Or, does trait perhaps refer to a specific way to utilizing meta-functions?
If they are not synonymous, please point me to some examples of traits which are not meta-functions or those of meta-functions which are not traits. An actually working piece of code, perhaps within STL or Boost libraries, would be appreciated rather than a contrived toy example.
I'd like to see how experts in this field of C++ programming use these terminologies. I'm not sure if there are authoritative definitions of them...
Thanks in advance!
Clarification: It's not that I looking for any examples of traits or meta-functions. I've been using tens (if not hundreds) of them at my day job.
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"Meta" is C++ terminology for template programming. A clear example is the Boost Meta Programming Library (MPL).
In this sense, a meta-function is a function whose domain is not objects but C++ constructs. The common inputs are therefore types, ordinary functions and other templates.
A simple meta-function is for example template<typename T> using Foo = Bar<T, int> which has as its input a type T and as its output a type Foo<T>. Trivial, yes, but ordinary functions can be trivial too.
A trait is a metafunction whose codomain is a constant expression, often boolean. E.g. is_foo<T>::value obviously is boolean. The oldest trait in this sense is sizeof(T) whose codomain is size_t.
The terms are not equivalent.
Meta functions
The term is not defined in the C++ Standard.
There seem to be two main contending definitions:
1) "meta-functions derive/calculate/map-to values or types (based on their arguments/parameters) at compile time", and/or
constexpr functions may or may not be included in any given person's definition/conception; there's functional overlap, but much existing writing mentioning meta functions predates or just doesn't also discuss constexpr functions, so it's hard to know whether any definition or examples given deliberately leaves room for or excludes them
2) "meta-functions are functions and/or classes utilising template meta-programming", which specifically means a core of code (the template) may be reused for different parameters, performing some code generation or transformation based thereon
it's not necessarily the case that a meta-programming's final product is a compile-time type or constant - it may be a function intended for use with runtime values or data
A small survey of top google results for "meta function c++"
"metafunctions accept types and compile-time constants as parameters and return types/constants. A metafunction, contrary to its name, is a class template."
"code...executed and its result used while...compiling. ...meta-functions can compute two things: values or types"
this article uses "meta function" to refer to class templates yielding compile-time values/types
Traits
The C++ Standard doesn't define "traits", but does define "traits class"es:
17.3.25 [defns.traits] traits class
a class that encapsulates a set of types and functions necessary for class templates and function templates to manipulate objects of types for which they are instantiated
[ Note: Traits classes defined in Clauses 21, 22 and 27 are character traits, which provide the character handling support needed by the string and iostream classes. —end note ]
I'd add that traits can have values, as well as types and functions - for example rank exposes ::value. Traits provide types/values offering insight into either the parameter type itself, or the behaviour desired of the system using the trait when that system's working on variables of that type.
The Standard Library's character traits contain example of runtime functionality: for example X::length(p), X::find(p, n, c).
The <type_traits> header in the C++ Standard Library is a good place to get an initial feel for what kind of things traits can be used for.
Traits are traditionally and typically (but now C++11 provides constexpr functions not necessarily) classes, as distinct from functions meta- or otherwise.
A trait might tell you if a parameter T is constant or not, whether it's serialisable, or whether it should be compressed when transmitted via TCP: whatever some other general-purpose code might need to customise its behaviour for the range of types it handles.
Sometimes traits will be provided by the system that uses them - other times they can be supplied by client code wishing to customise its behaviour on a per-type basis.
Traits may deduce things using various tests of the parameter types, or they may be hand-crafted specialisations hard-coding values for specific types.
This ACCU introduction to traits](http://accu.org/index.php/journals/442) is worth reading, and includes this quote from the creator of C++:
Think of a trait as a small object whose main purpose is to carry information used by another object or algorithm to determine "policy" or "implementation details". - Bjarne Stroustrup
Contrasting "meta function" with "traits"
Regardless of which definition of meta function you adopt, the criteria relates to implementation details: when the function can be evaluated, and/or whether it involves code generation/transformation. That contrasts with traits, where the key concept/requirement is the purpose to which they're put, not the common - but far from universal - implementation using template specialisations or instantiations, or any capacity for compile time vs run-time evaluation.
