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A Tour of C++ for experienced programmers, as if C++26 is the only version that ever existed.
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123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230231232233234235236237238239240241242243244245246247248249250251252253254255256257258259260261262263264265266267268269270271272273274275276277278279280281282283284285286287288289290291292293294295296297298299300301302303304305306307308309310311312313314315316317318319320321322323324325326327328329330331332333334335336337338339340341342343344345346347348349350351352353354355356357358359360361362363364365366<!DOCTYPE HTML><html lang="en" class="light sidebar-visible" dir="ltr"> <head> <!-- Book generated using mdBook --> <meta charset="UTF-8"> <title>Containers - A Tour of C++26</title>
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<h1 class="menu-title">A Tour of C++26</h1>
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<div id="mdbook-content" class="content"> <main> <h1 id="containers"><a class="header" href="#containers">Containers</a></h1><p>Containers provide storage for collections of objects. The Standard Library supplies a family of containers that differ in allocation strategy, ordering guarantees, and performance characteristics. Choose a container that matches the required usage pattern. The default choice is <code>std::vector</code> because it stores elements contiguously, which gives good cache locality and enables constant‑time random access.</p><h2 id="stdvector-the-default-container"><a class="header" href="#stdvector-the-default-container"><code>std::vector</code>: the default container</a></h2><p><code>std::vector<T></code> owns a dynamically allocated array of <code>T</code>. Elements are stored next to each other in memory. The implementation allocates a capacity that can be greater than the current size. When the size grows past the capacity, the vector allocates a new, greater block, copies or moves existing elements, and frees the old block. This reallocation invalidates all pointers, references, and iterators that refer to the previous storage.</p><p>Because reallocation can be expensive, two techniques reduce its impact:</p><ul><li><strong>Reserve</strong>: call <code>reserve(10)</code> before inserting ten or greater number of elements. The vector allocates space for at least <code>n</code> elements up front, so later <code>push_back</code> or <code>emplace_back</code> calls cannot trigger a reallocation.</li><li><strong>Emplace</strong>: <code>emplace_back(args…)</code> constructs a new element directly in the storage. The arguments are forwarded to the element’s constructor, avoiding an extra copy or move.</li></ul><p>Both techniques tie back to earlier chapters. Chapter 07 described the lifetime‑safety analysis that flags dangling pointers after a reallocation. Reserving ahead of time eliminates that risk for the most common case. Chapter 08 explained why passing arguments by value or reference matters. <code>emplace_back</code> constructs in place, satisfying the principle of “construct where you use”.</p><p>The vector grows geometrically, typically doubling capacity. This yields amortised constant‑time <code>push_back</code>. Use <code>reserve</code> when the final size is known to avoid intermediate reallocations and iterator invalidation.</p><p>Erasing an element from the middle of a <code>vector</code> shifts every later element down by one, so removal is <code>O(n)</code> in the number of elements after the erased position. <code>erase</code> and the C++20 <code>erase</code>/<code>erase_if</code> free functions return the new logical end. When you only need to drop the last few elements, <code>pop_back</code> or <code>resize</code> is cheaper.</p><p><code>std::vector<bool></code> is a partial specialization that packs bits, so <code>operator[]</code> returns a proxy object rather than a <code>bool&</code>. This breaks generic code that expects a real reference, and it is the one standard container that is not a true container. Prefer <code>std::vector<char></code> or <code>std::bitset</code> when you need a sequence of individual bit values with normal reference semantics.</p><pre><code class="language-cpp">#include <vector>#include <print>
int main() { std::vector<int> v; v.reserve(10); v.emplace_back(1); v.emplace_back(2); v.emplace_back(3); std::println("vector size: {}", v.size()); return 0;}</code></pre><h2 id="stdarray-fixed-size-on-the-stack"><a class="header" href="#stdarray-fixed-size-on-the-stack"><code>std::array</code>: fixed size on the stack</a></h2><p><code>std::array<T, N></code> stores exactly <code>N</code> objects of type <code>T</code>. The size is known at compile time, and the array lives in the surrounding object’s storage, which is the stack for local variables. No dynamic allocation occurs.</p><p>A classic C array (<code>T a[N]</code>) decays to a pointer when passed to a function, losing its size information. <code>std::array</code> retains its size via the member function <code>size()</code>. It also provides standard container interfaces (<code>begin()</code>, <code>end()</code>, <code>operator[]</code>), so generic algorithms work uniformly.</p><pre><code class="language-cpp">#include <array>#include <print>
