Re: Using a std::vector as a variable-length receive buffer
On Aug 12, 12:31 am, irotas <goo...@irotas.net> wrote:
Hi folks,
I'd like to use a std::vector as a buffer for the standard UNIX
sockets recv() function. However, I need the buffer to be dynamically
resizable, because the incoming messages will be of arbitrary length.
The protocol I'm using is fairly typical. Each message consists of two
packets; the first is a fixed 4-byte packet that indicates the length
of the second packet, the second being an arbitrary data packet.
I made the std::vector a class member variable so that the allocated
memory would be reusable between receptions. The buffer gets copied
immediately after every message is received, so that the next receive
can use the same buffer.
So here's the question. When I read that 4-byte length field, do I
need to use a 'resize()' on the std::vector to allocate space, or will
a 'reserve()' suffice? I don't really care if the std::vector knows
how many elements it contains, because I have that knowledge
externally.
I was reading the following two pages:http://www.cplusplus.com/reference/stl/vector/reserve.htmlhttp://www.cplusplus.com/reference/stl/vector/resize.html
According to those pages, 'reserve()' is O(1), and 'resize()' is O(N).
Therefore, if 'reserve()' will suffice here, it is preferable.
Basically what the question boils down to is whether or not
'reserve()' allocates its memory at the address that will point to the
0'th element after 'reserve()' returns.
Thanks,
Adam
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well it would, but it would be a very bad design.First there must be
at least one element in the vector, in order to be able copy the
buffer over it,secondly you cant use any STL algorithm or member
function. I vision something like this;
char* sizebuf[5]="";
std::vector<char> vec(1);
recv (s,sizebuf,4,flags); //assuming you have 4 bytes to read
int size=boost::lexical_cast<int>(sizebuf);
vec.reserve(size);
int len = recv(s,&vec[0],size,flags); //assuming len ==size
then what? you cant do anything with that vector it has size of 1,
best you can do is memcpy'ing size bytes somewhere else
so why have you needed vector in the first place?
hurcan
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