Examples/test-suite/cpp20_concepts.i

%module cpp20_concepts

// SWIG provides only basic parser level support for C++20 concept declarations
// and requires-clauses: both are silently consumed and produce no node in the
// parse tree, so a constrained template wraps as if it were unconstrained.

%inline %{
#include <concepts>

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

// A concept whose body refers to another concept (and uses sizeof in a
// parenthesised subexpression to exercise paren tracking in the skipper).
template<typename T>
concept SmallNumeric = Numeric<T> && (sizeof(T) <= 4);

// Function template with a trailing requires-clause.
template<typename T>
T cube(T x) requires Numeric<T> {
  return x * x * x;
}

// Function template with a prefix requires-clause - the constraint sits
// between the template head and the return type.
template<typename T>
requires Numeric<T>
T quad(T x) {
  return x * x * x * x;
}

// Prefix requires-clause with a compound constraint joined by '&&'.
template<typename T>
requires Numeric<T> && SmallNumeric<T>
T half(T x) {
  return x / 2;
}

// Prefix requires-clause with a parenthesised constraint subexpression.
template<typename T>
requires (Numeric<T> || std::same_as<T, bool>)
T identity(T x) {
  return x;
}

// Trailing requires-clause after a trailing return type, the trailing return
// type being the end of the declarator.
template<typename T>
auto negate_value(T x) -> T requires Numeric<T> {
  return -x;
}

// Prefix and trailing requires-clauses on the same trailing return type.
template<typename T>
requires Numeric<T>
auto triple(T x) -> T requires SmallNumeric<T> {
  return 3 * x;
}

// Trailing requires-clause whose constraint contains a requires-expression
// as a primary - the inner '{ t + t; }' is part of the constraint, not the
// function body.
template<typename T>
T add(T a, T b) requires requires (T t) { t + t; } {
  T result = a + b;
  return result;
}

// A concept whose body is itself a requires-expression.  Functionally
// equivalent to the Addable variable template in cpp20_variable_templates.i,
// but spelt as a named concept usable as a constraint.
template<typename T>
concept Summable = requires (T t) { t + t; };

template<typename T>
T sum_pair(T a, T b) requires Summable<T> {
  return a + b;
}

// A concept whose body is a requires-expression containing a compound
// requirement with a trailing return type constraint - '{ a + b } -> std::same_as<T>;'
template<typename T>
concept AddableSame = requires(T a, T b) {
  { a + b } -> std::same_as<T>;
};

template<typename T>
T add_same(T a, T b) requires AddableSame<T> {
  return a + b;
}

// Inline 'requires requires' with a compound requirement.
template<typename T>
T add_inline_same(T a, T b) requires requires(T x, T y) { { x + y } -> std::same_as<T>; } {
  return a + b;
}

// Concept body with multiple simple-requirements.
struct Engine {
  bool running = false;
  void start() { running = true; }
  void stop()  { running = false; }
};

template<typename T>
concept Machine = requires(T m) {
  m.start();
  m.stop();
};

template<typename T>
bool cycle(T& m) requires Machine<T> {
  m.start();
  m.stop();
  return true;
}

// Concept body with a type-requirement, a noexcept compound-requirement and
// a nested-requirement alongside a simple-requirement.
struct Container {
  using value_type = int;
  void push_back(int) noexcept {}
};

template<typename T>
concept BasicContainer = requires(T c, typename T::value_type v) {
  typename T::value_type;
  { c.push_back(v) } noexcept;
  requires sizeof(T) > 0;
};

template<typename T>
bool check_container(T c) requires BasicContainer<T> {
  return true;
}
%}

%template(cube_int)     cube<int>;
%template(cube_double)  cube<double>;
%template(quad_int)     quad<int>;
%template(quad_double)  quad<double>;
%template(half_int)     half<int>;
%template(identity_int) identity<int>;
%template(negate_value_int) negate_value<int>;
%template(triple_int)   triple<int>;
%template(add_int)      add<int>;
%template(sum_pair_int) sum_pair<int>;
%template(add_same_int) add_same<int>;
%template(add_inline_same_int) add_inline_same<int>;
%template(cycle_engine)        cycle<Engine>;
%template(check_container_c)   check_container<Container>;