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)"
这篇关于C ++编译器可以重新排序结构中的元素的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!