JNI C++ undefined reference to

Question

i have the following JNI bridge, the bridge compiles and JNI is known to be working but when compiling the C++ code i get errors. Not sure what to do. Android Studio Doesn't mark anything wrong with the code.

extern "C" {

class Singleton {
private:
    Singleton();

public:
    static Singleton &instance() {
        static Singleton INSTANCE;
        return INSTANCE;
    }

    std::string something = "123";

    void setSomething(std::string some) {
        this->something = std::move(some);
    }

    std::string getSomething() {
        return this->something;
    }
};

JNIEXPORT void JNICALL
Java_com_example_jni_JniGcode_init(JNIEnv *env, jobject jniGcode, jstring file_path) {

    Singleton::instance().getSomething();
    Singleton::instance().setSomething("blabla");
}


JNIEXPORT void JNICALL
Java_com_example_jni_JniGcode_something(JNIEnv *env, jobject thiz) {
    Singleton::instance().getSomething();
}

}

the error that im getting:

In function `Singleton::instance()':
/{redacted}/file.cpp:34: undefined reference to `Singleton::Singleton()'