Java與C++的函式參數傳遞比較

Java參數傳遞測試

Java的參數傳遞對於基本型態是複製，而物件型態則是類似C/C++的指標行為, 是複製物件的位址到函式中，所以在函式內assgin的新物件不影響外部變數所指的變數。

C++參數傳遞測試

而C++的參考則是會讓外部變數跟內部變數的狀態保持一致。

Java測試用程式碼
public class test {
    static class MyObject {
        public int iv = 0;
        public String is = "initial field";
        public MyObject io = this;
        public MyObject() {

        }
        public MyObject(int primitive, String string, MyObject object) {
            iv = primitive;
            is = string;
            io = object;
        }
    }
    static public void setSomething(int primitive, String string, MyObject object) {
        System.out.println("setSomething");
        primitive = 200;
        string = "do something";
        object = new MyObject();
    }
    static public void setSomething2(int primitive, String string, MyObject object) {
        System.out.println("setSomething2");
        primitive = 300;
        string = "do something 2";
        object.iv = 400;
        object.is = "set field";
        object.io = new MyObject();
    }
    public static void main(String[] args) {
        int pri = 100;
        String str = "initial value";
        MyObject obj = new MyObject();
        System.out.println("int pri = " + pri);
        System.out.println("String str = " + str + ", hash-code = " + Integer.toHexString(str.hashCode()));
        System.out.println("MyObect obj = " + obj + ", hash-code = " + Integer.toHexString(obj.hashCode()));
        System.out.println("        obj.iv = " + obj.iv);
        System.out.println("        obj.is = " + obj.is + ", hash-code = "  + Integer.toHexString(obj.is.hashCode()));
        System.out.println("        obj.io = " + obj.io + ", hash-code = "  + Integer.toHexString(obj.io.hashCode()));
        System.out.println();
        setSomething(pri, str, obj);
        System.out.println();
        System.out.println("int pri = " + pri);
        System.out.println("String str = " + str + ", hash-code = " + Integer.toHexString(str.hashCode()));
        System.out.println("MyObect obj = " + obj + ", hash-code = "  + Integer.toHexString(obj.hashCode()));
        System.out.println("        obj.iv = " + obj.iv);
        System.out.println("        obj.is = " + obj.is + ", hash-code = "  + Integer.toHexString(obj.is.hashCode()));
        System.out.println("        obj.io = " + obj.io + ", hash-code = "  + Integer.toHexString(obj.io.hashCode()));
        System.out.println();
        setSomething2(pri, str, obj);
        System.out.println();
        System.out.println("int pri = " + pri);
        System.out.println("String str = " + str + ", hash-code = " + Integer.toHexString(str.hashCode()));
        System.out.println("MyObect obj = " + obj + ", hash-code = "  + Integer.toHexString(obj.hashCode()));
        System.out.println("        obj.iv = " + obj.iv);
        System.out.println("        obj.is = " + obj.is + ", hash-code = "  + Integer.toHexString(obj.is.hashCode()));
        System.out.println("        obj.io = " + obj.io + ", hash-code = "  + Integer.toHexString(obj.io.hashCode()));
    }
}

C++測試用程式碼
#include <string>
#include <iostream>

using namespace std;

class MyObject {
public:
    int iv;
    string is;
    MyObject *io;
    MyObject() :
        iv(0), is("initial"), io(this)
    {
        // empty
    }
    MyObject(int primitive, string &string, MyObject &object) :
        iv(primitive), is(string), io(&object)
    {
        // empty
    }
    MyObject(const MyObject &obj)
    {
        cout << "MyObject copy constructor" << endl;
        iv = obj.iv;
        is = obj.is;
        iv = obj.iv;
    }
};

ostream & operator <<(ostream &out, MyObject& obj) {
    out << "MyObject@" << (void*) &obj;
    return out;
}

void setSomething(int primitive, string &string, MyObject &object) {
    cout << "setSomething" << endl;
    primitive = 200;
    string = "do something";
    object = MyObject();
}

void setSomething2(int primitive, string &string, MyObject &object) {
    cout << "setSomething2" << endl;
    primitive = 300;
    string = "do something 2";
    object.iv = 400;
    object.is = "set field";
    object.io = new MyObject();
}

int main(int argc, char** argv) {
    int pri = 100;
    string str = "initial value";
    MyObject obj = MyObject();

    cout << "int pri = " << pri << endl;
    cout << "string str = " << str << ", addr = " << (void*)&str << endl;
    cout << "MyObect obj = " << obj << ", addr = " << (void*)&obj << endl;
    cout << "        obj.iv = " << obj.iv << endl;
    cout << "        obj.is = " << obj.is << ", addr = " << (void*)&obj.is << endl;
    cout << "        obj.io = " << *(obj.io) << ", addr = " << (void*)&obj.io << endl;

    cout << endl;
    setSomething(pri, str, obj);
    cout << endl;

    cout << "int pri = " << pri << endl;
    cout << "string str = " << str << ", addr = " << (void*)&str << endl;
    cout << "MyObect obj = " << obj << ", addr = " << (void*)&obj << endl;
    cout << "        obj.iv = " << obj.iv << endl;
    cout << "        obj.is = " << obj.is << ", addr = " << (void*)&obj.is << endl;
    cout << "        obj.io = " << *(obj.io) << ", addr = " << (void*)&obj.io << endl;

    cout << endl;
    setSomething2(pri, str, obj);
    cout << endl;

    cout << "int pri = " << pri << endl;
    cout << "string str = " << str << ", addr = " << (void*)&str << endl;
    cout << "MyObect obj = " << obj << ", addr = " << (void*)&obj << endl;
    cout << "        obj.iv = " << obj.iv << endl;
    cout << "        obj.is = " << obj.is << ", addr = " << (void*)&obj.is << endl;
    cout << "        obj.io = " << *(obj.io) << ", addr = " << (void*)&obj.io << endl;

    return 0;
}