Question about std::shared_ptr callback and Pointer casting in JavaCPP

[y z (s1mple)]

Hello,

I'm using JavaCPP to create a native interface for a C++ library. I am encountering an issue when handling a callback that uses std::shared_ptr as a parameter.

The problem is that the Pointer received in the Java callback cannot be correctly cast to the generated MyString type, resulting in a NullPointerException.

Here is the relevant code and the error message:

C++ side:

class MyString { 

    std::string my_string; 

}; 

template <typename M0> using CallbackFunc = std::function<void(std::shared_ptr<M0>)>; 

void callback(CallbackFunc<MyString> func) { 

    std::shared_ptr<MyString> p = std::make_shared<MyString>(); 

    p->my_string = "hello"; 

    func(p); 

    return; 

}

JavaCPP Configuration:

// I'm mapping CallbackFunc to FunctionPointer 

public static class CallbackFunc extends FunctionPointer { 

    static { Loader.load(); } 

    public CallbackFunc(Pointer p) { super(p); } 

    protected CallbackFunc() { allocate(); } 

    private native void allocate(); 

    public native void call(@ByVal @Cast("std::shared_ptr<MyString>*") Pointer string); 

}

Java Test Code:

CallbackFunc cb = new CallbackFunc() { 

    public void call(Pointer str) { 

        MyString str1 = new MyString(str); 

        String str2 = str1.my_string().getString(); 

        System.out.println(str2); 

    } 

};
callback(cb);

Execution Result:

An exception occurred while executing the Java class. java.lang.NullPointerException: Cannot invoke "MyString.my_string()" because the return value of "MyString.my_string()" is null

My analysis:

I've found that if the C++ callback parameter is not wrapped in std::shared_ptr (e.g., MyString*), the conversion works correctly. The issue seems to be with how JavaCPP handles the std::shared_ptr object itself, possibly casting the Pointer to the smart pointer object's address instead of the managed MyString object's address.

What is the correct way to handle std::shared_ptr as a callback parameter in JavaCPP?

Thank you for your time.

[Samuel Audet]

Try using the @SharedPtr adapter annotation like this:

https://github.com/bytedeco/javacpp-presets/blob/master/pytorch/src/main/java/org/bytedeco/pytorch/NamedSharedModuleApplyFunction.java#L28

[y z]

Thank you so much for your help and excellent advice.

I followed your guidance and correctly implemented the @SharedPtr annotation, and it completely solved the NullPointerException issue.

Your explanation was very clear and helped me understand the correct way to handle std::shared_ptr in JavaCPP. I truly appreciate your support.

Samuel Audet <samuel...@gmail.com> 于2025年8月26日周二 16:48写道：

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