A metafunction (in the context of C++) is a function that produces a result that can be used at compile time. The result can either be types (which can only be obtained at compile time by definition, using techniques such as partial specialisation) or values (which can be computed at compile time, using the fact that template parameters can be integral values, not just types).
A trait is essentially a class (or object) that packages up information (policy contraints, type characteristics, implementation details) for use by another class (or object) at compile time. The information packaged up can consist of type information (e.g. typedefs) or properties of interest. For example, std::numeric_limits<T> (available through <limits>) is a "type trait" which provides information about arithmetic types (e.g. is T an integral type? is the set of values a T can represent bounded or finite? is T signed? etc). The information is packaged in a form so that metaprograms (i.e. template functions or classes) can use the information at compile time. For example, a template metafunction might use information from a type trait to ensure the template is only instantiated for unsigned integral types, and trigger a compilation error if an attempt is made to instantiate the template for other types.
In this sense, a trait is a type of metafunction that provides information at compile time for use by other metafunctions.
As far as I know, there aren't any formal definitions of these terms. So I'll provide the ones that I use.
Metafunctions are templates which return a type. Usually, the returned result is a type which contains a member type named "type" so it can be accessed via the suffix ::type. Example invoking a metafunction:
using new_type = std::common_type<int, char>::type;
(see http://en.cppreference.com/w/cpp/types/common_type)
A trait would be be special type of metafunction which returns a result convertible to bool. Example:
bool ic = std::is_convertible<char, int>::type;
Note that nowadays, the requirement that a metafunction result have a member name "type" is relaxed so some metafunctions might be defined in such a manner that the ::type can be dropped. Example:
bool ic = std::experimental::is_convertible_v<char, int>;
Related
According to cppreference C++20 now supports floating-point parameters in templates.
I am, however, unable to find any compiler support information on that site as well as on others.
Current gcc trunk just does it, the others are negative.
I would just like to know if this is a very low-priortity feature and/or when to expect it to become commonly supported.
The only related thing I can find is:
P0732R2 Class types in non-type template parameters. Kudos if anyone could briefly explain that instead.
It seems that the real question that can be answered here is about the history of this feature, so that whatever compiler support can be understood in context.
Limitations on non-type template parameter types
People have been wanting class-type non-type template parameters for a long time. The answers there are somewhat lacking; what really makes support for such template parameters (really, of non-trivial user-defined types) complicated is their unknown notion of identity: given
struct A {/*...*/};
template<A> struct X {};
constexpr A f() {/*...*/}
constexpr A g() {/*...*/}
X<f()> xf;
X<g()> &xg=xf; // OK?
how do we decide whether X<f()> and X<g()> are the same type? For integers, the answer seems intuitively obvious, but a class type might be something like std::vector<int>, in which case we might have
// C++23, if that
using A=std::vector<int>;
constexpr A f() {return {1,2,3};}
constexpr A g() {
A ret={1,2,3};
ret.reserve(1000);
return ret;
}
and it's not clear what to make of the fact that both objects contain the same values (and hence compare equal with ==) despite having very different behavior (e.g., for iterator invalidation).
P0732 Class types in non-type template parameters
It's true that this paper first added support for class-type non-type template parameters, in terms of the new <=> operator. The logic was that classes that defaulted that operator were "transparent to comparisons" (the term used was "strong structural equality") and so programmers and compilers could agree on a definition of identity.
P1185 <=> != ==
Later it was realized that == should be separately defaultable for performance reasons (e.g., it allows an early exit for comparing strings of different lengths), and the definition of strong structural equality was rewritten in terms of that operator (which comes for free along with a defaulted <=>). This doesn't affect this story, but the trail is incomplete without it.
P1714 NTTP are incomplete without float, double, and long double!
It was discovered that class-type NTTPs and the unrelated feature of constexpr std::bit_cast allowed a floating-point value to be smuggled into a template argument inside a type like std::array<std::byte,sizeof(float)>. The semantics that would result from such a trick would be that every representation of a float would be a different template argument, despite the fact that -0.0==0.0 and (given float nan=std::numeric_limits<float>::quiet_NaN();) nan!=nan. It was therefore proposed that floating-point values be allowed directly as template arguments, with those semantics, to avoid encouraging widespread adoption of such a hacky workaround.
At the time, there was a lot of confusion around the idea that (given template<auto> int vt;) x==y might differ from &vt<x>==&vt<y>), and the proposal was rejected as needing more analysis than could be afforded for C++20.
