重访按值致电与按引用通话

Question

let me first say. I know the title suspects that I'm asking a question which was answered here and around the internet many times. I did research indeed yet I just can't find a satisfying answer.

My question in the end comes down to this. Why is Java Call By Value and C++ Call By Reference (when using pointers) ?

Consider a method call in Java when passing references and in c++ when passing pointers. In the end in both cases I am able to make changes which are visible to the caller. Also in both cases I'm passing the address of the object to the function. In fact I'm copying ie making a call by value in c++ too when passing a pointer don't I?

You can simply verify that circumstance by running the following code:

#include <iostream>

void modify (int *i) {
    int a = 5;
    i = &a;
}

int main () {
    int b;
    int *i = &b;
    std::cout << i << std::endl;
    modify(i);
    std::cout << i << std::endl;
    return 0;
}

Which will print two times the same address.

For me it's just not enough to justify a call by reference with the property that you can do a swap function with it. I would like to know the very heart of what makes call by reference call by reference.

Thank you in advance.

Answer 1

My question in the end comes down to this. Why is Java Call By Value and C++ Call By Reference (when using pointers) ?

C++ is really call-by-value, not call-by-reference. The by-reference behavior you see in C++ is a side effect of the fact that you're passing a pointer: the pointer itself is copied by value. You can't compare these situations because Java doesn't have pointers.

Answer 2

The example you give is inappropriate use of call by reference. In the function modify

void modify (int *i) {
    int a = 5;
    i = &a;
}

You declare a local variable a which is on the stack. When it goes out of scope, its address is meaningless. So assigning address of a to i has no effect on what i points to.

So what makes call by reference work is you can modify the value of what i points to in the function. For example:

void modify (int *i) {
    int a = 5;
    *i = a;
}

Therefore in the main function, you can call it like:

int b = 2;
int *i = &b;
std::cout << *i << std::endl;  //This will print 2
modify(i);
std::cout << *i << std::endl;  //This will prints 5