优化C++中的成员变量顺序

Please note that there are some gotchas here. If you are using CPU-based atomic operations (which the atomic types in C++0x generally will), then you may find that the CPU locks the entire cache line in order to lock the field. Then, if you have several atomic fields close together, with different threads running on different cores and operating on different fields at the same time, you will find that all those atomic operations are serialised because they all lock the same memory location even though they're operating on different fields. Had they been operating on different cache lines then they would have worked in parallel, and run faster. In fact, as Glen (via Herb Sutter) points out in his answer, on a coherent-cache architecture this happens even without atomic operations, and can utterly ruin your day. So locality of reference is not necessarily a good thing where multiple cores are involved, even if they share cache. You can expect it to be, on grounds that cache misses usually are a source of lost speed, but be horribly wrong in your particular case.

现在，除了区分常用字段和不常用字段之外，对象越小，它占用的内存(因此缓存越少)就越少.这几乎是一个好消息，至少在您没有激烈争用的情况下.对象的大小取决于其中的字段，以及必须在字段之间插入的任何填充，以确保它们与架构正确对齐.C++(有时)根据字段的声明顺序对字段必须出现在对象中的顺序进行了限制.这是为了使低级编程更容易.因此，如果您的对象包含:

struct some_stuff {
    double d;   // I expect double is 64bit IEEE, it might not be
    uint64_t l; // 8 bytes, could be 8-aligned or 4-aligned, I don't know
    uint32_t i; // 4 bytes, usually 4-aligned
    int32_t j;  // same
    short s;    // usually 2 bytes, could be 2-aligned or unaligned, I don't know
    char c[4];  // array 4 chars, 4 bytes big but "never" needs 4-alignment
    char d;     // 1 byte, any alignment
};