5 ms·

Coming from C++, if "passing by pointer value" is considered "passing by value", then what is the opposite? Not passing at all? I mean, there's always going to

by chi3 8y ago

Coming from C++, if "passing by pointer value" is considered "passing by value", then what is the opposite? Not passing at all? I mean, there's always going to be _some_ kind of value being passed?

lmm 8y ago
If you write foo(&a), both "you pass a by reference" and "you pass &a by value" are fair descriptions of what happens. The confusion arises because Java doesn't explicitly distinguish between a and &a; if you write "Foo a = new Foo();" in Java, the behaviour is similar to "Foo *a = new Foo();" in C++. So if we ask how "a" is passed, is the "a" we're talking about the reference or the value?
- chi3 8y ago
  Ah, I see the reasoning. I only did basic Java in university so I might be remembering incorrectly, but aren't variables storing non-primitive values called "References"? Seems weird to me to say that Java "passes by value" when all objects being passed are actually references to objects.
  - lmm 8y ago
    > I only did basic Java in university so I might be remembering incorrectly, but aren't variables storing non-primitive values called "References"? Technically yes, but since the language doesn't let you talk about references directly, the concept rarely comes up. Like, in theory you could say "a is a reference that refers to a value of type Foo", but people don't actually say that; they just say "a is of type Foo". > Seems weird to me to say that Java "passes by value" when all objects being passed are actually references to objects. Indeed, completely agreed.
    - emidln 8y ago
      You do see this kind of language in Clojure, where you can talk about a var separately from its value.
- zamalek 8y ago
  The pointer value is passed (which is a valid contention), but that is what it has been defined as for decades.
TheCoelacanth 8y ago
In C++, terms void f(Foo x); // pass-by-value void f(Foo* x); // pass-pointer-by-value void f(Foo& x); // pass-by-reference In the last example, you aren't passing in a value, you are essentially creating an alias for some variable in the calling scope.
- chi3 8y ago
  But it's still just a pointer underneath? It's more or less just syntactical sugar which makes it dereferenced it for you, and I'd argue that it only makes sense to use the phrase in certain contexts. The only real difference between the last two is that the last one shouldn't be sent in as null, isn't it? The test in OPs artictle doesn't really make sense either, because you can switch the values using pointers as well.
colanderman 8y ago
My made-up term "pass-by-pointer-value" just emphasizes the fact that in Java, Python, and sundry other OOP languages, you are always passing around a pointer to an object, even if it is implicit. This is why e.g. equality usually checks object identity (pointer value) in these languages (Python being a notable exception). C++ just forces you to be explicit about this, as others have pointed out.

by chi3 8y ago

Coming from C++, if "passing by pointer value" is considered "passing by value", then what is the opposite? Not passing at all? I mean, there's always going to be _some_ kind of value being passed?

lmm 8y ago

If you write foo(&a), both "you pass a by reference" and "you pass &a by value" are fair descriptions of what happens. The confusion arises because Java doesn't explicitly distinguish between a and &a; if you write "Foo a = new Foo();" in Java, the behaviour is similar to "Foo *a = new Foo();" in C++. So if we ask how "a" is passed, is the "a" we're talking about the reference or the value?

chi3 8y ago
Ah, I see the reasoning. I only did basic Java in university so I might be remembering incorrectly, but aren't variables storing non-primitive values called "References"? Seems weird to me to say that Java "passes by value" when all objects being passed are actually references to objects.
- lmm 8y ago
  > I only did basic Java in university so I might be remembering incorrectly, but aren't variables storing non-primitive values called "References"? Technically yes, but since the language doesn't let you talk about references directly, the concept rarely comes up. Like, in theory you could say "a is a reference that refers to a value of type Foo", but people don't actually say that; they just say "a is of type Foo". > Seems weird to me to say that Java "passes by value" when all objects being passed are actually references to objects. Indeed, completely agreed.
  - emidln 8y ago
    You do see this kind of language in Clojure, where you can talk about a var separately from its value.
zamalek 8y ago
The pointer value is passed (which is a valid contention), but that is what it has been defined as for decades.

TheCoelacanth 8y ago

In C++, terms void f(Foo x); // pass-by-value void f(Foo* x); // pass-pointer-by-value void f(Foo& x); // pass-by-reference In the last example, you aren't passing in a value, you are essentially creating an alias for some variable in the calling scope.

chi3 8y ago
But it's still just a pointer underneath? It's more or less just syntactical sugar which makes it dereferenced it for you, and I'd argue that it only makes sense to use the phrase in certain contexts. The only real difference between the last two is that the last one shouldn't be sent in as null, isn't it? The test in OPs artictle doesn't really make sense either, because you can switch the values using pointers as well.

colanderman 8y ago

My made-up term "pass-by-pointer-value" just emphasizes the fact that in Java, Python, and sundry other OOP languages, you are always passing around a pointer to an object, even if it is implicit. This is why e.g. equality usually checks object identity (pointer value) in these languages (Python being a notable exception). C++ just forces you to be explicit about this, as others have pointed out.