error: constexpr variable 'nOff' must be initialized by a constant expression

Asked Dec 22 '17 at 13:20

Active Dec 22 '17 at 13:23

Viewed 633 times

clang compile fail, but not g++

/Users/xx/xxx/xxxx/common/src/memorychunkpn.hpp:438:28: error: constexpr variable 'nOff' must be initialized by a constant expression
            constexpr auto nOff = (size_t)(uintptr_t)(&((*((InnMemoryChunk *)(0))).Data[0]));

InnMemoryChunk:

  typedef struct _InnMemoryChunk{
  bool In;            // this to indicate the chunk is allocated in the memory pool
                      // actually we can compare pointer range to decide

  size_t NodeID;      // node in its memory pool, use it for offset calculation
  size_t PoolID;      //
  size_t BranchID;    //

  uint8_t Data[4];    // offset to the data field, size is always ok
}InnMemoryChunk;

Anyone could help please?

edited Dec 22 '17 at 13:23

Baum mit Augen

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asked Dec 22 '17 at 13:20

ryancheung

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That's undefined behavior anyways. Use the standard `offsetof`. – Baum mit Augen Dec 22 '17 at 13:24
You mean replacing `size_t` with `offsetof` ? – ryancheung Dec 22 '17 at 13:25
No, I mean replacing that entire pointer mess with `offsetof`. Check out the docs, it's quite straight forward. – Baum mit Augen Dec 22 '17 at 13:26
It's c++14 here, I included `cstddef` header, but still no offsetof found. – ryancheung Dec 22 '17 at 13:28
Should exist in said header. MCVE (in a new question) or it didn't happen. – Baum mit Augen Dec 22 '17 at 13:29
Also OT: `_InnMemoryChunk` is an implementation reserved identifier, you cannot use it. – Baum mit Augen Dec 22 '17 at 13:30
Yeah, It worked, `offsetof(InnMemoryChunk, Data)` pretty straightforward – ryancheung Dec 22 '17 at 13:31

error: constexpr variable 'nOff' must be initialized by a constant expression

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