Re: help for back_inserter and end()
On 26 Mar, 15:14, "Jess" <w...@hotmail.com> wrote:
Yes, the back_inserter is used when inserting items at the end of a
container (hence the name). You can not use v.end() instead, it is of
the wrong type (not an output iterator).
Can I generalize this to say if we use an output iterator to copy/
assign/add elements, then the size of the container will be increased
automatically?
Nearly. If you use an *insert* iterator then the size of the container
will be increased automatically. An insert iterator is just one kind
of output iterator.
On the other hand, input iterator should only be used
for reading, is this right?
Again no. That statement is unnecessarily restrictive. Here's a
variation of your program. You originally passed back_inserter(v) to
copy as the third argument. You've already seen that changing that to
v.end() gives undefined behaviour. But here I have changed it to
v.begin() (which is an input iterator)
#include<iostream>
#include<vector>
#include<iterator>
#include<algorithm>
using namespace std;
int main(){
vector<int> v;
v.push_back(0);
v.push_back(1);
v.push_back(2);
vector<int> w;
w.push_back(3);
w.push_back(4);
copy(w.begin(),w.end(),v.begin());
}
After the call to copy, v contains the sequence [3, 4, 2]. The first
two elements have been copied from the vector w and overwritten what
was there before. The third element is untouched by the copy. This
behaviour is well defined and standard, and if you want to copy into
some existing space and overwrite the pre-existing data, this is a
perfectly acceptable way of doing so.
Note that it only works if the size of w <= the size of v. If w had
more elements than v, the call to copy would attempt to write past the
end of v - undefined behaviour again.
Have you got this book? It lives up to its title as both a tutorial
and reference extremely well.
http://www.josuttis.com/libbook/
Gavin Deane