C++ Concepts and Constraints | Type Requirements in C++20

The Problem with Unconstrained Templates

Templates in C++ can accept any type — which means bad type errors appear at instantiation, buried deep in the implementation:

template<typename T>
T sum(const std::vector<T>& v) {
    T result{};
    for (const auto& x : v) result += x;  // error here if T has no +=
    return result;
}

// Calling with a type that doesn't support +=
std::vector<std::string> words = {"hello", "world"};
sum(words);
// Error: no match for 'operator+=' for 'std::string'
// ... in instantiation of 'T sum<T>(const std::vector<T>&) [with T=std::string]'
// ... (many more lines)

With a concept, the error appears at the call site with a clear message:

template<typename T>
concept Addable = requires(T a, T b) {
    { a + b } -> std::convertible_to<T>;
    { a += b };
};

template<Addable T>
T sum(const std::vector<T>& v) { /* ... */ }

sum(words);
// Error: constraints not satisfied for 'sum' — 'std::string' does not satisfy 'Addable'

---

Defining Concepts

A concept is a compile-time predicate on template parameters:

#include <concepts>

// Simple type trait concept
template<typename T>
concept Integral = std::is_integral_v<T>;

// Requires expression — checks that specific operations compile
template<typename T>
concept Addable = requires(T a, T b) {
    a + b;      // expression must be valid
    a += b;     // this too
};

// With return type constraint
template<typename T>
concept Comparable = requires(T a, T b) {
    { a < b } -> std::convertible_to<bool>;   // must return something convertible to bool
    { a == b } -> std::same_as<bool>;          // must return exactly bool
};

// Compound concept
template<typename T>
concept Numeric = std::integral<T> || std::floating_point<T>;

// Nested requirements
template<typename T>
concept Sortable = requires(T container) {
    typename T::value_type;                       // type must exist
    { container.begin() } -> std::forward_iterator;
    { container.end() }   -> std::forward_iterator;
    requires std::totally_ordered<typename T::value_type>;  // nested requires
};

---

Standard Library Concepts

<concepts> provides a rich set of ready-made concepts:

#include <concepts>

// Type categories
std::integral<T>              // int, long, char, bool, ...
std::floating_point<T>        // float, double, long double
std::signed_integral<T>       // signed integer types
std::unsigned_integral<T>     // unsigned integer types
std::arithmetic<T>            // integral or floating_point

// Relationships
std::same_as<T, U>            // T and U are the same type
std::derived_from<T, Base>    // T is derived from Base
std::convertible_to<T, U>     // T is implicitly convertible to U
std::common_with<T, U>        // T and U share a common type

// Comparison
std::equality_comparable<T>          // T has == and !=
std::totally_ordered<T>             // T has <, <=, >, >=
std::three_way_comparable<T>        // T has <=>

// Callable
std::invocable<F, Args...>          // F can be called with Args
std::regular_invocable<F, Args...>  // same + equality preserving
std::predicate<F, Args...>          // F returns bool-like

// Object concepts
std::copyable<T>                    // copy constructible and assignable
std::movable<T>                     // move constructible and assignable
std::regular<T>                     // copyable, equality comparable

---

Four Ways to Apply a Concept

template<typename T>
concept Printable = requires(std::ostream& os, T t) {
    { os << t } -> std::same_as<std::ostream&>;
};

// 1. Abbreviated function template (simplest)
void print(const Printable auto& value) {
    std::cout << value << '\n';
}

// 2. Requires clause after template parameters
template<typename T>
requires Printable<T>
void print(const T& value) {
    std::cout << value << '\n';
}

// 3. Constrained template parameter (classic concept syntax)
template<Printable T>
void print(const T& value) {
    std::cout << value << '\n';
}

// 4. Inline requires expression (for one-off constraints)
template<typename T>
requires requires(std::ostream& os, T t) { os << t; }
void print(const T& value) {
    std::cout << value << '\n';
}

All four are equivalent. Style 1 (abbreviated) is the most concise for simple cases; style 2 (requires clause) handles complex multi-concept constraints most readably.

---

Requires Expressions in Detail

A requires expression tests whether operations on template parameters are valid at compile time:

template<typename T>
concept Container = requires(T c, const T cc) {
    // Type requirements — nested type must exist
    typename T::value_type;
    typename T::iterator;

    // Simple expression — must compile (return type not checked)
    c.clear();

    // Compound expression — {expr} -> type constraint
    { c.size() } -> std::convertible_to<std::size_t>;
    { c.begin() } -> std::same_as<typename T::iterator>;
    { cc.begin() } -> std::same_as<typename T::const_iterator>;
    { c.empty() } -> std::convertible_to<bool>;

    // Nested requirement — must evaluate to true
    requires std::copyable<typename T::value_type>;
};

// Test it
static_assert(Container<std::vector<int>>);    // passes
static_assert(Container<std::list<double>>);   // passes
// static_assert(Container<int>);              // fails — int has no .size() etc.