P1907R0 Inconsistencies with non-type template parameters
It turns out that == has a lot of problems in this area. Even enumerations (which have always been allowed as template parameter types) can overload ==, and using them as template arguments simply ignores that overload entirely. (This is more or less necessary: such an operator might be defined in some translation units and not others, or might be defined differently, or have internal linkage, etc.) Moreover, what an implementation needs to do with a template argument is canonicalize it: to compare one template argument (in, say, a call) to another (in, say, an explicit specialization) would require that the latter had somehow already been identified in terms of the former while somehow allowing the possibility that they might differ.
This notion of identity already differs from == for other types as well. Even P0732 recognized that references (which can also be the type of template parameters) aren't compared with ==, since of course x==y does not imply that &x==&y. Less widely appreciated was that pointers-to-members also violate this correspondence: because of their different behavior in constant evaluation, pointers to different members of a union are distinct as template arguments despite comparing ==, and pointers-to-members that have been cast to point into a base class have similar behavior (although their comparison is unspecified and hence disallowed as a direct component of constant evaluation).
In fact, in November 2019 GCC had already implemented basic support for class-type NTTPs without requiring any comparison operator.
P1837 Remove NTTPs of class type from C++20
These incongruities were so numerous that it had already been proposed that the entire feature be postponed until C++23. In the face of so many problems in so popular a feature, a small group was commissioned to specify the significant changes necessary to save it.
P1907R1 (structural types)
These stories about template arguments of class type and of floating-point type reconverge in the revision of P1907R0 which retained its name but replaced its body with a solution to National Body comments that had also been filed on the same subject. The (new) idea was to recognize that comparisons had never really been germane, and that the only consistent model for template argument identity was that two arguments were different if there was any means of distinguishing them during constant evaluation (which has the aforementioned power to distinguish pointers-to-members, etc.). After all, if two template arguments produce the same specialization, that specialization must have one behavior, and it must be the same as would be obtained from using either of the arguments directly.
While it would be desirable to support a wide range of class types, the only ones that could be reliably supported by what was a new feature introduced (or rather rewritten) at almost the last possible moment for C++20 were those where every value that could be distinguished by the implementation could be distinguished by its clients—hence, only those that have all public members (that recursively have this property). The restrictions on such structural types are not quite as strong as those on an aggregate, since any construction process is permissible so long as it is constexpr. It also has plausible extensions for future language versions to support more class types, perhaps even std::vector<T>—again, by canonicalization (or serialization) rather than by comparison (which cannot support such extensions).
The general solution
This newfound understanding has no relationship to anything else in C++20; class-type NTTPs using this model could have been part of C++11 (which introduced constant expressions of class type). Support was immediately extended to unions, but the logic is not limited to classes at all; it also established that the longstanding prohibitions of template arguments that were pointers to subobjects or that had floating-point type had also been motivated by confusion about == and were unnecessary. (While this doesn't allow string literals to be template arguments for technical reasons, it does allow const char* template arguments that point to the first character of static character arrays.)
In other words, the forces that motivated P1714 were finally recognized as inevitable mathematical consequences of the fundamental behavior of templates and floating-point template arguments became part of C++20 after all. However, neither floating-point nor class-type NTTPs were actually specified for C++20 by their original proposals, complicating "compiler support" documentation.
In Rust, the main tool for abstraction are traits. In C++, there are two tools for abstractions: abstract classes and templates. To get rid of some of the disadvantages of using templates (e.g. hard to read error messages), C++ introduced concepts which are "named sets of requirements".
Both features seem to be fairly similar:
Defining a trait/concept is done by listing requirements.
Both can be used to bound/restrict generic/template type parameters.
Rust traits and C++ templates with concepts are both monomorphized (I know Rust traits can also be used with dynamic dispatch, but that's a different story).
But from what I understand, there are also notable differences. For example, C++'s concepts seem to define a set of expressions that have to be valid instead of listing function signatures. But there is a lot of different and confusing information out there (maybe because concepts only land in C++20?). That's why I'd like to know: what exactly are the differences between and the similarities of C++'s concepts and Rust's traits?
Are there features that are only offered by either concepts or traits? E.g. what about Rust's associated types and consts? Or bounding a type by multiple traits/concepts?
