CODEWITHSUNDEEP
c++cstructsizeof
duong_dajgja
duong_dajgja

Reputation: 4276

Difference between sizeof(empty struct) and sizeof(struct with empty array)?

I have two structs defined as follows:

struct EmptyStruct{

};

struct StructEmptyArr{
    int arr[0];
};

int main(void){
    printf("sizeof(EmptyStruct) = %ld\n", sizeof(EmptyStruct));
    printf("sizeof(StructEmptyArr) = %ld\n", sizeof(StructEmptyArr));

    return 0;
}

Compiled with gcc (g++) 4.8.4 on Ubuntu 14.04, x64.

Output (for both gcc and g++):

sizeof(EmptyStruct) = 1
sizeof(StructEmptyArr) = 0

I can understand why sizeof(EmptyStruct) equals to 1 but cannot understand why sizeof(StructEmptyArr) equals to 0. Why are there differences between two?

Upvotes: 35

Views: 3232

Answers (2)

haccks
haccks

Reputation: 106012

In C, the behavior of a program is undefined if a struct is defined without any named member.

C11-§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.

GCC allows an empty struct as an extension and its size will be 0.

For C++, the standard doesn't allow an object of size 0 and therefore sizof(EmptyStruct) returns a value of 1.
Arrays of zero length are not supported by standard C++¹, but are supported as an extension by GNU and the sizeof operator will return 0 if applied.

1. § 8.5.1-footnote 107) C++ does not have zero length arrays.

Upvotes: 40

user5368518
user5368518

Reputation:

https://gcc.gnu.org/onlinedocs/gcc/Empty-Structures.html

G++ treats empty structures as if they had a single member of type char.

https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html

Zero-length arrays are allowed in GNU C. They are very useful as the last element of a structure that is really a header for a variable-length object.

Upvotes: 5

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