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I'm currently writing a Photoshop PSD file parser to extract some specific metadata. Skipping over possibly big chunks of irrelevant data suggests some random access. And I need to support PSB as well as PSD - PSB has the same basic file structure but with bigger limits and with some size fields as 64-bits.

With the Photoshop file format specs. publicly available, this should be a trivial job, but I've never needed to worry about large files before, at least while using ifstream. I've always got away with ignoring streampos etc and just using size_t for file positions and offsets. I'm not getting away with that now.

I wasn't expecting that to be a big deal, and it isn't really, but it's turning out to be awkward and messy - I assume that means I'm fighting the library rather than doing what it expects.

First issue - there are three position/offset/size types...

  • streampos
  • streamoff
  • streamsize

This seems unnecessary to me. A position in a file is an offset from the start. The size of a file is also the offset from the start to the end. That's why we specify positions, offsets and sizes in vector with the same type - size_t.

In fact the easiest way I've found to determine the size of a file gives me a streampos - seek to the end of the file and query the position...

myfile.seekg (0, ios::end);
streampos myfilesize = myfile.tellg ();

The streampos type is also an instance of a template class fpos (which using GCC 4.7.0 seems to be 16 bytes - I haven't checked whether I'm building 32-bit or 64-bit code but either way that seems oversize). Why not just use an integer type?

Some annoyances...

streampos p;
p++;               //  Error - streampos doesn't have operator++
p = p + 1;         //  Warning - ambiguous (using 1ul or 1ull doesn't seem to fix it)

Those warning issues happen with comparisons and elsewhere too.

Basically, I'm getting a lot of irritating noise to work around these - lot's of casts to streampos or streamoff etc.

I have the feeling I'm being an idiot and missing something obvious, but my Google-skills are failing me - I haven't found an example that shows what I'm doing wrong.

So - what am I doing wrong? Is there an idiomatic style to doing random access with position calculations that avoids all this noise?

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到目前为止，我发现的是...

  • std::streampos不是一个简单的整数，因为它可能需要在没有 64 位整数类型作为标准的平台上支持大文件。事实上，它几乎可以肯定早于 PC 编译器对 64 位整数的广泛支持——Borland C++在 5.02 版中获得了它们，例如 1997 年的 IIRC——在第一个标准之前不久。long long直到 C++11 才成为标准。

  • std::streamsize并且std::streamoff都是整数类型的 typedef。但是，streamoff始终是有符号类型。这对于可能是向后的相对搜索很重要。我还没有找到关于必须签名还是未签名的任何要求streamsize——我猜它至少有可能是未签名的。

  • 从 中添加或减去时streampos，您应该始终添加或减去 a streamoff。在需要的地方进行类型转换，例如在添加或减去文字数字时。

  • 与往常一样，当有些东西乱七八糟时，把乱七八糟的东西包起来藏起来。

于 2013-05-16T20:28:19.227 回答