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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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123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230231232233234235236237238239240241242243244245246247248249250251252253254255256257258259260261262263264265266267268269270271272273274275276277278279280281282283284285286287288289290291292293294295296297298299300301302303304305306307308309310311312313314315316317318319320321322323324325326327328329330331332333334335336337338339340341342343344345346347348349350351352353354355356357358359360361362363364365366367368369370371372373374375376377378379380381382383384385386387388389390391392393394395396397398399400401402403404405406407408409410411412413414415416417418419420421422423<!DOCTYPE HTML><html lang="en" class="light sidebar-visible" dir="ltr"> <head> <!-- Book generated using mdBook --> <meta charset="UTF-8"> <title>Concepts: the constraint language - 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="concepts-the-constraint-language"><a class="header" href="#concepts-the-constraint-language">Concepts: the constraint language</a></h1><h2 id="why-concepts"><a class="header" href="#why-concepts">Why concepts</a></h2><p>Templates allow algorithms to work with any type that satisfies a set of requirements. Before C++20 those requirements were expressed with SFINAE tricks such as <code>std::enable_if</code> and trait metafunctions. The constraints were hidden inside long type expressions, and a failure produced a cascade of template‑instantiation diagnostics that were difficult to read. Concepts replace that style with explicit, named predicates. A concept is part of the function signature. The compiler checks it before it attempts to instantiate the template. If the requirement is not met, the diagnostic cites the concept name and the offending type. The error becomes clear and the intent obvious.</p><p>Concepts also act as in‑code documentation: the concept name (e.g., <code>std::integral</code> or <code>std::range</code>) states the precondition next to the declaration, removing the need for separate <code>enable_if</code> blocks. A concept is a compile‑time <code>bool</code> value, usable in <code>if constexpr</code> or <code>static_assert</code>, and it drives overload resolution and in‑body branching. Because the compiler checks the constraint before template instantiation, failures appear at the call site with the concept name and offending type, providing earlier, clearer diagnostics than a <code>static_assert</code> inside the function body.</p><h2 id="the-requires-clause-and-requires-expression"><a class="header" href="#the-requires-clause-and-requires-expression">The <code>requires</code> clause and <code>requires</code> expression</a></h2><p>A <em>requires clause</em> follows a template declaration and names one or more concepts that must be satisfied.</p><pre><code class="language-cpp">template<std::integral T>void f(T t) { std::cout << "integral: " << t << '\n';}</code></pre><p>A <em>requires expression</em> appears inside a template body and describes the operations that must exist for a particular set of types.</p><pre><code class="language-cpp">template<typename T>requires requires (T a) { { a + a } -> std::same_as<T>; }T twice(T a) { return a + a; }</code></pre><p>If the predicate evaluates to <code>false</code>, the overload is removed from overload resolution. The following example demonstrates two overloads, one for integral types and one for floating‑point types, and prints which overload ran.</p><pre><code class="language-cpp">#include <iostream>#include <type_traits>
// Overload for integral typestemplate<std::integral T>void show(T value) { (void)value; std::cout << "integral overload\n";}
// Overload for floating‑point typestemplate<std::floating_point T>void show(T value) { (void)value; std::cout << "floating overload\n";}
int main() { show(42); // integral show(3.14); // floating return 0;}</code></pre><p>The <code>requires</code> expression can be combined with logical operators to form richer constraints:</p><pre><code class="language-cpp">template<typename T>requires (std::integral<T> && sizeof(T) >= 4)void process(T value) { std::cout << "'integral (sizeof >= 4)'} " << value << '\n';}</code></pre><p>The compiler evaluates the whole Boolean expression before overload resolution, eliminating surprising matches.</p><p>The <code>requires</code> clause and the <code>requires</code> expression are distinct tools. The clause (<code>requires std::integral<T></code>) attaches a constraint to a declaration. The expression (the <code>requires (T a) { ... }</code> block) describes the operations a type must support and can test return types with <code>-> std::same_as<U></code>. A <code>requires</code> expression can also guard a single member function, so a class template can offer an operation only when the element type supports it.</p><h2 id="defining-a-concept"><a class="header" href="#defining-a-concept">Defining a concept</a></h2><p>A concept is a named predicate. It can be written with the <code>concept</code> keyword and a <em>requires clause</em> that enumerates the required expressions.</p><pre><code class="language-cpp">#include <iostream>#include <type_traits>
