C++ sizeof空结构在C中为0，在C++中为1，为什么？
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sizeof empty structure is 0 in C and 1 in C++ why?
提问by Ninad Page
Possible Duplicates:
Empty class in C++
What is the size of an empty struct in C?
可能的重复项：
C++ 中的空类 C
中空结构的大小是多少？
I read somewhere that size of an empty struct in C++ is 1. So I thought of verifying it.
Unfortunately I saved it as a C file and used <stdio.h>header and I was surprised to see the output. It was 0.
我在某处读到 C++ 中空结构的大小是 1。所以我想验证它。不幸的是，我将它保存为 C 文件并使用了<stdio.h>标头，我很惊讶地看到输出。它是 0。
That means
这意味着
struct Empty {
};
int main(void)
{
printf("%d",(int)sizeof(Empty));
}
was printing 0 when compiled as a C file and 1 when compiled as a C++ file. I want to know the reason. I read that sizeof empty struct in c++ is not zero because if the size were 0 two objects of the class would have the same address which is not possible. Where am I wrong?
编译为 C 文件时打印 0，编译为 C++ 文件时打印 1。我想知道原因。我读到 c++ 中的 sizeof empty struct 不为零，因为如果大小为 0，则该类的两个对象将具有相同的地址，这是不可能的。我哪里错了？
回答by Prasoon Saurav
You cannot have an empty structure in C. It is a syntactic constraint violation. However gcc permits an empty structure in C as an extension. Furthermore the behaviour is undefinedif the structure does not have any named member because
C 中不能有空结构。这是违反语法约束的。但是 gcc 允许 C 中的空结构作为扩展。此外，如果结构没有任何命名成员，则行为未定义，因为
C99says :
C99说：
If the struct-declaration-list contains no named members, the behavior is undefined.
如果 struct-declaration-list 不包含命名成员，则行为未定义。
So
所以
struct Empty {}; //constraint violation
struct Empty {int :0 ;}; //no named member, the behaviour is undefined.
And yes size of an empty struct is C++ cannot be zero:)
是的，空结构的大小是 C++不能为零:)
回答by vog
There are several good reasons. Among others, this is to ensure that pointer arithmetics over pointers to that structure don't lead to an infinite loop. More information:
有几个很好的理由。其中，这是为了确保指向该结构的指针上的指针算术不会导致无限循环。更多信息：
http://bytes.com/topic/c/insights/660463-sizeof-empty-class-structure-1-a
http://bytes.com/topic/c/insights/660463-sizeof-empty-class-structure-1-a
回答by Necrolis
Here is a wonderful article describing why this occurs, and more pertinently, a (safe) way around it :)
这是一篇精彩的文章，描述了为什么会发生这种情况，更恰当地说，是一种（安全的）解决方法:)
