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#include <experimental/meta>
#include <experimental/compiler>
using namespace std::experimental;
//====================================================================
// Library code: implementing the metaclass (once)
consteval void interface(meta::info source) {
for (meta::info mem : meta::member_range(source)) {
meta::compiler.require(!meta::is_data_member(mem), "interfaces may not contain data");
meta::compiler.require(!meta::is_copy(mem) && !meta::is_move(mem),
"interfaces may not copy or move; consider"
" a virtual clone() instead");
if (meta::has_default_access(mem))
meta::make_public(mem);
meta::compiler.require(meta::is_public(mem), "interface functions must be public");
meta::make_pure_virtual(mem);
-> mem;
}
-> fragment struct X { virtual ~X() noexcept {} };
};
//====================================================================
// User code: using the metaclass to write a type (many times)
class(interface) Shape {
int area() const;
void scale_by(double factor);
};
// try putting any of these lines into Shape to see "interface" rules
// enforced => using the metaclass name to declare intent makes
// this code more robust to such changes under maintenance
//
// int i; // error: interfaces may not contain data
// private: void g(); // error: interface functions must be public
// Shape(const Shape&); // error: interfaces may not copy or move;
// // consider a virtual clone() instead
consteval {
meta::compiler.print(reflexpr(Shape));
}
//====================================================================
// And then continue to use it as "just a class" as always... this is
// normal code just as if we'd written Shape not using a metaclass
class Circle : public Shape {
public:
int area() const override { return 1; }
void scale_by(double factor) override { }
};
consteval {
meta::compiler.print(reflexpr(Circle));
}
#include <memory>
int main() {
std::unique_ptr<Shape> shape = std::make_unique<Circle>();
shape->area();
}
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