Size of empty struct in rust

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I'm from C++ background . in C++ , empty class has size of 1 Byte. This is because C++ requires every object to have a unique memory address, even if it doesn't have any data or member variables. But in Rust empty struct has size Zero. considering Rust is a similar language like C++ then why the size of empty struct in rust is zero.

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According to The Rust Reference, the default representation for composite types (structs, enums, etc) in Rust obey the following three conditions:

  1. The fields are properly aligned.
  2. The fields do not overlap.
  3. The alignment of the type is at least the maximum alignment of its fields.

Zero-size structs don't break these rules, so they are allowed to exist to save space.

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Also have a look at the ZST section in the nomicon.

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In Rust, an empty struct (also known as a zero-sized type or ZST) has a size of zero bytes, which is different from the behavior in C++ where an empty class has a size of at least one byte. There are several reasons for this distinction:

  1. Address uniqueness: While C++ mandates that every object must have a unique memory address, Rust doesn't impose this requirement. As a result, Rust's compiler can optimize empty structs to have zero size.

  2. Language design: Rust's design enables the use of zero-sized types, whereas C++ does not. ZSTs in Rust serve various purposes, such as implementing marker traits, tags, and other compile-time constructs. These types are primarily utilized for type-level programming and don't incur any runtime overhead.

  3. Optimization benefits: The Rust compiler is designed to optimize empty structs to have zero size, which can result in improved performance and reduced memory usage. This is especially beneficial when dealing with a large number of empty structs in your code.

It's worth noting that even though Rust's empty structs have a size of zero bytes, the compiler can still ensure unique addresses for distinct instances of an empty struct when required, such as when used as a field in another struct or when dynamically allocated on the heap. This is achieved through compiler optimizations and layout calculations.

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Also note that C++20 added [[no_unique_address]] to allow for true zero sized fields.

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(Also, base class subobjects in C++ have always been permitted (since at least C++98) to have zero additional size, if they have no fields. It's only the most derived object which must have nonzero size.)

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I guess the ploy worked a tiny bit too well if people are asking such questions.

The best short characterisation of Rust would be: “an ML dialect in a C++ trenchcoat”.

Rust borrowed lots of it's syntax from C++, that's true. That's why it have enums and not coproducts, e.g.

But if we are talking about semantic then it doesn't follow C or C++ rules. In particular it's unit type is not special (unlike C/C++ void) and that one, naturally, have size zero.

And once you have allowed one type to have zero size it's not that hard to allow more zero-sized types.

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I'm not sure what you mean by this, but ZSTs as fields are still zero-sized and can have the same address as another field, Box of ZST doesn't allocate (and typically collections don't either) and their pointers-to-ZST can be the same, etc. Pointer identity is pretty meaningless in the face of ZSTs (and also fraught in the face of wide pointers).

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