c pass array by value?

Question

I have this homework problem:

Part A

Write a C program that tests whether or not the following data types are passed by reference or by value, and prints what it discovers out to the terminal:

• int
• array of ints

Hypothetically, if your program discovers that an int is passed by value and an array of ints is passed by value, then it should produce output like: int: pass-by-value array of ints: pass-by-value

The int I understand, but what I don't get is:

1. I thought the only way to pass an array was to pass the address of the first value in the array
2. does this count as passing by value or passing by reference? ( Pass an array to a function by value confused me)
3. Hints on how I can do this? I'm assuming something like passing the reference to another function, manipulating it, and seeing if it changed, but I'm not sure....

Edit: might help if someone explained this to me with a concrete example, such as: Say I have an array of length 10 ints stored at memory location 0 (yeah, I know, not real life, but for the sake of the example...). What would it look like if that array was passed by reference? What would it look like if it was passed by value?

Answer 1

Does this count as passing by value or passing by reference?

When you say "passing an array to a function", Actually a pointer to the first element is passed to the function. This allows the called function to modify the contents of the array. Since there is no copy of the array being made it makes sense to say that arrays are passed by reference.

Hints on how I can do this?

The test should be:

1. Create an local array in main().
2. Fill it with a known pattern
3. Print the contents of array
4. Pass the array to a function
5. Inside function body modify the contents of the array
6. Print the array inside the function
7. In main() print the contents of the local array again
8. If outputs in 6 and 7 match. You have a proof.

How do you pass an array by value?

Only possible way of passing an array by value is by wrapping it in a structure.
Online Sample:

#include <iostream>

struct myArrayWrapper 
{
    int m_array[5];
};

void doSomething(myArrayWrapper a) 
{
    int* A = a.m_array;

    //Display array contents
    std::cout<<"\nIn Function Before Modification\n";
    for (size_t j = 0; j < 5; ++j)
       std::cout << ' ' << A[j];
    std::cout << std::endl;

     //Modify the array
     for (size_t j = 0; j < 5; ++j)
       A[j] = 100;

    std::cout<<"\nIn Function After Modification\n";
    //Display array contents
    for (size_t j = 0; j < 5; ++j)
       std::cout << ' ' << A[j];
    std::cout << std::endl;

}

int main()
{
    myArrayWrapper obj;
    obj.m_array[0] = 0;
    obj.m_array[1] = 1;
    obj.m_array[2] = 2;
    obj.m_array[3] = 3;
    obj.m_array[4] = 4;
    doSomething(obj);

    //Display array contents
    std::cout<<"\nIn Main\n";
    for (size_t j = 0; j < 5; ++j)
       std::cout << ' ' << obj.m_array[j];
    std::cout << std::endl;

    return 0; 
}

Output:

In Function Before Modification
 0 1 2 3 4

In Function After Modification
 100 100 100 100 100

In Main
 0 1 2 3 4

Answer 2

C only pass arguments to functions by value.

From the mighty Kernighan & Ritchie, 2nd edition:

(1.8, Call by value) "In C all function arguments are passed by "value"