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    Value Semantics vs. Reference Semantics Value vs. Reference Semantics Values/Refs

    C++ Terminology

    object = set of bits interpreted according to a type

    Value Semantics

    Reference Semantics
    focuses on values focuses on object identities
    Value Variables Reference Variables
    • refer to objects itself, i.e., one specific memory location
    • exactly one value variable per object
    • lifetimes of variable and associated object are identical
    • may refer to any memory location that holds a type-compatible object
    • many reference variables per object possible
    • lifetimes of variable and referenced object are independent
    Value Members Reference Members
    • are usually stored as part of the containing object
    • refer to objects with the same lifetime as the containing object
    • refer to separately stored objects
    • have independent lifetimes from referenced objects
    Deep Copying Shallow Copying
    • produces a new, independent object; object (member) values are copied
    • produces a new reference to the same object
    Deep Assignment Shallow Assignment
    • makes value of target object equal to that of the source object
    • is either done by (recursively) copy-assigning members or alternatively by transferring ownership, i.e., (recursively) moving the source's value into the target
    • target variable is redirected to refer to the same object as the source variable
    Equality-Based Comparison Identity-Based Comparison
    • objects are equal, if their (member) values are equal
    • objects are identical, if their memory addresses are equal

    Example: Classes Classes

    C++ (value semantics) C++

    struct Point2d { int x;  int y; };
Point2d a {1,2};
Point2d b = a;  // deep copy
cout <<"a: "<< a.x <<' '<< a.y <<"  "
     <<"b: "<< b.x <<' '<< b.y <<'\n';
if (a == b) cout << "equal\n";

    a.x = 8;
cout <<"a: "<< a.x <<' '<< a.y <<"  "
     <<"b: "<< b.x <<' '<< b.y <<'\n';
if (a != b) cout << "different\n";
    
a: 1 2  b: 1 2
equal

    a: 8 2  b: 1 2
different

    Python (reference semantics) Python

    class Point2d:
  def __init__ (self, x, y):
    self.x = x
    self.y = y
a = Point2d(1,2)
b = a  // shallow copy
print("a:", a.x, a.y, " b:", b.x, b.y)

    a.x = 8;
print("a:", a.x, a.y, " b:", b.x, b.y)
if a == b: print("equal")
if a is b: print("identical")
    
a: 1 2  b: 1 2

    a: 8 2  b: 8 2
equal
identical

    Example: Fundamental Types Fundamentals

    Most programming languages use value semantics for fundamental types (also called primitives in some languages). So there is no difference between, e.g., C++ and Python/Java/C#/Swift in that regard.

    C++ (value semantics)

    int a = 2;
int b = a;
cout <<"a: "<< a <<"  b: "<< b <<'\n';

    a = 8;
cout <<"a: "<< a <<"  b: "<< b <<'\n';
    
a: 2  b: 2

    a: 8  b: 2

    Python (value semantics)

    a = 2
b = a
print("a:",a," b:",b)

    a = 8
print("a:",a," b:",b)
    
a: 2  b: 2

    a: 8  b: 2

    Example: Lists Lists

    C++ (value semantics) C++

    auto print = [](std::string_view s, auto const& v){
  cout << s << ": ";
  for (auto x : v) cout << x << ' ';
  cout << '\n';
};

    vector<int> a {1,2,3,4};
vector<int> b = a;  // deep copy
print("a", a); print("b", b);
if (a == b) cout << "equal\n";

    a[0] = 9;
print("a", a); print("b", b);
if (a != b) cout << "different\n";
    
a: 1 2 3 4
b: 1 2 3 4
equal

    a: 9 2 3 4
b: 1 2 3 4

    different

    Python (reference semantics) Python

    a = [1,2,3,4]
b = a  // shallow copy
print("a:", a, " b:", b)

    a[0] = 9;
print("a:", a, " b:", b)
if a == b: print("equal")
if a is b: print("identical")
    
a: [1,2,3,4]  b: [1,2,3,4]

    a: [9,2,3,4]  b: [9,2,3,4]
equal
identical

    Language Situation Languages

    C++, C

    • default: value semantics for fundamental types and user-defined types
    • reference semantics for C-arrays and C-strings – which is one reason to avoid them in C++
    • compiler auto-generates deep copying and deep assignment operations for struct/class
    • as of C++20 user can request compiler to also auto-generate deep comparisons
    • optional reference semantics (through references/pointers) for all types

    Rust

    • default: value semantics for all types, but only fundamental types are copied and user-defined types are moved
    • user defined types can be made copyable by deriving the Copy trait and implementing Clone
    • optional reference semantics (through references/pointers) for all types

    Python

    • default: value semantics for primitive types and reference semantics for lists, dictionaries and class types
    • no first-class support for user-defined value types
    • if needed, deep copying and deep comparison can be done by manually implementing special methods (__deepcopy__, __eq__, …)

    Java

    • default: value semantics for primitive types and reference semantics for class types
    • reference semantics for primitive types can be achieved by boxing it inside a class
    • no first-class support for user-defined value types
    • if needed, deep copying and deep comparison can be done by manually implementing special methods (clone, equals, …)

    Javascript

    • default: value semantics for primitive types and reference semantics for object types
    • no first-class support for user-defined value types
    • deep copying usually done by either converting object to JSON string and parsing it back into a new object or with a library like rfdc or lodash

    Related …

    Last updated: 2021-04-10

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