Zero size struct

问题:

I noticed that when compiled with GCC 4.6 sizeof(Foo) is 0 and sizeof(Bar) is 1. For some reason adding an empty array into an empty structure made its size 0. I thought that the sizes of both structures must be the same. What is going on here?

struct Foo {     char x[]; };  struct Bar {}; 

回答1:

Neither struct declaration is allowed by the C standard. 6.7.2.1 (8) in n1570:

And paragraph 18 in the same section:

As a special case, the last element of a structure with more than one named member

(emphasis mine)

Flexible array members are not allowed in C++, so the code is not valid C++ either.

As it is not valid code, the values reported by sizeof for these are meaningless.

回答2:

The sizeof operator never yields 0, even for an empty class.

as you can see here on msdn

furthermore the msdn is stating:

The sizeof operator cannot be used with the following operands:

• Functions. (However, sizeof can be applied to pointers to functions.)
• Bit fields.
• Undefined classes.
• The type void.
• Dynamically allocated arrays.
• External arrays.
• Incomplete types.
• Parenthesized names of incomplete types.

回答3:

C and C++ do not permit zero-sized objects.

gcc does support them as an extension. If you compile with the proper options, such as

gcc -std=c99 -pedantic -Wall -Wextra 

gcc will at least warn you about them; g++ has similar options.

文章来源: Zero size struct