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See this page as a slide show

CS253 Begin And End

Half-open interval

• .begin() returns an iterator that “points” to the first element
• .end() returns an iterator that “points” one past the last element
• They form a half-open interval.
• Remember, iterators are not pointers.

.begin() and .end()

vector<int> v = {11, 22, 33, 44, 55};
for (vector<int>::iterator it = v.begin(); it != v.end(); ++it)
    cout << *it << ' ';

11 22 33 44 55 

or:

vector<int> v = {11, 22, 33, 44, 55};
for (auto it = v.begin(); it != v.end(); ++it)
    cout << *it << ' ';

11 22 33 44 55 

++it is probably faster than it++.

const

Why does this fail?

class Foo {
    int sum() const {
        int total = 0;
        for (vector<int>::iterator it = data.begin(); it != data.end(); ++it)
            total += *it;
        return total;
    }
    vector<int> data;
};

c.cc:4: error: conversion from '__normal_iterator<const int*,[...]>' to 
   non-scalar type '__normal_iterator<int*,[...]>' requested

const

• .sum() is a const method.
• Therefore, inside .sum(), data is a const vector<int>.
  • Let that sink in for a moment.
  • How is a const method implemented, anyway?
  • It’s not that the compiler checks for changes to member data inside a const method.
  • Instead, the compiler simply regards all data members as const when compiling a const method.
• Therefore, data.begin() and data.end() return iterators of type const_iterator, not type iterator.
  • They’re overloaded methods!
• You can’t assign const_iterator to iterator.

Restate the problem

This is the same problem:

const vector<int> v = {11, 22, 33, 44, 55};
for (vector<int>::iterator it = v.begin(); it != v.end(); ++it)
    cout << *it << ' ';

c.cc:2: error: conversion from '__normal_iterator<const int*,[...]>' to 
   non-scalar type '__normal_iterator<int*,[...]>' requested

Solution #1

One solution—use the correct type:

const vector<int> v = {11, 22, 33, 44, 55};
for (vector<int>::const_iterator it = v.begin(); it != v.end(); ++it)
    cout << *it << ' ';

11 22 33 44 55 

Solution #2

A better solution—let auto figure it out:

const vector<int> v = {11, 22, 33, 44, 55};
for (auto it = v.begin(); it != v.end(); ++it)
    cout << *it << ' ';

11 22 33 44 55 

Solution #3

The best solution—avoid all of this:

const vector<int> v = {11, 22, 33, 44, 55};
for (auto val : v)
    cout << val << ' ';

11 22 33 44 55 

.cbegin() and .cend()

• .begin() is an overloaded method. It returns a const_iterator if the object is const, iterator otherwise.
• .end() is an overloaded method. It returns a const_iterator if the object is const, iterator otherwise.
• .cbegin(): like .begin(), but always returns const_iterator
• .cend(): like .end(), but always returns const_iterator
  
  

  vector<int> v = {11, 22, 33, 44, 55};
  for (auto it = v.cbegin(); it != v.cend(); ++it)
      cout << *it << ' ';
  

  11 22 33 44 55 

.rbegin() and .rend()

• .rbegin(): like .begin(), but goes the other way
• .rend(): like .end(), but goes the other way
• .crbegin(): like .cbegin(), but goes the other way
• .crend(): like .cend(), but goes the other way
  
  

  vector<int> v = {11, 22, 33, 44, 55};
  for (auto it = v.rbegin(); it != v.rend(); ++it)
      cout << *it << ' ';
  

  55 44 33 22 11 

vector<int> v = {11, 22, 33, 44, 55};
for (auto it = v.crbegin(); it != v.crend(); ++it)
    cout << *it << ' ';

55 44 33 22 11 

Don’t get too excited

• Not all containers have the reversed versions.
  • For example, forward_list, as a singly-linked list, doesn’t have reverse iterators.
• For some containers, iterator and const_iterator are the same.
  • A set has inherently-constant iterators, because it’s in order. If you could change the values via iterators, then it wouldn’t be in order any more.
  • An unordered_set, a hash, has its own special order, and changing the values in place would be bad.

Plain old data types:

How about old C-style data?

int a[] = {11, 22, 33, 44, 55};
for (auto it = a.begin(); it != a.end(); ++it)
    cout << *it << ' ';

c.cc:2: error: request for member 'begin' in 'a', which is of non-class type 
   'int [5]'

That failed miserably. Perhaps it’s because arrays are not objects.

Free functions

Fortunately, there exist begin() and end() free functions (not methods).

int a[] = {11, 22, 33, 44, 55};
for (auto it = begin(a); it != end(a); ++it)
    cout << *it << ' ';

11 22 33 44 55 

These work for objects, as well:

set<int> s = {11, 22, 33, 44, 55};
for (auto it = begin(s); it != end(s); ++it)
    cout << *it << ' ';

11 22 33 44 55 

What use is that? Generality for the sake of generality?

for loop

Consider any for loop:

forward_list<int> fl = {11,22,33,44,55};
for (double v : fl)
    cout << v << ' ';

11 22 33 44 55 

The compiler turns this into:

forward_list<int> fl = {11,22,33,44,55};
const auto toofar = end(fl);
for (auto it = begin(fl); it != toofar; ++it) {
    double v = *it;
    cout << v << ' ';
}

11 22 33 44 55 

Which works for any container type, even C-style arrays.

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