Re: change vector type?
* tf, on 04.10.2010 23:19:
Is there any way I could "change" a vector's type in O(1) time and memory?
I work with a lot of large 3D data, commonly stored raw or close to that
in files. It's common to read such data into a char or unsigned char
array, because of course it's binary data and we're talking bytes at
that level (i.e. istream::read requires it, of course).
Not quite. There's no requirement that your buffer is actually typed as an array
of char or unsigned char. It only needs to be POD. :-)
So, I think here's the point where you want to do a little reinterpret_cast.
Let istream::read think that it's storing into a char array.
After reading such data in, though, I want to process it somehow. I
could always just convert the data:
extern std::vector<char> data;
// external knowledge tells me this is 16bit, unsigned data.
std::vector<unsigned short> typed_data(data.size()/2);
for(size_t i=0; i < data.size()/2; ++i) {
// just pretend the endianness here is fine for a moment.
typed_data[i] = (data[2*i+1] << 8) | (data[2*i]);
}
I can also write my methods to accept unsigned short* and just
reinterpret_cast &data.at(0).
Both are pretty miserable. The first requires a dumb O(n) pass over the
data, and the latter means I can't take advantage of nice C++ features.
The real 'solution' that I tend to adopt is to have methods accept the
'2' above (and the 'external knowledge', typically as template
parameters) as a parameter and reconstruct the type as it performs the
operation. Of course, this basically means that all standard and even
3rd party algorithms are useless in my domain, which is pretty harsh.
I guess what I'd like is some way to say:
extern std::vector<char> data;
std::vector<mytype> typed_data(data.size() / sizeof(mytype));
typed_data.set_pointer(reinterpret_cast<mytype*>(&data[0]));
I'm aware of the lifetime issues I could get into w/ such a situation;
still, that's surely the lesser of two evils here.
Other thoughts?
The problem seems to be that you have allowed raw data into your app when you
already know at the app boundary what that raw data represents.
And a solution is to convert at the app boundary, which means, when reading a
file or writing to a file.
Then you only need to handle correctly typed data within the application.
Cheers & hth.,
- Alf
--
blog at <url: http://alfps.wordpress.com>
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