Which headers in the C++ standard library are guaranteed to include another header?

Question

The C++ standard library headers may include each other in unspecified ways, so programmers generally shouldn't depend on one header including another. In a few cases, however, a header is guaranteed to include another header, or make available certain functions that would otherwise require inclusion of another header. What are those cases?

Answer 1

This answer ignores C headers - both the <meow.h> and <cmeow> ones. Of the C++ library headers (all references are to N4659):

<initializer_list> is guaranteed to be included by:

• <utility> (§23.2.1 [utility.syn])
• <string> (§24.3.1 [string.syn])
• <array> (§26.3.2 [array.syn])
• <deque> (§26.3.3 [deque.syn])
• <forward_list> (§26.3.4 [forward_list.syn])
• <list> (§26.3.5 [list.syn])
• <vector> (§26.3.6 [vector.syn])
• <map> (§26.4.2 [associative.map.syn])
• <set> (§26.4.3 [associative.set.syn])
• <unordered_map> (§26.5.2 [unord.map.syn])
• <unordered_set> (§26.5.3 [unord.set.syn])
• <queue> (§26.6.2 [queue.syn])
• <stack> (§26.6.3 [stack.syn])
• <algorithm> (§28.2 [algorithms.syn])
• <random> (§29.6.2 [rand.synopsis])
• <valarray> (§29.7.1 [valarray.syn])
• <regex> (§31.4 [re.syn])

<iostream> is guaranteed to include <ios>, <streambuf>, <istream>, and <ostream> (§30.4.1 [iostream.syn]).

<ios> is guaranteed to include <iosfwd> (§30.5.1 [ios.syn]).

<bitset> is guaranteed to include <string> and <iosfwd> (§23.9.1 [bitset.syn]).

The free function templates std::begin, std::end, the C++14 c-, r-, and cr- versions, and the C++17 free function templates std::size, std::empty and std::data nominally reside in <iterator>, but are also available if any of the following headers is included: <array>, <deque>, <forward_list>, <list>, <map>, <regex>, <set>, <string>, <unordered_map>, <unordered_set>, and <vector> (§27.7 [iterator.range], §27.8 [iterator.container]).

When <string_view> is included, the *begin and *end functions, and the two generic std::swap overloads defined in [utility.swap] (swap(T&, T&) and swap(T (&a)[N], T (&b)[N])) are guaranteed to be available. size/empty/data, however, are not. (§24.4.1 [string.view.synop]).

Answer 2

Here are the mandatory includes for C++20, taken from N4860.

compare is included in:

• array
• chrono
• coroutine
• deque
• filesystem
• forward_list
• iterator
• list
• map
• memory
• optional
• queue
• ranges
• regex
• set
• stack
• string
• string_view
• system_error
• thread
• tuple
• typeindex
• unordered_map
• unordered_set
• utility
• variant
• vector

initializer_list is included in:

• algorithm
• array
• deque
• forward_list
• list
• map
• queue
• random
• ranges
• regex
• set
• stack
• string
• thread
• unordered_map
• unordered_set
• utility
• valarray
• vector

string is included in:

• bitset

iosfwd is included in:

• bitset
• ios

concepts is included in:

• iterator

iterator is included in:

• ranges

ios, streambuf, istream are included in:

• iostream

ostream is included in:

• iostream
• syncstream

cinttypes is included in:

• cstdint