C ++编译器可以重新排序结构中的元素 [英] Can a C++ compiler re-order elements in a struct

问题描述

C ++编译器（特别是g ++）可以重新排序结构的内部元素吗？

Can a C++ compiler (specifically g++) re-order the internal elements of a struct?

我看到一些奇怪的行为，如下所示：

I'm seeing some strange behaviour where I have a structure that contains something like the following:

Struct SomeStruct{
   ...
   ...
   long someLong;
   long someLongArray[25];
   unsigned long someUnsignedLong;
   unsigned long someUnsignedLongArray[8];
   unsigned long int someUnsignedLongInt;
   ...
   ...
};

当我将输出写入文件时， someUnsignedLongArray （即 someLongArray [] 中的元素出现在 someUnsignedLong 之后， someUnsignedLongArray [] 出现在 someLong 之后）。是否可能？

When I write output this to file, the order of someUnsignedLongArray and someLongArray seem to be reversed (i.e. the elements in someLongArray[] appear after someUnsignedLong and the elements of someUnsignedLongArray[] appear after someLong). Is this possible??

感谢

---

更新：
根据请求，我使用以下内容写出结构：

Update: As requested, I am writing out the structure using the following:

int fd = open(fspec,O_RDWR|O_CREAT|O_TRUNC,0666);
int writeRes =  write(fd,(char *)&someStruct,sizeof(SomeStruct));

为了完整性，这里是完整的结构：

For completeness, here is the full struct:

struct SomeStruct{
byte someByte;
byte someByteArray[6];
char someChar;
char someCharArray[5];
char someCharArrayArray[3][5];
short someShort;
signed short someShortArray[2];
unsigned short someUnsignedShort;
unsigned short someUnsignedShortArray[8];
int someInt;
int someIntArray[3];
int someIntArrayArrayArrayArray[4][3][2][6];
int *pSomeInt;
unsigned int someUnsignedInt;
unsigned int someUnsignedIntArray[9];
long someLong;
long someLongArray[25];
unsigned long someUnsignedLong;
unsigned long someUnsignedLongArray[8];
unsigned long int someUnsignedLongInt;
long long someLongLong;
long long someLongLongArray[5];
bool someBool;
bool someBoolArray[3];
unsigned long long someUnsignedLongLong;
unsigned long long someUnsignedLongLongArray[5];
unsigned long long someUnsignedLongLongArrayArray[5][2];
unsigned long long int *pSomeUnsignedLongLongInt;
};

推荐答案

通常不能对元素重新排序。

It normally can't reorder elements, no.

如果存在将它们分隔开的访问说明符，则例外：

An exception is if there's an access specifier separating them:

struct Foo {    
  A a;
  B b;
  C c;
private:
  D d;
  E e;
  F f;
};

a，b和c保证按此顺序存储，d，e和f保证按顺序存储。但是不能保证a，b和c相对于d，e和f的存储位置。

a, b and c are guaranteed to be stored in this order, and d, e and f are guaranteed to be stored in order. But there is no guarantees about where a, b and c are stored relative to d, e and f.

另一件要记住的事情是编译器可以插入

Another thing to keep in mind is that the compiler can insert as much padding as it likes, even if it doesn't reorder anything.

以下是标准的相关部分：

Here's the relevant part of the standard:

第9.2.12节：

宣布没有$ b的
（非联合）类的非静态数据成员$ b intervening access-specifier是分配的
，以便以后的成员在类
对象中有
个更高的地址。由
访问限定符分隔的
非静态数据成员的分配顺序未指定
（11.1）

Nonstatic data members of a (non-union) class declared without an intervening access-specifier are allocated so that later members have higher addresses within a class object. The order of allocation of nonstatic data members separated by an access-specifier is unspecified (11.1)"

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