int main() { std::array<int,5> a{{1,2,3,4,5}}; std::println("array size: {}", a.size()); std::println("first element: {}", a[0]); return 0;}</code></pre><p>The example fills an <code>std::array<int,5></code> with the values 1 through 5, prints the size, and prints the first element.</p><p><code>std::array</code> is an aggregate, so it can be copy‑assigned and returned by value with no hidden allocation, and its size is part of the type. Prefer it over <code>std::vector</code> when <code>N</code> is small and fixed, because the data sits in the parent object with no pointer chase. Reach for <code>std::vector</code> once the size is dynamic or larger than a few dozen elements.</p><h2 id="sequence-containers-with-different-insertion-properties"><a class="header" href="#sequence-containers-with-different-insertion-properties">Sequence containers with different insertion properties</a></h2><div class="table-wrapper"><table><thead><tr><th>Container</th><th>Typical use</th><th>Insertion cost</th><th>Iterator invalidation</th></tr></thead><tbody><tr><td><code>std::deque</code></td><td>Push or pop at both ends without moving existing elements</td><td>Amortised constant time at either end</td><td>Inserting at the front or back does not invalidate existing iterators. Inserting in the middle can invalidate.</td></tr><tr><td><code>std::list</code></td><td>Frequent insertion or removal in the middle of a long list</td><td>Constant time anywhere</td><td>No iterator is invalidated by insertion or removal, except for the iterator that is removed.</td></tr><tr><td><code>std::forward_list</code></td><td>Singly‑linked list, minimal memory overhead</td><td>Constant time insertion after a known iterator</td><td>Same rules as <code>std::list</code>. No iterator invalidation except for erased elements.</td></tr></tbody></table></div><p>All three store elements non‑contiguously. The lack of cache locality makes them slower for tight loops that iterate over a large number of elements. Use them only when the insertion pattern outweighs the cache penalty.</p><pre><code class="language-cpp">// Example of a deque that pushes at both ends.std::deque<int> dq;for (int i = 0; i < 5; ++i) dq.push_back(i);for (int i = 5; i < 10; ++i) dq.push_front(i);</code></pre><p>A <code>std::deque</code> stores elements in fixed‑size chunks rather than one block, which is why pushing at the front never moves the existing elements and never invalidates their iterators. The trade‑off is a double indirection on access and a larger per‑element overhead than <code>vector</code>.</p><h2 id="ordered-associative-containers-stdmap-and-stdset"><a class="header" href="#ordered-associative-containers-stdmap-and-stdset">Ordered associative containers: <code>std::map</code> and <code>std::set</code></a></h2><p><code>std::map<Key, Value></code> and <code>std::set<Key></code> are implemented as red‑black trees. They keep keys in sorted order defined by <code>operator<</code> (or a custom comparator). Lookup, insertion, and removal take <code>O(log n)</code> time. Iterators remain valid across insertions and deletions, except when the element itself is erased.</p><p>Use these containers when ordered iteration, range queries, or stable iterator validity are required.</p><p>Use <code>operator[]</code> to find or insert, and <code>at()</code> when a missing key must raise <code>std::out_of_range</code> instead of creating a default. The ordering follows <code>operator<</code> on the key by default. A custom comparator changes both the order and the equality test. <code>std::multimap</code> and <code>std::multiset</code> permit duplicate keys when that is needed.</p><p><code>std::set</code> stores keys only and is the right choice when you need a sorted, deduplicated collection rather than key‑value pairs. Both <code>map</code> and <code>set</code> gain a <code>contains</code> member in C++20 that tests membership without constructing an iterator.</p><pre><code class="language-cpp">std::map<std::string, int> word_counts;word_counts["apple"] = 3;word_counts["banana"] = 5;for (const auto& [w, c] : word_counts) { std::println("{}: {}", w, c);}</code></pre><h2 id="unordered-associative-containers-stdunordered_map-and-stdunordered_set"><a class="header" href="#unordered-associative-containers-stdunordered_map-and-stdunordered_set">Unordered associative containers: <code>std::unordered_map</code> and <code>std::unordered_set</code></a></h2><p><code>std::unordered_map<Key, Value></code> and <code>std::unordered_set<Key></code> are hash tables. Average‑case lookup, insertion, and removal are constant time. The containers do <strong>not</strong> preserve any ordering of keys. The key type must be hashable. The standard library provides <code>std::hash</code> for fundamental types and for <code>std::string</code>. Custom types need a specialization of <code>std::hash</code> and an equality operator.