C++ compiler upgrades on infoarena

We now have the $--std=c++0x$ compiler option enabled on infoarena. We also updated our g++ compiler to version 4.8.

We now have the $--std=c++0x$ compiler option enabled on infoarena. We also updated our $g++$ compiler to $4.8$.

h2. What does it mean?

$C++$ users can now use a bunch of cool features, some of which are described below briefly. Keep in mind that these features are *not* yet available at OJI, ONI, etc., so don't use them at any of these competitions unless they are allowed explicitly by the regulations.

$C++$ users can now use a bunch of cool features, some of which are briefly described below. Keep in mind that these features are *not* yet available at $OJI$, $ONI$, etc., so don't use them at any of these competitions unless they are allowed explicitly by the regulations.

h3. auto

h3. 1. auto

You can now let the compiler guess the type of some variable:

You can now let the compiler infer the type of your variables with $auto$:

== code(cpp) |
auto a = 45;

auto c = vector<int>(10);
==

$auto$ can also be used with $const auto$ or $const auto&$. It works in the same way other types would work. In most cases, $auto$ cannot be used in function signatures.

$auto$ can also be used with $const auto$ or $const auto&$. In most cases, $auto$ cannot be used in function signatures.

h3.

h3. 2. range-based for loops
 
In $C++11$, you can write less code to iterate over every element in a list of elements:
 
== code(cpp) |
int array[5] = {1, 2, 3, 4, 5};
for (int x: array) {
  cout << x << endl;
}
==
 
If you want to modify the elements in the list, you need to get a reference to the current element:
 
== code(cpp) |
double array[5] = {1.5, 2.7, 3.9};
for (auto &x: array) {
  x *= 2;
}
==
 
Note: This code compiles without using $&$, but the original array is not modified unless a reference is used.
 
h3. 3. initializer lists
 
Simple one-line initializations with lists of constant values:
 
== code(cpp) |
vector<pair<int, int>> dirs = {{-1, 0}, {0, -1}, {1, 0}, {0, 1}};
==
 
Note that in $C++11$ you no longer need to introduce a space between closing right angle brackets $(>>)$.
 
h3. 4. unordered_set, unordered_map
 
These are hash-based implementations of the well known $set$ and $map$ containers; the average time complexity on most operations is $O(1)$. (These containers were previously available on infoarena under $tr1$, but only a small fraction of users were actually using them.)
 
== code(cpp) |
#include <unordered_map>
...
using namespace std;
...
unordered_map<string, int> age = {
  {"john", 18}, {"mary", 21}, {"anna", 19}
};
cout << "Anna is " << age["anna"] << " years old." << endl;
==
 
Using STL vectors, pairs or user defined objects as keys is a little trickier because you also need to provide a hash function.
 
== code(cpp) |
#include <unordered_set>
#include <vector>
 
using namespace std;
 
// Some user defined magic constants used for hashing.
const int P = 666013;
 
// Courtesy of Adrian Budau
struct myhash {
  size_t operator()(const vector<int>& v) const {
    size_t value = 1;
    for (auto x: v) {
      value = value * P + hash<int>()(x);
    }
    return value;
  }
};
...
unordered_set<vector<int>, myhash> s;
vector<int> v = {1, 2, 3};
s.insert(v);
==
 
You don't need to worry about collisions, $unordered_set$ and $unordered_map$ will take care of them for you.
 
h3. 5. lambda functions
 
You can now define anonymous functions to write less code. A common application is sorting according to multiple criteria.
 
== code(cpp) |
#include <algorithm>
 
using namespace std;
 
...
vector<int> a = {5, 3, 1, 3};
vector<int> b = {6, 1, 7, 2};
vector<int> ind = {0, 1, 2, 3};
 
sort(ind.begin(), ind.end(), [&a, &b](int x, int y) -> bool {
  return a[x] < a[y] || (a[x] == a[y] && b[x] < b[y]);
});
 
for (int i: ind) {
  cout << a[i] << " " << b[i] << endl;
}
==
 
(Often, a cleaner alternative to using lambda functions for sorting is to define a class holding all the data for each entry and to implement the $<$ operator.)
 
_Let us know in the comments section what are the $C++11$ features that you use for programming contests. Code snippets illustrating your favorite tricks are welcome._

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