Cool template

#include <bits/stdc++.h>

#define DEBUG false

#define _(x) {if (DEBUG) cout << "[Line:" << __LINE__ << "] " <<  #x << " = " << x << " " << endl;}

const double E = 1e-8;
const double PI = acos(-1);

using namespace std;

int main() {
    // for fast cin and cout
    ios::sync_with_stdio(false);

    int a = 15;
    _(a)   // prints value of a when DEBUG is true
}

The #include <bits/stdc++.h> includes all the common includes for IO, strings, math, algorithms and STL template classes.

#include <iostream>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <climits>
#include <stack>
#include <queue>
#include <vector>
#include <set>
#include <map>
#include <list>
#include <cassert>
#include <unordered_map>