</p><p>Because hash tables store elements in buckets, iterator invalidation rules differ from ordered containers: inserting does not invalidate iterators, but rehashing (which can occur when the load factor exceeds a threshold) invalidates all iterators.</p><p>An <code>unordered_map</code> has a higher per‑operation constant cost than a vector search over tiny sets, and it allocates buckets, so prefer it only once the keyed lookup actually pays off. <code>reserve</code> and <code>max_load_factor</code> let you tune the bucket count before a bulk insert.</p><p>Iteration order over an <code>unordered_map</code> is not specified and can change between runs, so never depend on it. The <code>contains</code> member tests membership in average constant time.</p><pre><code class="language-cpp">std::unordered_map<int, std::string> id_to_name{{1, "Alice"}, {2, "Bob"}};if (auto it = id_to_name.find(1); it != id_to_name.end()) { std::println("Found {}", it->second);}</code></pre><h2 id="stdspan-a-nonowning-view-over-contiguous-data"><a class="header" href="#stdspan-a-nonowning-view-over-contiguous-data"><code>std::span</code>: a non‑owning view over contiguous data</a></h2><p>A <code>std::span<T></code> is a lightweight object that refers to a contiguous sequence of <code>T</code>. It does <strong>not</strong> own the elements. It only stores a pointer and a length. Because it is non‑owning, it can be created from multiple sources: a <code>std::vector<T></code>, a <code>std::array<T, N></code>, or a raw C array.</p><p>Using <code>std::span</code> allows algorithms to accept any of those sources without copying. The container’s lifetime must outlive the span. Otherwise the span becomes dangling, which the lifetime‑safety analysis flags.</p><pre><code class="language-cpp">#include <vector>#include <array>#include <span>#include <print>
void print_span(std::span<const int> s) { std::print("span elements:"); for (int v : s) { std::print(" {}", v); } std::println("");}
int main() { std::vector<int> v{1, 2, 3}; std::array<int,3> a{{4,5,6}}; print_span(v); print_span(a); return 0;}</code></pre><p>The <code>print_span</code> function takes a <code>std::span<const int></code> and prints each element. The <code>main</code> function calls it with a <code>std::vector<int></code> and an <code>std::array<int,3></code>, demonstrating the uniform interface.</p><p>A <code>std::span</code> can carry a compile‑time extent (<code>std::span<T, N></code>) or a dynamic one. The static form lets the compiler prove bounds in some algorithms. <code>first</code>, <code>last</code>, and <code>subspan</code> produce new spans over subranges without copying, which is how zero‑copy parsing pipelines stay allocation free.</p><p>A span exposes <code>size</code>, <code>empty</code>, and <code>data</code>, and it converts to a <code>std::vector</code> only through an explicit constructor, never by accident. This keeps ownership explicit at every call site.</p><h2 id="choosing-the-right-container"><a class="header" href="#choosing-the-right-container">Choosing the right container</a></h2><p>When you start a new piece of code, follow this decision guide:</p><ol><li><strong>Begin with <code>std::vector</code>.</strong> Its contiguous storage gives the best cache performance for most workloads.</li><li><strong>Switch to <code>std::array</code></strong> if the number of elements is known at compile time and the total size fits within typical stack limits (e.g., ≤ 1 KB).</li><li><strong>Select an associative container</strong> when you need O(1) average‑case lookup by key. Choose <code>std::map</code> if you require ordered iteration or range queries. Choose <code>std::unordered_map</code> for constant‑time performance when ordering does not matter.</li><li><strong>Consider <code>std::deque</code></strong> only when you need efficient insertion or removal at both ends and cannot accept the iterator invalidation of a vector during growth.</li><li><strong>Use <code>std::list</code> or <code>std::forward_list</code></strong> only when you must insert or erase frequently in the middle of a large sequence and the cache penalty is tolerable.</li><li><strong>Use <code>std::span</code></strong> to write generic algorithms that operate on any contiguous view without taking ownership.</li></ol><p>Prefer contiguous, owning containers such as <code>std::vector</code> (default) or <code>std::array</code> when size is fixed. Use <code>std::span</code> for non‑owning views and <code>std::pmr</code> allocators to customise allocation without changing the container type. The same cache‑locality reasoning applies to <code>std::string</code>, while <code>std::string_view</code> follows the view pattern described earlier.</p><h2 id="try-this"><a class="header" href="#try-this">Try this</a></h2><p>Write a program that:</p><ol><li>Declares a <code>std::vector<int></code>.</li><li>Calls <code>reserve(10)</code>.</li><li>Uses <code>emplace_back</code> to add the values 1, 2, 3.</li><li>Creates a <code>std::span<int></code> that references the vector.</li><li>Computes and prints the sum of the elements in the span.</li></ol><p>No solution is provided. The reader must fill in the code.</p>
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