generally provided through static members or static functions
(and static functions are a bit easier to use in header-only C++11).

Author: addaleax

napi-inl.h

@@ -2607,8 +2607,8 @@ inline Error::Error(napi_env env, napi_value value) : ObjectReference(env, nullp
 
       // property flag that we attach to show the error object is wrapped
       napi_property_descriptor wrapObjFlag = {
-          ERROR_WRAP_VALUE,  // Unique GUID identifier since Symbol isn't a
-                             // viable option
+          ERROR_WRAP_VALUE(),  // Unique GUID identifier since Symbol isn't a
+                               // viable option
           nullptr,
           nullptr,
           nullptr,
@@ -2648,15 +2648,17 @@ inline Object Error::Value() const {
     // We are checking if the object is wrapped
     bool isWrappedObject = false;
 
-    status = napi_has_property(
-        _env, refValue, String::From(_env, ERROR_WRAP_VALUE), &isWrappedObject);
+    status = napi_has_property(_env,
+                               refValue,
+                               String::From(_env, ERROR_WRAP_VALUE()),
+                               &isWrappedObject);
 
     // Don't care about status
     if (isWrappedObject) {
       napi_value unwrappedValue;
       status = napi_get_property(_env,
                                  refValue,
-                                 String::From(_env, ERROR_WRAP_VALUE),
+                                 String::From(_env, ERROR_WRAP_VALUE()),
                                  &unwrappedValue);
       NAPI_THROW_IF_FAILED(_env, status, Object());
 
@@ -2772,6 +2774,10 @@ inline const char* Error::what() const NAPI_NOEXCEPT {
 
 #endif // NAPI_CPP_EXCEPTIONS
 
+inline const char* Error::ERROR_WRAP_VALUE() NAPI_NOEXCEPT {
+  return "4bda9e7e-4913-4dbc-95de-891cbf66598e-errorVal";
+}
+
 template <typename TError>
 inline TError Error::New(napi_env env,
                          const char* message,

napi.h

@@ -1720,8 +1720,7 @@ namespace Napi {
    /// !endcond
 
   private:
-   const char* ERROR_WRAP_VALUE =
-       "4bda9e7e-4913-4dbc-95de-891cbf66598e-errorVal";
+   static inline const char* ERROR_WRAP_VALUE() NAPI_NOEXCEPT;
    mutable std::string _message;
   };