Disclaimer: I have not yet used concepts, all I know about them was gleaned from the various proposals and cppreference, so take this answer with a grain of salt.
Run-Time Polymorphism
Rust Traits are used both for Compile-Time Polymorphism and, sometimes, Run-Time Polymorphism; Concepts are only about Compile-Time Polymorphism.
Structural vs Nominal.
The greatest difference between Concepts and Traits is that Concepts use structural typing whereas Traits use nominal typing:
In C++ a type never explicitly satisfies a Concept; it may "accidentally" satisfy it if it happens to satisfy all requirements.
In Rust a specific syntactic construct impl Trait for Type is used to explicitly indicates that a type implements a Trait.
There are a number of consequences; in general Nominal Typing is better from a maintainability point of view -- adding a requirement to a Trait -- whereas Structural Typing is better a bridging 3rd party libraries -- a type from library A can satisfy a Concept from library B without them being aware of each other.
Constraints
Traits are mandatory:
No method can be called on a variable of a generic type without this type being required to implement a trait providing the method.
Concepts are entirely optional:
A method can be called on a variable of a generic type without this type being required to satisfy any Concept, or being constrained in any way.
A method can be called on a variable of a generic type satisfying a Concept (or several) without that method being specified by any Concept or Constraint.
Constraints (see note) can be entirely ad-hoc, and specify requirements without using a named Concept; and once again, they are entirely optional.
Note: a Constraint is introduced by a requires clause and specifies either ad-hoc requirements or requirements based on Concepts.
Requirements
The set of expressible requirements is different:
Concepts/Constraints work by substitution, so allow the whole breadth of the languages; requirements include: nested types/constants/variables, methods, fields, ability to be used as an argument of another function/method, ability to used as a generic argument of another type, and combinations thereof.
Traits, by contrast, only allow a small set of requirements: associated types/constants, and methods.
Overload Selection
Rust has no concept of ad-hoc overloading, overloading only occurs by Traits and specialization is not possible yet.
C++ Constraints can be used to "order" overloads from least specific to most specific, so the compiler can automatically select the most specific overload for which requirements are satisfied.
Note: prior to this, either SFINAE or tag-dispatching would be used in C++ to achieve the selection; calisthenics were required to work with open-ended overload sets.
Disjunction
How to use this feature is not quite clear to me yet.
The requirement mechanisms in Rust are purely additive (conjunctions, aka &&), in contrast, in C++ requires clauses can contain disjunctions (aka ||).
In C++11, the std::unique_lock constructor is overloaded to accept the type tags defer_lock_t, try_to_lock_t, and adopt_lock_t:
unique_lock( mutex_type& m, std::defer_lock_t t );
unique_lock( mutex_type& m, std::try_to_lock_t t );
unique_lock( mutex_type& m, std::adopt_lock_t t );
These are empty classes (type tags) defined as follows:
struct defer_lock_t { };
struct try_to_lock_t { };
struct adopt_lock_t { };
This allows the user to disambiguate between the three constructors by passing one of the pre-defined instances of these classes:
constexpr std::defer_lock_t defer_lock {};
constexpr std::try_to_lock_t try_to_lock {};
constexpr std::adopt_lock_t adopt_lock {};
I am surprised that this is not implemented as an enum. As far as I can tell, using an enum would:
be simpler to implement
not change the syntax
allow the argument to be changed at runtime (albeit not very useful in this case).
(probably) could be inlined by the compiler with no performance hit
Why does the standard library use type tags, instead of an enum, to disambiguate these constructors? Perhaps more importantly, should I also prefer to use type tags in this situation when writing my own C++ code?
Tag dispatching
It is a technique known as tag dispatching. It allows the appropriate constructor to be called given the behaviour required by the client.
The reason for tags is that the types used for the tags are thus unrelated and will not conflict during overload resolution. Types (and not values as in the case of enums) are used to resolve overloaded functions. In addition, tags can be used to resolve calls that would otherwise have been ambiguous; in this case the tags would typically be based on some type trait(s).
Tag dispatching with templates means that only code that is required to be used given the construction is required to be implemented.
Tag dispatching allows for easier reading code (in my opinion at least) and simpler library code; the constructor won't have a switch statement and the invariants can be established in the initialiser list, based on these arguments, before executing the constructor itself. Sure, your milage may vary but this has been my general experience using tags.
