sizeof空结构在C中为0，在C++中为1，为什么? [英] sizeof empty structure is 0 in C and 1 in C++ why?

问题描述

可能的重复:
C++ 中的空类
C 中空结构的大小是多少?

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.是 0.

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.

这意味着

struct Empty {

};

int main(void)
{
  printf("%d",(int)sizeof(Empty));
}

在编译为 C 文件时打印 0，在编译为 C++ 文件时打印 1.我想知道原因.我读到 C++ 中的 sizeof 空结构不为零，因为如果大小为 0 那么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?

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 then two objects of the class would have the same address which is not possible. Where am I wrong?

推荐答案

C 中不能有空结构.这是违反语法约束的.然而，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

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 undefined if the structure does not have any named member because

C99 说:

如果结构声明列表不包含命名成员，则行为未定义.

If the struct-declaration-list contains no named members, the behavior is undefined.

所以

struct Empty {}; //constraint violation

struct Empty {int :0 ;}; //no named member, the behaviour is undefined.

是的，空结构的大小是 C++ 不能为零 :)

And yes size of an empty struct is C++ cannot be zero :)

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