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Thinking in C++ Vol 2 - Practical Programming
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A plethora of iterators

An iterator is an abstraction for genericity. It works with different types of containers without knowing the underlying structure of those containers. Most containers support iterators,[102] so you can say:


to produce the iterator types for a container. Every container has a begin( ) member function that produces an iterator indicating the beginning of the elements in the container, and an end( ) member function that produces an iterator which is the past-the-end marker of the container. If the container is const begin( ) and end( ) produce const iterators, which disallow changing the elements pointed to (because the appropriate operators are const).

All iterators can advance within their sequence (via operator++) and allow == and != comparisons. Thus, to move an iterator it forward without running it off the end, you say something like:

while(it != pastEnd) {
// Do something

where pastEnd is the past-the-end marker produced by the container s end( ) member function.

An iterator can be used to produce the container element that it is currently selecting via the dereferencing operator (operator*). This can take two forms. If it is an iterator traversing a container, and f( ) is a member function of the type of objects held in the container, you can say either:




Knowing this, you can create a template that works with any container. Here, the apply( ) function template calls a member function for every object in the container, using a pointer to member that is passed as an argument:

//: C07:Apply.cpp
// Using simple iteration.
#include <iostream>
#include <vector>
#include <iterator>
using namespace std;
template<class Cont, class PtrMemFun>
void apply(Cont& c, PtrMemFun f) {
typename Cont::iterator it = c.begin();
while(it != c.end()) {
((*it).*f)(); // Alternate form
class Z {
int i;
Z(int ii) : i(ii) {}
void g() { ++i; }
friend ostream& operator<<(ostream& os, const Z& z) {
return os << z.i;
int main() {
ostream_iterator<Z> out(cout, " ");
vector<Z> vz;
for(int i = 0; i < 10; i++)
copy(vz.begin(), vz.end(), out);
cout << endl;
apply(vz, &Z::g);
copy(vz.begin(), vz.end(), out);
} ///:~

You can t use operator-> here because the resulting statement would be:


which attempts to use the iterator s operator->*, which is not provided by the iterator classes.[103]

It is much easier to use either for_each( ) or transform( ) to apply functions to sequences, as you saw in the previous chapter.

Thinking in C++ Vol 2 - Practical Programming
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire