A strong type is a type used in place of another type to carry specific meaning through its name.
This project experiments with strong types in C++. All components are in the namespace fluent. You can find a collection of blog posts explaining the rationale of the library and usages for strong types on Fluent C++.
It central piece is the templated class NamedType, which can be used to declare a strong type with a typedef-like syntax:
using Width = NamedType<double, struct WidthTag>;
using Height = NamedType<double, struct HeightTag>;which can be used to make interfaces more expressive and more robust. Note how the below constructor shows in which order it expects its parameters:
class Rectangle
{
public:
Rectangle(Width width, Height height) : width_(width.get()), height_(height.get()) {}
double getWidth() const { return width_; }
double getHeight() const { return height_; }
private:
double width_;
double height_;
};Strong types are about better expressing your intentions, both to the compiler and to other human developers.
This implementation of strong types can be used to add strong typing over generic or unknown types such as lambdas:
template<typename Function>
using Comparator = NamedType<Function, struct ComparatorTag>;
template <typename Function>
void performAction(Comparator<Function> comp)
{
comp.get()();
}
performAction(make_named<Comparator>([](){ std::cout << "compare\n"; }));The NamedType class is designed so that the following usage:
using FamilyNameRef = NamedType<std:string&, struct FamilyNameRefTag>;behaves like a reference on an std::string, strongly typed.
You can declare which functionalities should be inherited from the underlying type. So far, only basic operators are taken into account. For instance, to inherit from operator+ and operator<<, you can declare the strong type:
using Meter = NamedType<double, MeterTag, Addable, Printable>There is one special skill, FunctionCallable, that lets the strong type be converted in the underlying type. This has the effect of removing the need to call .get() to get the underlying value. And MethodCallable enables operator-> on the strong type to invoke methods on the underlying type.
The skill Callable is the union of FunctionCallable and MethodCallable.
By their nature strong types can play the role of named parameters:
using FirstName = NamedType<std::string, struct FirstNameTag>;
using LastName = NamedType<std::string, struct LastNameTag>;
void displayName(FirstName const& theFirstName, LastName const& theLastName);
// Call site
displayName(FirstName("John"), LastName("Doe"));But the nested type argument allows to emulate a named argument syntax:
using FirstName = NamedType<std::string, struct FirstNameTag>;
using LastName = NamedType<std::string, struct LastNameTag>;
static const FirstName::argument firstName;
static const LastName::argument lastName;
void displayName(FirstName const& theFirstName, LastName const& theLastName);
// Call site
displayName(firstName = "John", lastName = "Doe");You can have a look at tests.cpp for usage examples.
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