The article has a section about that.

For me, a struct in C/C# can be modified and is passed by copy while a value class can not be modified and is passed by value.

I do not think you can do stack allocation in Java.

▲

kuhsaft 2 hours ago | parent | next [-]

Like @layer8 said, pass by copy and pass by value are the same.

C# copies C++ behavior where you can pass a struct by value or reference, and you can mark the parameter as readonly. C# also has in/out parameters. Essentially, you can program in C# exactly like you would in C++.

https://learn.microsoft.com/en-us/dotnet/csharp/language-ref...

The footgun with C# structs are that you can accidentally box them onto the heap. To avoid that you can define `ref struct`s that cannot be boxed. `ref struct`s follow the C# disposable pattern.

https://learn.microsoft.com/en-us/dotnet/csharp/language-ref...

▲

layer8 8 hours ago | parent | prev [-]

I don’t see a difference between pass by copy and pass by value.

The mutability difference is that part of a struct can be modified in place, which value classes can’t: the value of a complete value-class variable (or array slot) can only be modified (reassigned) as a whole. This is presumably because object references to value-class objects can be created, and those objects should be immutable so their identity doesn’t matter.

▲

_old_dude_ 8 hours ago | parent | next [-]

I think pass by copy is a consequence of being modifiable.

The other solution is to stack allocate and pass a pointer but as i said, unlike in C#, i do not think it's possible to do that in Java.

In Go, you can stack allocate but when you send a pointer (that escapes), the compiler will heap allocate the object.

▲

layer8 8 hours ago | parent [-]

My point is that pass by copy and pass by value do the same thing, they copy the value representation. In other words, pass by copy means exactly pass by value.

▲

gf000 7 hours ago | parent | next [-]

Actually, Java only has pass-by-value, even for reference types. (The same way as C does).

People really misuse/misunderstand this term: Java objects are passed by their pointers ("references") being copied.

The alternative is pass by reference, which is done by e.g. c++, rust, who actually have references (Java doesn't). A good litmus test is whether you can write a swap method that actually changes your local variables.

	▲	steveklabnik 2 hours ago \| parent [-]
		Rust is also “pass reference by value,” not pass by reference.

▲

_old_dude_ 7 hours ago | parent | prev [-]

For me, the difference is that if methods are inlined, the compiler is still required to do a copy for structs but not for value classes.

I do not know how this is called.

▲

gf000 8 hours ago | parent | prev [-]

I think that's mostly a semantic difference - Java avoided the problem of strange lifetimes, captures, tearing by fixing the semantics as immutable value objects, while C# has to deal with these issues.

But under the hood it can (and will) do a modification in place.