---

Custom Concepts: Real Examples

Hashable Type

template<typename T>
concept Hashable = requires(T t) {
    { std::hash<T>{}(t) } -> std::convertible_to<std::size_t>;
};

template<Hashable K, typename V>
class HashMap {
    std::unordered_map<K, V> data_;
public:
    void insert(const K& key, const V& value) { data_[key] = value; }
    std::optional<V> get(const K& key) const {
        auto it = data_.find(key);
        if (it == data_.end()) return std::nullopt;
        return it->second;
    }
};

HashMap<std::string, int> wordCount;  // OK — string is Hashable
// HashMap<std::vector<int>, int> map;  // error — vector is not Hashable

Serializable Type

template<typename T>
concept Serializable = requires(T t, std::ostream& os, std::istream& is) {
    { t.serialize(os) }   -> std::same_as<void>;
    { T::deserialize(is) } -> std::same_as<T>;
};

template<Serializable T>
void saveToFile(const T& obj, const std::string& path) {
    std::ofstream f(path);
    obj.serialize(f);
}

Iterator Concepts

template<typename It>
concept InputIter = requires(It it) {
    *it;        // dereferenceable
    ++it;       // pre-increment
    it != it;   // comparable
};

template<InputIter It>
auto accumulate(It first, It last) {
    using T = std::remove_cvref_t<decltype(*first)>;
    T sum{};
    for (; first != last; ++first) sum += *first;
    return sum;
}

---

Concepts for Overload Resolution

Concepts can select between overloads — more specific constraints win:

#include <concepts>
#include <iostream>

// Generic fallback
template<typename T>
void process(T val) {
    std::cout << "Generic: " << val << '\n';
}

// More constrained — selected when T is integral
template<std::integral T>
void process(T val) {
    std::cout << "Integer: " << val << " (bits: " << sizeof(T)*8 << ")\n";
}

// Even more constrained — selected when T is exactly int
void process(int val) {
    std::cout << "Exact int: " << val << '\n';
}

int main() {
    process(3.14);    // Generic: 3.14
    process(42L);     // Integer: 42 (bits: 64)
    process(42);      // Exact int: 42
    process("hello"); // Generic: hello
}

---

Concepts vs SFINAE

The same constraint, old and new style:

// SFINAE (pre-C++20) — cryptic, easy to get wrong
template<typename T,
         std::enable_if_t<std::is_integral_v<T> &&
                          std::is_signed_v<T>, int> = 0>
T negate(T value) { return -value; }

// Concepts (C++20) — readable, compile errors at call site
template<std::signed_integral T>
T negate(T value) { return -value; }

// Abbreviated — even shorter
auto negate(std::signed_integral auto value) { return -value; }

Error message comparison for a bad call like negate(3.14f):

• SFINAE: ~30 lines about template substitution failure, enable_if internals
• Concepts: 2 lines — “float does not satisfy signed_integral”

---

Compiler Support

# GCC 10+
g++ -std=c++20 main.cpp

# Clang 10+
clang++ -std=c++20 main.cpp

# MSVC 2019 16.8+
cl /std:c++20 main.cpp

---

Key Takeaways

• Concepts are compile-time predicates on template parameters — they constrain which types a template accepts
• Use requires expressions to specify that certain operations on T must compile, with optional return type constraints
• Standard <concepts> header provides std::integral, std::copyable, std::invocable, and many more
• The four application styles (abbreviated, requires clause, constrained parameter, inline requires) are equivalent — choose for readability
• Concepts improve overload resolution — the most constrained overload wins
• No runtime cost — all checks are compile-time
• Error messages are dramatically shorter and appear at the call site, not deep in template instantiation

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자주 묻는 질문 (FAQ)

Q. 이 내용을 실무에서 언제 쓰나요?

A. C++20 concepts and constraints: requires clauses, standard concepts, custom concepts, and how they replace verbose SFINA… 실무에서는 위 본문의 예제와 선택 가이드를 참고해 적용하면 됩니다.

Q. 선행으로 읽으면 좋은 글은?

A. 각 글 하단의 이전 글 또는 관련 글 링크를 따라가면 순서대로 배울 수 있습니다. C++ 시리즈 목차에서 전체 흐름을 확인할 수 있습니다.

Q. 더 깊이 공부하려면?

A. cppreference와 해당 라이브러리 공식 문서를 참고하세요. 글 말미의 참고 자료 링크도 활용하면 좋습니다.

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같이 보면 좋은 글 (내부 링크)

이 주제와 연결되는 다른 글입니다.

• [C++20 Modules](/en/blog/cpp-series-24-1-modules-basics/
• [C++ Copy Initialization: The = Form, explicit, and Copy](/en/blog/cpp-copy-initialization/
• [C++ map vs unordered_map: When to Use Each and How They Work](/en/blog/cpp-stl-map-unordered-map/

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