// Concept that requires additiontemplate<typename T>concept Addable = requires (T a, T b) { a + b; };
// Function using the concepttemplate<Addable T>T add(T a, T b) { return a + b;}
int main() { std::cout << "int add: " << add(2, 3) << std::endl; std::cout << "string add: " << add(std::string{"hi"}, std::string{"!"}) << std::endl; return 0;}</code></pre><p>The <code>Addable</code> concept checks that the binary <code>+</code> operator is valid for two operands of type <code>T</code>. The accompanying <code>add</code> function uses the concept as a constraint, so any type that models <code>Addable</code> can be added safely. Because the concept name appears directly in the signature, the intent is clear at the call site.</p><p>Concepts can also be expressed as Boolean formulas of other concepts. This enables a small vocabulary of high‑level requirements while reusing primitive concepts from <code><concepts></code>:</p><pre><code class="language-cpp">template<typename T>concept Number = std::integral<T> || std::floating_point<T>;</code></pre><p>A library author can build more expressive constraints by layering concepts.</p><p>A <code>requires</code> expression enumerates several checks separated by semicolons inside the brace, each a statement that must be valid. Return-type constraints use an arrow: <code>{ a + b } -> std::same_as<T></code> demands that <code>a + b</code> not only compiles but also yields a type convertible to <code>T</code>. Because a concept reduces to a <code>constexpr bool</code>, you can combine concepts with <code>&&</code>, <code>||</code>, and <code>!</code> to build richer requirements without writing a new named concept.</p><p>The same Boolean nature lets a function branch on a concept with <code>if constexpr</code>, selecting an implementation without writing separate overloads. <code>if constexpr (std::is_pointer_v<T>) { /* pointer path */ } else { /* value path */ }</code> picks the branch at compile time and discards the unused one, so the body need not be valid for every <code>T</code>. This replaces the old tag-dispatch and SFINAE patterns for in-body selection.</p><h2 id="subsumption"><a class="header" href="#subsumption">Subsumption</a></h2><p>When two concepts overlap, the more specific one wins. This rule is called <em>subsumption</em> and mirrors goal ordering in Prolog: the most specific rule is chosen before a more generic one. The compiler builds a partial ordering of candidate functions based on the subsumption relationship of their constraints.</p><pre><code class="language-cpp">#include <iostream>#include <type_traits>
// Overload for integral typestemplate<std::integral T>void foo(T) { std::cout << "integral overload" << std::endl;}
// Overload for signed integral types (more specific)template<std::signed_integral T>void foo(T) { std::cout << "signed integral overload" << std::endl;}
int main() { unsigned int u = 1; int s = -1; foo(u); // should select integral overload foo(s); // should select signed integral overload return 0;}</code></pre><p>Subsumption removes ambiguity from overload sets. When two constrained overloads both match, the compiler prefers the overload whose constraint subsumes the other. For example, <code>std::signed_integral</code> implies <code>std::integral</code>. Calls with a signed type select the <code>std::signed_integral</code> overload, while unsigned calls select the generic overload. This deterministic ordering eliminates the ambiguous‑overload errors common with SFINAE tricks.</p><h2 id="terse-syntax"><a class="header" href="#terse-syntax">Terse syntax</a></h2><p>C++26 allows constraints to appear directly on a parameter type or on a plain <code>auto</code> placeholder.</p><pre><code class="language-cpp">void show(std::integral auto x) { std::cout << x << '\n'; }std::integral auto n = 5;</code></pre><p>The same pattern works for references, forwarding references, and ranges. The example below uses a constrained variable, a constrained parameter, and a call to <code>std::ranges::sort</code>.</p><pre><code class="language-cpp">#include <iostream>#include <vector>#include <algorithm>#include <ranges>
// Constrained parametervoid show(std::integral auto n) { std::cout << "value: " << n << '\n';}
int main() { std::integral auto n = 7; // constrained variable show(n); std::vector<int> v = {5, 2, 9, 1, 4}; std::ranges::sort(v); std::cout << "sorted:"; for (int x : v) std::cout << ' ' << x; std::cout << std::endl; return 0;}</code></pre><p>These terse declarations keep the constraint next to the entity it describes, improving readability. Because the constraint travels with the parameter, a constrained lambda passed to <code>std::ranges::sort</code> is checked the moment the call is written, not when the algorithm instantiates. They also work inside generic lambdas. This enables concise constraint specifications:</p><pre><code class="language-cpp">auto cmp = [] (std::totally_ordered auto const& a, std::totally_ordered auto const& b) { return a < b;};</code></pre><p>Constraints also attach to return types through the same <code>auto</code> syntax: <code>std::integral auto square(std::integral auto x) { return x * x }</code> constrains both the parameter and the result. Algorithm authors lean on <code>std::predicate</code> and <code>std::relation</code> to describe the callables they accept, so a sorting routine declares <code>std::predicate<std::weak_ordering, T, T></code> instead of a vague template parameter.</p><h2 id="constrained-algorithms-and-ranges"><a class="header" href="#constrained-algorithms-and-ranges">Constrained algorithms and ranges</a></h2><p>Standard algorithms already carry concept requirements. <code>std::ranges::sort</code> requires <code>std::sortable</code>. <code>std::ranges::find</code> requires <code>std::range</code> and an <code>std::indirectly_comparable</code> predicate. The compiler checks those concepts before instantiating the algorithm, so a call that does not meet the requirements fails at the call site with a clear diagnostic.</p><pre><code class="language-cpp">#include <iostream>#include <vector>#include <algorithm>#include <ranges>
