After reading and discussing @Christophe’s and @Theraot’s excellent answers, and drawing inspiration from Bjarne Stroustrup’s and Phil Goodwin’s definitions, I finally came to the following set of definitions, that are close but slightly different (more general) from the previous authors’:
- Value/reference semantics. — An independency/dependency relation between objects.
- Value/reference type. — A type that provides copies in value/reference semantics relation.
- Value/reference object. — An instance of a value/reference type.
Sufficient conditions for value semantics:
- the objects are deep copies of one another, or
- the objects do not hold references and are shallow copies of one another, or
- the objects do not hold references to mutable objects, hold references to immutable objects and are shallow copies of one another, or
- the objects hold references to mutable objects, hold references to immutable objects and are mutable-deep and immutable-shallow copies of one another.
Sufficient condition for reference semantics:
- the objects hold references to mutable objects and are shallow copies of one another.
Feel free to give your feedback in comments.
Playground
Memory layout resulting from assignment in C++:
int i{3}; // i:3
int j{i}; // i:3 j:3 (copy of i: j)
int* p{&i}; // i:3 p:&i (alias of i: *p)
int* q{p}; // i:3 p:&i q:&i (copy of p: q, alias of i: *q)
int* r{new int{*p}}; // i:3 p:&i *r:3 r:_ (copy of i: *r)
int** s{&p}; // i:3 p:&i s:&p (alias of p: *s)
int** t{s}; // i:3 p:&i s:&p t:&p (copy of s: t, alias of p: *t)
int** u{new int*{*s}}; // i:3 p:&i s:&p *u:&p u:_ (copy of s: *u, alias of p: **u)
Here i and j are in value semantics relation, p an q are in reference semantics relation, p and r are in value semantics relation, s and t are in reference semantics relation, and s and u are in reference semantics relation.