C++ 编译器(特别是 g++)可以重新排序结构的内部元素吗?
我看到一些奇怪的行为,我的结构包含以下内容:
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 和 someLongArray 的顺序似乎颠倒了(即 someLongArray[] 中的元素出现在 someUnsignedLong 之后,someUnsignedLongArray[] 的元素出现在 someLong 之后)。这可能吗??
谢谢
更新: 根据要求,我正在使用以下内容写出结构:
int fd = open(fspec,O_RDWR|O_CREAT|O_TRUNC,0666);
int writeRes = write(fd,(char *)&someStruct,sizeof(SomeStruct));
为了完整起见,这里是完整的结构:
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;
};
最佳答案
它通常不能重新排序元素,不。
如果有一个访问说明符将它们分开,则异常(exception):
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 的存储位置
和 f
.
要记住的另一件事是编译器可以根据需要插入尽可能多的填充,即使它不重新排序任何内容。
这是标准的相关部分:
第 9.2.12 节:
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)"
关于c++ - C++ 编译器可以重新排序结构中的元素吗,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/916600/