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strong_type

An additive strong typedef library for C++14/17/20 using the Boost Software License 1.0

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Very much inspired by @foonathan's type_safe library, but aim is slightly different. Limit scope for type safety only. No runtime checks. Also strive for a higher level abstraction of the needed functionality. The idea is to suffer no runtime penalty, but to capture misuse at compile time (for example accidentally subtracting from a handle, or swapping two parameters in a function call) while still being easy to use for inexperienced programmers.

Example use:

#include <strong_type.hpp>
using myint = strong::type<int, struct my_int_>;

myint is a very basic handle. You can initialize it. You can do equal/not-equal comparison with other instances of the same type, and you can access its underlying int instance with value_of(variable).

To get the underlying type of a strong type, use typename strong::underlying_type<mytype>::type, or the convenience alias strong::underlying_type_t<mytype>. If mytype is not a strong::type, they give mytype.

using otherint = strong::type<int, struct other_int_>;

otherint is a distinct type from myint. If a function takes an argument of type myint, you can't pass it an instance of otherint, and vice-versa. You also can't cross-assign, cross-create or cross-compare.

To access more functionality, you add modifiers. For example:

using ordered_int = strong::type<int, struct ordered_int_, strong::ordered>;

Type ordered_int now supports relational order comparisons, like <, (provided the underlying type, int this case int, does.) Type ordered_int can thus be used as key in std::map<> or std::set<>.

Modifiers are:

  • strong::default_constructible. The strong type is not default constructible by default. This modifier enables a default constructor which uses a default constructor of the underlying type.

  • strong::equality provides operators == and !=. The strong type can be then compared for equality or inequality.

  • strong::equality_with<Ts...> provides operators == and != between the strong type and each of the types Ts.... Note! While Ts can include other strong types, it can not refer to the strong type being defined. Use strong::equality for that.

  • strong::ordered provides operators '<', '<=', '>=' and '>'. The strong type offers the same ordering relatin as the underlying type.

  • strong::ordered_with<Ts...> provides operators '<', '<=', '>=' and '>' between the strong type and each of the types Ts.... Note! While Ts can include other strong types, it cannot refer to the strong type being defined. Use strong::ordered for that.

  • strong::semiregular. This gives you default constructible, move/copy constructible, move/copy assignable and swappable. A decent default for many types.

  • strong::regular. Same as regular and also equality comparable. A good default base for most types.

  • strong::unique. Make the type move constructible and move assignable but not copy constructible nor copy assignable.

  • strong::ostreamable, strong::istreamable, strong::iostreamable, which provide the default iostream integrations (as handled by the underlying type.) Provide your own operators instead if you prefer that.

  • strong::incrementable, strong::decrementable, strong::bicrementable. Support operator++ and operator--. bicrementable is obviously a made- up word for the occasion, but I think its meaning is clear.

  • strong::boolean provides explicit operator bool() const, providing the underlying type supports it.

  • strong::convertible_to<Ts...> provides an explicit operator Ts() const for each type Ts, providing the underlying type supports it.

  • strong::implicitly_convertible_to<Ts...> provides an operator Ts() const for each type Ts, providing the underlying type supports it.

  • strong::hashable allows std::hash<> on the type (forwards to the underlying type,) to allow use in std::unordered_set<> and std::unordered_map<>

  • strong::difference allows instances to be subtracted and added (yielding a strong::difference,) divided (yielding the base type), or multiplied or divided with the base type, yielding another strong::difference. A strong::difference is also strong::ordered

  • strong::affine_point<D> allows instances to be subtracted (yielding a D) or to add or subtract a D to an instance. See Affine Space. Examples of one dimentional affine points are pointer (with D being ptrdiff_t,) or std::time_point<> (with std::duration<> as D.) An example of a multidimensional affine point is a coordinate (with a vector type for D.) It is natural that D is of a strong::difference type. This is a good name from a mathematical point of view, but perhaps a bit too academic, and not well aligned with the other names.

  • strong::pointer allows operator* and operator->, and comparisons with nullptr providing the underlying type supports it.

  • strong::arithmetic allows addition, subtraction, multiplication, division and remainder of instances.

  • strong::bitarithmetic allows bitwise &, bitwise |, bitwise ^ and shift operations.

  • strong::indexed<D> allows use of the subscript operator[] on type D. This also allows member function at(D), providing the underlying type supports it. A lame version indexed<> allows subscript on any type that works.

  • strong::iterator adds functionality needed depending on iterator category. If the iterator type is a random_access_iterator, the strong type is strong::indexed<> and strong::affine_point<difference_type>. It should be possible to specify the index type and affine_point type.

  • strong::range adds the functionality needed to iterate over the elements. the iterator types are using the same tag as using in the range. Only implements types iterator and const_iterator, and thus .begin(), .end(), .cbegin(), .cend(), .begin() const and .end() const.

For modifier strong::arithmetic, the type trait std::is_arithmetic<> is true.

For modifier strong::iterator, the type trait std::iterator_traits mirrors the traits of the underlying iterator type.

Miscellaneous:

  • strong::type provides a non-member swap() function as a friend, which swaps underlying values using.

  • strong::underlying_type<Type> is T for strong::type<T, Tag, Ms...> and public descendants, and Type for other types.

  • strong::uninitialized can be used to construct instances of strong::type<T...> without initializing the value. This is only possible if the underlying type is trivially default constructible, for example:

    void init(int*);
    void function() {
        strong::type<int, struct int_tag> x(strong::uninitialized);
        // x will have an unspecified value
        init(&value_of(x)); // hopefully the init() function assigns a value
    }

To build the self-test program:

cmake <strong_type_dir> -DCMAKE_BUILD_TYPE=Debug
cmake --build . --target self_test

N.B. Microsoft Visual Studio MSVC compiler < 19.22 does not handle constexpr correctly. Those found to cause trouble are disabled for those versions.

Other libraries:

Library Author
type_safe Jonathan Müller
NamedType Jonathan Boccara
strong_typedef Anthony Williams (justsoftwaresolutions)

Presentations about defining and using strong types

Strong Types for Strong Interfaces Jonathan Boccara from MeetingC++ 2017
Strong Types in C++ Barney Dellar from C++OnSea 2019
Type Safe C++? - LOL! - ;-) Björn Fahller from ACCU 2018
Curiously Coupled Types Adi Shavit & Björn Fahller from NDC{Oslo} 2019

Discussions, pull-requests, flames are welcome.

@bjorn_fahller

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An additive strong typedef library for C++14/17/20

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