Re: The most efficient way for fill a vector from an array?
towi <towitowi@gmail.com> wrote:
I wonder, what is the most efficient STL way to move the values from
an array into a vector of the same kind?
* "move" means, that I dont care about the array afterwards.
* It should be a platform independent way, probably a way how it could
be done
with general C++ features and STL methods if the implementors chose
to implement it.
int arr[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
std::vector<int> vec(arr, arr + 10);
My general idea would be that there might be something like
(pseudocode):
std::vector<int> vec;
int arr[10] = { 1,2,3, 2,3,4, 3,4,5, 10} ;
std::move( vec.begin(), vec.end(), arr, arr+10); // vec := arr
I am trying to find the best line[s] for my invended "std::move()". I
imagined, there might be something in the STL that would have allowed
by design their implementors to do some very careful pointer-magic, if
they choose to, like that pseudocode specialization here:
template<>
move<T>(vector<T>::interator dst_start, vector<T>::iterator
dst_stop, T* src_start, T* src_stop)
{
if(//dst_start and dst_stop cover the whole of the dst//) {
//set the internal data-pointer of trg to src_start//
} else {
std::swap_ranges(...);
}
}
The above is simply not possible. Even if the above worked as you
wanted, what would happen to the vector if I then called push_back? The
vector would try to delete[] the memory it has, but that is an error.
Maybe there is a general performance-cheap solution I can not see -- a
way the designers of the STL or C++ thought, they leave the library or
language implementors the "door" open for good performance tweaking,
if they set their mind to it.
You could write your own vector like class that was able to use memory
that it didn't own...
The hard fact reason for my question is, that I want to read a file
that I could do with a single read()-call into an array[], but I want
it to go into a vector<>, instead. And because it's huge, I want to do
it without twice the memory and without an unneccessary copy.
That's simple, I do it all the time:
// assume the following exists:
int read(unsigned char* buffer, int size);
// reads contents of a file into buffer, up to size bytes.
vector<unsigned char> buffer(/*size of file*/);
read(&buffer[0], buffer.size());
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