Boost.org has a write up on the tag dispatching technique. It has a history of use that seems to go back at least as far as the SGI STL.
Why use it?
Why does the standard library use type tags, instead of an enum, to disambiguate these constructors?
Types would be more powerful and flexible when used during overload resolution and the possible implementation than enums; bear in mind the enums were originally unscoped and limited in how they could be used (by contrast to the tags).
Additional noteworthy reasons for tags;
Compile time decisions can be made over which constructor to use, and not runtime.
Disallows more "hacky" code where a integer is cast to the enum type with a value that is not catered for - design decisions would need to be made out to handle this and then code implemented to cater for any resultant exceptions or errors.
Keep in mind that the shared_lock and lock_guard also use these tags, but in the case of the lock_guard, only the adopt_lock is used. An enumeration would introduce more potential error conditions.
I think precedence and history also plays a role here. Given the wide spread use in the Standard Library and elsewhere; it is unlikely to change how situations, such as the original example, are implemented in the library.
Perhaps more importantly, should I also prefer to use type tags in this situation when writing my own C++ code?
This is essentially a design decision. Both can and should be used to target the problems they solve. I have used tags to "route" data and types to the correct function; particular when the implementation would be incompatible at compile time and if there are any overload resolutions in play.
The Standard Library std::advance is often given as an example of how tag dispatching can be used to implement and optimise an algorithm based on traits (or characteristics) of the types used (in this case when the iterators are random access iterators).
It is a powerful technique when used appropriately and should not be ignored. If using enums, favour the newer scoped enums over the older unscoped ones.
Using these tags enables you to take advantage of the type system of the language. This is closely related to template meta-programming. Simply speaking, using these tags allows the dispatch decision concerning which constructor to invoke to be made statically at compile time. This leaves room for compiler optimization, improves run-time efficiency, and makes template meta-programming with std::unique_lock easier. This is possible, because the tags are of different static types. With an enum, this cannot be done, for the value of an enum cannot be foreseen at compile time. Note that, using tags for differentiating purposes is a common template meta-programming technique. Just see those iterator tags used by the standard library.
The point is that if you want to add another function using enum, you should edit your enum, then rebuild all projects, which use your functions and enum. In addition there will be one function taking enum as argument and using switch or something. This will bring excess code into your application.
Otherwise if you use overloaded functions with tags, you can easily add another tag and add another overloaded function, without touching old ones. This is more back-compatible.
I suspect it was optimization. Notice that using a type (as is) the correct version is selected at compile time. As you point out using an enum is (potentially) selected in some conditional statement (maybe a switch) at run-time.
In many implementations locks are acquired and released at extremely high frequency and maybe designers thought with branch prediction and the implied memory synchronization events that might be a significant issue.
The flaw in my argument (which you also point out) is that the constructor is likely to be inline and it is likely that the condition would be optimized away anyway.
Notice that using 'dummy' parameters is the closest possible analogue to actually providing named constructors.
This method is called tag dispatching (I may be wrong). Enum type with different values is just one type in compile time and enum values can't be used to overload constructor. So with enum it will be one constructor with switch statement in it. Tag dispatching is equivalent to switch statement in compile time. Each tag type specify: what this constructor would do, how it will try to acquire the lock. You should use type tags, when you want to make decision in compile time and use enum to make decision in run-time.
Because, in std::unique_lock<Mutex>, you don't want to force Mutex to have a lock or try_lock method if it may never need to be called.
If it accepted an enum parameter, then both of those methods would need to be present.
Will it be possible to specialize std::optional for user-defined types? If not, is it too late to propose this to the standard?
My use case for this is an integer-like class that represents a value within a range. For instance, you could have an integer that lies somewhere in the range [0, 10]. Many of my applications are sensitive to even a single byte of overhead, so I would be unable to use a non-specialized std::optional due to the extra bool. However, a specialization for std::optional would be trivial for an integer that has a range smaller than its underlying type. We could simply store the value 11 in my example. This should provide no space or time overhead over a non-optional value.
Am I allowed to create this specialization in namespace std?
The general rule in 17.6.4.2.1 [namespace.std]/1 applies:
A program may add a template specialization for any standard library template to namespace std only if the declaration depends on a user-defined type and the specialization meets the standard library requirements for the original template and is not explicitly
prohibited.
