Re: [Boost-users] Compress to buffer

Subject: Re: [Boost-users] Compress to buffer
From: Kenneth Adam Miller (kennethadammiller_at_[hidden])
Date: 2014-01-15 00:20:35

 ah that's a really good explanation, thanks so much. After staring at code
for hours, I start to question things like the laws of simply being.

On Tue, Jan 14, 2014 at 11:14 PM, Gavin Lambert <gavinl_at_[hidden]>wrote:

> On 15/01/2014 14:46, Quoth Kenneth Adam Miller:
>
>
>>              From what I had thought though, if my string was empty with
>>             0's,
>>
>>
>>         Right here's your mistake. string(30000,0) creates a non empty
>>         string. It's a string of 30k nulls.
>>
> [...]
>
>
>> Ok, so supposing I just use a vector; do I use vector<byte>? Is it more
>> efficient to use vector< word_size_of_computer > since any write to it
>> would obviously be faster or will the compiler catch this? Also, when I
>> grab all of the bytes from vector, does it automatically detect when the
>> end is? Like, one of the purposes of having 0's all through the string
>> was so that it was always terminated. In the case of vector, if I
>> push_back 3 times, and then I output, will I get 3 bytes?
>>
>
> You can still use a string if you want, but you're not initialising the
> pool correctly.
>
>   std::string *p1 = new std::string(30000, 0);
>
> This declaration creates a string that is 30k characters long, where all
> characters happen to be 0.  (It is legal for a string to contain embedded
> nulls.)  If you appended to this string, you would have a 30k+1 character
> string, and it *might* do a memory allocation.  This is obviously not what
> you want.
>
>   std::string *p2 = new std::string();
>   p2->reserve(30000);
>
> This declaration creates an empty string with an initial capacity of at
> least 30k characters; if you append a char to it then the string will be 1
> char long.  It should not allocate any further memory unless you try to
> append more characters than that.  (Though test it, as the STL is permitted
> to ignore this.)
>
> Also string will automatically ensure that it is always null-terminated
> when viewed from .c_str(), so you normally don't need to worry about
> filling with zeroes or anything like that, and you should not try to append
> a zero yourself unless you actually want that in the string data.
>
> But since you're talking about compressed data here, it will no longer be
> printable characters and could potentially contain a 0 as actual data.
>  Either vector or string can cope with this just fine (vector is typically
> more efficient at appending, but less efficient at copying) but either way
> you'll need to use the .size() to determine how much data is actually in
> there instead of relying on any termination.
>
>
>
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