Use of C++ objects in SNI function

MicroEJ SDK
1

Hi,

Is it possible to use a C++ code/objects in an SNI C function?
If so, could you please point me to any documentation or an example that shows how to use it?

Thanks

2

Hi,

Yes, you can call C++ functions or use C++ objects from an SNI C function.

To do that, in your C++ source file, you need to surround the SNI function definition with the extern "C" { ... } directive.

It is worth noting the C++ objects fields and methods cannot be accessed directly from the Java code.
In Java, you can retrieve, store, and use a reference to a C++ object using pointers. These pointers must be stored as Java long fields.

Here is an example of a Java code that declares native methods that use C++ objects:

public class TestCPP {

	public static void main(String[] args) {

		long cppObject = newCppObject();

		cppPrintHelloWorld(cppObject);
	}

	/**
	 * Allocates and returns a C++ object.
	 */
	public static native long newCppObject();

	/**
	 * Calls a C++ object's method.
	 */
	public static native void cppPrintHelloWorld(long cppObject);
}

And the C++ code that defines a C++ class and the SNI wrappers to access it:

CppObject::CppObject() {
}

void CppObject::printHelloWorld() {
    std::cout << "Hello world C++"<<std::endl;
}

/* Implement here the SNI wrapper functions to call C++ object's methods*/
#ifdef __cplusplus
extern "C" {
#endif

int64_t Java_com_mycompany_TestCPP_newCppObject() {
	void *cppObjectPtr = new CppObject();
	return (int64_t)cppObjectPtr;
}

void Java_com_mycompany_TestCPP_cppPrintHelloWorld(int64_t cppObjectPtr) {

    CppObject* cppObject = static_cast<HelloWorld *>((void*)cppObjectPtr);
    cppObject->printHelloWorld();
}

#ifdef __cplusplus
}
#endif

Regards,
Jerome