So I would say it's allowed.
N.B. optional will not be part of the C++14 standard, it will be included in a separate Technical Specification on library fundamentals, so there is time to change the rule if my interpretation is wrong.
If you are after a library that efficiently packs the value and the "no-value" flag into one memory location, I recommend looking at compact_optional. It does exactly this.
It does not specialize boost::optional or std::experimental::optional but it can wrap them inside, giving you a uniform interface, with optimizations where possible and a fallback to 'classical' optional where needed.
I've asked about the same thing, regarding specializing optional<bool> and optional<tribool> among other examples, to only use one byte. While the "legality" of doing such things was not under discussion, I do think that one should not, in theory, be allowed to specialize optional<T> in contrast to eg.: hash (which is explicitly allowed).
I don't have the logs with me but part of the rationale is that the interface treats access to the data as access to a pointer or reference, meaning that if you use a different data structure in the internals, some of the invariants of access might change; not to mention providing the interface with access to the data might require something like reinterpret_cast<(some_reference_type)>. Using a uint8_t to store a optional-bool, for example, would impose several extra requirements on the interface of optional<bool> that are different to the ones of optional<T>. What should the return type of operator* be, for example?
Basically, I'm guessing the idea is to avoid the whole vector<bool> fiasco again.
In your example, it might not be too bad, as the access type is still your_integer_type& (or pointer). But in that case, simply designing your integer type to allow for a "zombie" or "undetermined" value instead of relying on optional<> to do the job for you, with its extra overhead and requirements, might be the safest choice.
Make it easy to opt-in to space savings
I have decided that this is a useful thing to do, but a full specialization is a little more work than necessary (for instance, getting operator= correct).
I have posted on the Boost mailing list a way to simplify the task of specializing, especially when you only want to specialize some instantiations of a class template.
http://boost.2283326.n4.nabble.com/optional-Specializing-optional-to-save-space-td4680362.html
My current interface involves a special tag type used to 'unlock' access to particular functions. I have creatively named this type optional_tag. Only optional can construct an optional_tag. For a type to opt-in to a space-efficient representation, it needs the following member functions:
T(optional_tag) constructs an uninitialized value
initialize(optional_tag, Args && ...) constructs an object when there may be one in existence already
uninitialize(optional_tag) destroys the contained object
is_initialized(optional_tag) checks whether the object is currently in an initialized state
By always requiring the optional_tag parameter, we do not limit any function signatures. This is why, for instance, we cannot use operator bool() as the test, because the type may want that operator for other reasons.
An advantage of this over some other possible methods of implementing it is that you can make it work with any type that can naturally support such a state. It does not add any requirements such as having a move constructor.
You can see a full code implementation of the idea at
https://bitbucket.org/davidstone/bounded_integer/src/8c5e7567f0d8b3a04cc98142060a020b58b2a00f/bounded_integer/detail/optional/optional.hpp?at=default&fileviewer=file-view-default
and for a class using the specialization:
https://bitbucket.org/davidstone/bounded_integer/src/8c5e7567f0d8b3a04cc98142060a020b58b2a00f/bounded_integer/detail/class.hpp?at=default&fileviewer=file-view-default
(lines 220 through 242)
An alternative approach
This is in contrast to my previous implementation, which required users to specialize a class template. You can see the old version here:
https://bitbucket.org/davidstone/bounded_integer/src/2defec41add2079ba023c2c6d118ed8a274423c8/bounded_integer/detail/optional/optional.hpp
and
https://bitbucket.org/davidstone/bounded_integer/src/2defec41add2079ba023c2c6d118ed8a274423c8/bounded_integer/detail/optional/specialization.hpp
The problem with this approach is that it is simply more work for the user. Rather than adding four member functions, the user must go into a new namespace and specialize a template.
In practice, all specializations would have an in_place_t constructor that forwards all arguments to the underlying type. The optional_tag approach, on the other hand, can just use the underlying type's constructors directly.
In the specialize optional_storage approach, the user also has the responsibility of adding proper reference-qualified overloads of a value function. In the optional_tag approach, we already have the value so we do not have to pull it out.
optional_storage also required standardizing as part of the interface of optional two helper classes, only one of which the user is supposed to specialize (and sometimes delegate their specialization to the other).
