A value of type "System::String ^" cannot be used to initialize an entity of type "jstring"

Question

So as the title says this the error I get while running this code, which is written in C++/CLI:

String^ somethig(const char* string)
{
  // Some code

}
 JNIEXPORT jstring JNICALL Java_Class_nativeClass
 (JNIEnv * env, jobject jobj,jstring string)
{
jboolean isCopyString;
const char *c_string = env->GetStringUTFChars(string, &isCopyString);
jstring example = somethig(c_string);
return example;
}

I get the error on line:

jstring example = something(c_string); 

Specifically at example. Now I know what the error means, I just don't know how to get jstring to accept the function something. Is there a conversion method that works with functions to make it an acceptable type for jstring?

Thanks in advance.

Answer 1

The method env->NewString() will take a Unicode buffer and return a jstring. You'll need to convert from String^ to wchar_t*, and then pass that to env->NewString(). (Or env->NewStringUTF() to convert a UTF8-encoded char*, which will work for any ASCII char* as well.)

However, if you need to create a jstring, then can you modify somethig to return the jstring directly, or modify the input char*? Without seeing how somethig is implemented, taking a trip through String^ seems unnecessary.

Edit

Here's the conversion code you would use. Again, if you can modify something, there are better solutions than this.

You need to convert the String^ to a wchar_t using StringToHGlobalUni, then you can convert that to a jstring using NewString. Don't forget, you must free the wchar_t* using FreeHGlobal.

String^ managedString = something(...);
wchar_t* unmanagedWideString = Marshal::StringToHGlobalUni(managedString).ToPointer();
jstring example = env->NewString(unmanagedWideString, managedString->Length);
Marshal::FreeHGlobal(unmanagedWideString);
return example;

Disclaimer: I'm not a JNI expert, and I didn't check my syntax in a compiler.