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According to me, it is zero but there seems to be bit confusion here
I have tested it with gcc compiler and it gives me zero as output. I know that in C++, size of an empty class is 1. Let me know if I am missing anything here.
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Empty struct in C is undefined behaviour (refer C17 spec, section 6.7. 2.1 ): If the struct-declaration-list does not contain any named members, either directly or via an anonymous structure or anonymous union, the behavior is undefined.
The size of the entire structure is 8 bytes. On knowing the structured padding, it is easier to redesign or rewrite the structure.
An empty structIt occupies zero bytes of storage. var s struct{} fmt.Println(unsafe.Sizeof(s)) // prints 0. As the empty struct consumes zero bytes, it follows that it needs no padding. Thus a struct comprised of empty structs also consumes no storage.
It's worth noting that empty structs are only somewhat supported in C and disallowed in C99. Empty structs are supported in C++ but different compilers implement them with varying results (for sizeof and struct offsets), especially once you start throwing inheritance into the mix.
A struct cannot be empty in C because the syntax forbids it. Furthermore, there is a semantic constraint that makes behavior undefined if a struct has no named member:
struct-or-union-specifier: struct-or-union identifieropt { struct-declaration-list } struct-or-union identifier struct-or-union: struct union struct-declaration-list: struct-declaration struct-declaration-list struct-declaration struct-declaration: specifier-qualifier-list struct-declarator-list ; /* type-specifier or qualifier required here! */ specifier-qualifier-list: type-specifier specifier-qualifier-listopt type-qualifier specifier-qualifier-listopt struct-declarator-list: struct-declarator struct-declarator-list , struct-declarator struct-declarator: declarator declaratoropt : constant-expression If you write
struct identifier { }; It will give you a diagnostic message, because you violate syntactic rules. If you write
struct identifier { int : 0; }; Then you have a non-empty struct with no named members, thus making behavior undefined, and not requiring a diagnostic:
If the struct-declaration-list contains no named members, the behavior is undefined.
Notice that the following is disallowed because a flexible array member cannot be the first member:
struct identifier { type ident[]; }; The C grammar doesn't allow the contents of a struct to be empty - there has to be at least an unnamed bitfield or a named member (as far as the grammar is concerned - I'm not sure if a struct that contains only an unnamed bitfield is otherwise valid).
Support for empty structs in C are an extension in GCC.
In C++ and empty struct/class member-specification is explicitly permitted, but the size is defined to be 1 - unless as part of the empty base optimization the compiler is allowed to make an empty base class take no space in the derived class.
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