Are pointers considered a method of calling by reference in C?

Question

In my University\'s C programming class, the professor and subsequent book written by her uses the term call or pass by reference when referring to poin

Answer 1

"Passing by reference" is a concept. Yes, you are passing the value of a pointer to the function, but in that instance the value of that pointer is being used to reference the variable.

Someone else used a screwdriver analogy to explain that it is wrong to refer to the passing of pointers as passing by reference, saying that you can screw a screw with a coin but that that doesn't mean you would call the coin a screw driver. I would say that is a great analogy, but they come to the wrong conclusion. In fact, while you wouldn't claim a coin was a screwdriver, you would still say that you screwed the screw in with it. i.e. even though pointers are not the same as c++ references, what you are using them to do IS passing by reference.

Answer 2

you cannot pass by reference in C but can use pointers as an alternative

Yup, thats correct.

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To elaborate a little more. Whatever you pass as an argument to c functions, it is passed by values only. Whether it be a variable's value or the variable address.

What makes the difference is what you are sending.

When we pass-by-value we are passing the value of the variable to a function. When we pass-by-reference we are passing an alias of the variable to a function. C can pass a pointer into a function but that is still pass-by-value. It is copying the value of the pointer, the address, into the function.

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• If you are sending the value of a variable, then only the value will be received by the function, and changing that won't effect the original value.

• If you are sending the address of a variable, then also only the value(the address in this case) is sent, but since you have the address of a variable it can be used to change the original value.

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As an example, we can see some C++ code to understand the real difference between call-by-value and call-by-reference. Taken from this website.

// Program to sort two numbers using call by reference. 
// Smallest number is output first.

#include <iostream>
using namespace std;

// Function prototype for call by reference
void swap(float &x, float &y);

int main()
{
   float a, b;

   cout << "Enter 2 numbers: " << endl;
   cin >> a >> b;
   if(a>b) 
     swap(a,b); // This looks just like a call-by-value, but in fact
                // it's a call by reference (because of the "&" in the
                // function prototype

   // Variable a contains value of smallest number
   cout << "Sorted numbers: ";
   cout << a << " " << b << endl;
   return 0;
}

// A function definition for call by reference
// The variables x and y will have their values changed.

void swap(float &x, float &y)
// Swaps x and y data of calling function
{
   float temp;

   temp = x;
   x = y;
   y = temp;
}

In this C++ example, reference variable(which is not present in C) is being used. To quote this website,

"A reference is an alias, or an alternate name to an existing variable...",

and

"The main use of references is acting as function formal parameters to support pass-by-reference..."

This is different then the use of pointers as function parameters because,

"A pointer variable (or pointer in short) is basically the same as the other variables, which can store a piece of data. Unlike normal variable which stores a value (such as an int, a double, a char), a pointer stores a memory address."

So, essentially when one is sending address and receiving through pointers, one is sending the value only, but when one is sending/receiving a reference variable, one is sending an alias, or a reference.

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**UPDATE : 11 November, 2015**

There has been a long debate in the C Chatroom, and after reading comments and answers to this question, i have realized that there can be another way to look at this question, another perspective that is.

Lets look at some simple C code

int i;
int *p = &i;
*p = 123;

In this scenario, one can use the terminology that, p's value is a reference to i. So, if that is the case, then if we send the same pointer (int* p) to a function, one can argue that, since i's reference is sent to the function, and thus this can be called pass-by-reference.

So, its a matter of terminology and way of looking at the scenario.

I would not completely disagree with that argument. But for a person who completely follows the book and rules, this would be wrong.

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NOTE: Update inspired by this chat.