Built-in

(For the sake of brevity, this article leaves out details about how these data structures are implemented—for example, dynamic array doubling for vector or balanced binary search trees for set. Feel free to look these up yourself if you’re curious!)

As you may know already, most languages come with a standard library with helpful utilities. This includes C++. C++ without its standard library is actually a very minimal language. For example, we wouldn’t have any way to take in input without <iostream>!

Is there really no way?

There is a way, but it requires invoking system calls manually, which is tedious and far beyond what you need for IOI. Low-level programming is outside the scope of the contest, so you do not need to learn this for anything practical.

Anyway, every C++ program you’ll write will probably look somewhat like this:

#include <bits/stdc++.h>

using namespace std;

int main() {
  
}

Let’s examine the line #include <bits/stdc++.h>. It uses an include directive. The header bits/stdc++.h itself consists of a list of all the standard C++ headers.

bits/stdc++.h

// C++ includes used for precompiling -*- C++ -*-

// Copyright (C) 2003-2025 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @file stdc++.h
 *  This is an implementation file for a precompiled header.
 */

// 17.4.1.2 Headers

// C
#ifndef _GLIBCXX_NO_ASSERT
#include <cassert>
#endif
#include <cctype>
#include <cfloat>
#include <climits>
#include <csetjmp>
#include <cstdarg>
#include <cstddef>
#include <cstdlib>

#if __cplusplus >= 201103L
#include <cstdint>
#if __cplusplus < 201703L
#include <ciso646>
#endif
#endif

// C++
// #include <bitset>
// #include <complex>
#include <algorithm>
#include <bitset>
#include <functional>
#include <iterator>
#include <limits>
#include <memory>
#include <new>
#include <numeric>
#include <typeinfo>
#include <utility>

#if __cplusplus >= 201103L
#include <array>
#include <atomic>
#include <initializer_list>
#include <ratio>
#include <scoped_allocator>
#include <tuple>
#include <typeindex>
#include <type_traits>
#endif

#if __cplusplus >= 201402L
#endif

#if __cplusplus >= 201703L
#include <any>
// #include <execution>
#include <optional>
#include <variant>
#include <string_view>
#endif

#if __cplusplus >= 202002L
#include <bit>
#include <compare>
#include <concepts>
#include <numbers>
#include <ranges>
#include <span>
#include <source_location>
#include <version>
#if __cpp_impl_coroutine
# include <coroutine>
#endif
#endif

#if __cplusplus > 202002L
#include <expected>
#include <stdatomic.h>
#endif

#if _GLIBCXX_HOSTED
// C
#ifndef _GLIBCXX_NO_ASSERT
#include <cassert>
#endif
#include <cctype>
#include <cerrno>
#include <cfloat>
#include <climits>
#include <clocale>
#include <cmath>
#include <csetjmp>
#include <csignal>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <cwchar>
#include <cwctype>

#if __cplusplus >= 201103L
#include <cfenv>
#include <cinttypes>
#include <cstdint>
#include <cuchar>
#if __cplusplus < 201703L
#include <ccomplex>
#include <cstdalign>
#include <cstdbool>
#include <ctgmath>
#endif
#endif

// C++
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>

#if __cplusplus >= 201103L
#include <array>
#include <atomic>
#include <chrono>
#include <codecvt>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <tuple>
#include <typeindex>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#endif

#if __cplusplus >= 201402L
#include <shared_mutex>
#endif

#if __cplusplus >= 201703L
#include <any>
#include <charconv>
// #include <execution>
#include <filesystem>
#include <optional>
#include <memory_resource>
#include <variant>
#endif

#if __cplusplus >= 202002L
#include <barrier>
#include <bit>
#include <compare>
#include <concepts>
#include <format>
#include <latch>
#include <numbers>
#include <ranges>
#include <span>
#include <stop_token>
#include <semaphore>
#include <source_location>
#include <syncstream>
#include <version>
#endif

#if __cplusplus > 202002L
#include <expected>
#include <flat_map>
#include <flat_set>
#include <generator>
#include <print>
#include <spanstream>
#include <stacktrace>
#include <stdatomic.h>
#include <stdfloat>
#endif

#if __cplusplus > 202302L
#include <text_encoding>
#include <stdbit.h>
#include <stdckdint.h>
#endif

#endif // HOSTED

The C++ standard library is large, but we only need a small portion of it. In this article, we will focus on the data structures it provides.