int main() { std::vector<int> v = {1, 2, 3, 4, 5}; auto it = std::ranges::find(v, 3); if (it != v.end()) { std::cout << "found" << std::endl; } else { std::cout << "not found" << std::endl; } return 0;}</code></pre><p>The program searches a <code>std::vector<int></code> for a value. Because the container satisfies <code>std::range</code>, the call compiles. A raw array fails the <code>range</code> concept, and the compiler reports that the concept is not satisfied. Since the concepts appear in the algorithm’s signature, users see the preconditions directly without consulting external documentation or static assertions.</p><h2 id="library-concept-vocabulary"><a class="header" href="#library-concept-vocabulary">Library concept vocabulary</a></h2><p>The header <code><concepts></code> provides the core building blocks used throughout the standard library:</p><ul><li><code>std::integral</code>: all integral types.</li><li><code>std::floating_point</code>: all floating‑point types.</li><li><code>std::same_as<T, U></code>: two types are identical.</li><li><code>std::convertible_to<From, To></code>: implicit conversion is possible.</li><li><code>std::invocable<F, Args...></code>: a callable can be invoked with the given arguments.</li><li><code>std::range<R></code>: a type provides <code>begin</code> and <code>end</code> iterators.</li></ul><p>The iterator library adds concepts such as <code>std::input_iterator</code>, <code>std::forward_iterator</code>, <code>std::random_access_iterator</code>, and <code>std::contiguous_iterator</code>. The ranges library refines those with concepts like <code>std::sized_range</code> and <code>std::view</code>. These names become part of the public interface. Reading an algorithm signature tells you exactly which properties the arguments must provide.</p><p>Several concepts describe callables rather than types. <code>std::predicate<P, Args...></code> is true when <code>P</code> can be called on <code>Args...</code> and the result is convertible to <code>bool</code>, which is exactly what <code>std::ranges::find_if</code> requires of its comparator. <code>std::relation</code> and <code>std::strict_weak_order</code> capture the ordering contracts that sorting and set operations depend on. Using them in your own signatures lets the compiler reject a comparator that returns the wrong category before any element is compared.</p><p>The <code>std::predicate</code> and <code>std::relation</code> concepts are exactly the constraints behind the algorithms from chapters 12 and 13, so the range pipelines written there were concept-checked whether or not the signatures made it explicit.</p><p>Concepts are the modern replacement for the SFINAE machinery that chapter 21 teaches as reading fluency. Where an old signature used <code>std::enable_if</code> in a return type, a new signature writes the concept in the parameter list. The behaviour is equivalent, but the new form is checkable before instantiation and readable at a glance.</p><h2 id="reading-concept-diagnostics"><a class="header" href="#reading-concept-diagnostics">Reading concept diagnostics</a></h2><p>When a constraint fails, the compiler prints the name of the concept and the type that caused the failure. For example, compiling the following code:</p><pre><code class="language-cpp">template<std::integral T>void g(T) {}
g("text");</code></pre><p>produces a diagnostic similar to:</p><blockquote><p>error: static assertion failed: constraints not satisfiedrequired from ‘std::integral<char const*>’note: substitution failed for ‘T = char const*’The message shows that <code>std::integral</code> was the offending concept and that <code>char const*</code> does not satisfy it. This is far clearer than the cascades of template‑instantiation errors that appeared before concepts.</p></blockquote><p>When user‑defined concepts are involved, the diagnostic includes the failed sub‑requirement. This makes it possible to pinpoint exactly which operation is missing. For instance, if <code>Addable</code> were violated because <code>+</code> returned the wrong type, the compiler points to the <code>{ a + b } -> std::same_as<T></code> clause.</p><h2 id="try-this"><a class="header" href="#try-this">Try this</a></h2><p>Define a concept <code>Comparable</code> that requires both <code>a < b</code> and <code>a == b</code> to be valid. Then write a <code>min</code> function constrained by <code>Comparable</code>. Call the function with two <code>int</code> values and with two <code>std::string</code> values.</p>
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