Type safe primitives in C++

phantomtypes.cpp

// Phantom is a phatom type. It means that it is not used
// during runtime, but play type safety roles during compile
// time.
template <typename Phantom>
struct StrongId {
    int id;

    // Conversion int -> StrongId<T> must to be explicit
    explicit StrongId(auto id_) : id(id_) {}

    // Conversion StrongId<T> -> int is implicit
    constexpr operator auto() const { return id; }
};

// We can use the type being defined to define strong primitives
// like int. In fact we can generalize StrongId to accept another
// type parameter and use any time instead of 'int', but I'm not
// doing this here for sake of simplicity.
//
// Here we use the struct being defined to define strongly typed
// ids. These ids CANNOT be swapped by accident because the program
// is ill-typed if you do so.
struct Store { StrongId<Store> id; };
struct Product { StrongId<Product> id; };
// Even if 'Product' is an incomplete type
// inside its own definition, this does not matter much because the
// type is not used at runtime, so its size does not matter in this case.

// Note: Incomplete type is a struct or class that is declared but not
// defined, for example `struct foo;` declares a struct but does not
// defines it. Because is not defined its size is unknown, and because of
// that there are things that you cannot do with it, like allocating it,
// (how much bytes would you allocate?), but all this is off-topic

// Back to phantom types, this just an example function accepting an int
void findById(int id) {}

// This function cannot be called with a StrongId<Store> by accident
void findProductById(StrongId<Product> productId) { 
     // operator int() converts it to int automatically
    findById(productId);
}

void testStrongId(Product product, Store store) {
    // You can't pass a simple int to it, it need to be wrapped
    findProductById(1); // Type error: No matching function for call to 'findProductById'
    findProductById(product.id); // ok
    findProductById(store.id); //  Type error: No matching function for call to 'findProductById'

}