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Off Topic => General Interest => Computers & Programming General => Topic started by: Hooman on October 07, 2017, 08:43:10 PM

Title: Call-By-Sharing
Post by: Hooman on October 07, 2017, 08:43:10 PM
I came across a somewhat less common programming term: Call-By-Sharing (https://en.wikipedia.org/wiki/Evaluation_strategy#Call_by_sharing)

One characterization is:
Call-by-value, but object variables are understood to contain a reference to the actual object.

If you mutate a passed object, the change is seen by the caller. If you re-assign a value to a passed variable, it is not seen by the caller.

Python examples from the Wikipedia article:
Mutation is seen by caller:
Code: [Select]
def f(l):
m = []
print(m)  # => [1]

Re-assignment is not seen by caller:
Code: [Select]
def f(l):
    l = [1]
m = []
print(m)  # => []

Applies to:
Python, Iota, Java (for object references), Ruby, JavaScript, Scheme, OCaml, AppleScript, and many others

Another characterization:
Primitive values use pass-by-value, while object variables use pass-by-reference.

Despite the name, for C++, this is more like pointer syntax than reference syntax. If you assigned to a reference (after the initial assignment that sets the address/object identity), it will copy over the original object. If you assign to a pointer, the pointer now holds a new distinct object, and the old object continues to exist in memory.
Code: [Select]
#include <iostream>

void f(int a, int& b, int* c, int* d)
  a = 5;  // Assignment of value
  b = 6;  // Assignment of reference (overwrites/mutates original object)
  *c = 7; // Assignment through a reference (mutating original through a reference)
  d = new int{8}; // Assignment of pointer (new object identity, separate from old)

int main()
  int a = 1;
  int b = 2;
  int c = 3;
  int d = 4;

  f(a, b, &c, &d);

  std::cout << a << b << c << d << std::endl;  // => 1674

There is also this nicely answered question on StackOverflow for JavaScript:
Is JavaScript a Pass-By-Reference or Pass-By-Value Language? (https://stackoverflow.com/questions/518000/is-javascript-a-pass-by-reference-or-pass-by-value-language)