The difference between this and compact_optional
compact_optional is a way of saying "Treat this special sentinel value as the type being not present, almost like a NaN". It requires the user to know that the type they are working with has some special sentinel. An easily specializable optional is a way of saying "My type does not need extra space to store the not present state, but that state is not a normal value." It does not require anyone to know about the optimization to take advantage of it; everyone who uses the type gets it for free.
The future
My goal is to get this first into boost::optional, and then part of the std::optional proposal. Until then, you can always use bounded::optional, although it has a few other (intentional) interface differences.
I don't see how allowing or not allowing some particular bit pattern to represent the unengaged state falls under anything the standard covers.
If you were trying to convince a library vendor to do this, it would require an implementation, exhaustive tests to show you haven't inadvertently blown any of the requirements of optional (or accidentally invoked undefined behavior) and extensive benchmarking to show this makes a notable difference in real world (and not just contrived) situations.
Of course, you can do whatever you want to your own code.
I want to explain the question in detail. In many languages with strong type systems (like Felix, Ocaml, Haskell) you can define a polymorphic list by composing type constructors. Here's the Felix definition:
typedef list[T] = 1 + T * list[T];
typedef list[T] = (1 + T * self) as self;
In Ocaml:
type 'a list = Empty | Cons ('a, 'a list)
In C, this is recursive but neither polymorphic nor compositional:
struct int_list { int elt; struct int_list *next; };
In C++ it would be done like this, if C++ supported type recursion:
struct unit {};
template<typename T>
using list<T> = variant< unit, tuple<T, list<T>> >;
given a suitable definition for tuple (aka pair) and variant (but not the broken one used in Boost). Alternatively:
using list<T> = variant< unit, tuple<T, &list<T>> >;
might be acceptable given a slightly different definition of variant. It was not possible to even write this in C++ < C++11 because without template typedefs, there's no way to get polymorphism, and without a sane syntax for typedefs, there's no way to get the target type in scope. The using syntax above solves both these problems, however this does not imply recursion is permitted.
In particular please note that allowing recursion has a major impact on the ABI, i.e. on name mangling (it can't be done unless the name mangling scheme allows for representation of fixpoints).
My question: is required to work in C++11?
[Assuming the expansion doesn't result in an infinitely large struct]
Edit: just to be clear, the requirement is for general structural typing. Templates provide precisely that, for example
pair<int, double>
pair<int, pair <long, double> >
are anonymously (structurally) typed, and pair is clearly polymorphic. However recursion in C++ < C++11 cannot be stated, not even with a pointer. In C++11 you can state the recursion, albeit with a template typedef (with the new using syntax the expression on the LHS of the = sign is in scope on the RHS).
Structural (anonymous) typing with polymorphism and recursion are minimal requirements for a type system.
Any modern type system must support polynomial type functors or the type system is too clumbsy to do any kind of high level programming. The combinators required for this are usually stated by type theoreticians like:
1 | * | + | fix
where 1 is the unit type, * is tuple formation, + is variant formation, and fix is recursion. The idea is simply that:
if t is a type and u is a type then t + u and t * u are also types
In C++, struct unit{} is 1, tuple is *, variant is + and fixpoints might be obtained with the using = syntax. It's not quite anonymous typing because the fixpoint would require a template typedef.
Edit: Just an example of polymorphic type constructor in C:
T* // pointer formation
T (*)(U) // one argument function type
T[2] // array
Unfortunately in C, function values aren't compositional, and pointer formation is subject to lvalue constraint, and the syntactic rules for type composition are not themselves compositional, but here we can say:
if T is a type T* is a type
if T and U are types, T (*)(U) is a type
if T is a type T[2] is a type
so these type constuctors (combinators) can be applied recursively to get new types without having to create a new intermediate type. In C++ we can easily fix the syntactic problem:
template<typename T> using ptr<T> = T*;
template<typename T, typename U> using fun<T,U> = T (*)(U);
template<typename T> using arr2<T> = T[2];
so now you can write:
arr2<fun<double, ptr<int>>>
and the syntax is compositional, as well as the typing.
No, that is not possible. Even indirect recursion through alias templates is forbidden.
C++11, 4.5.7/3:
The type-id in an alias template declaration shall not refer to the alias template being declared. The type produced by an alias template specialization shall not directly or indirectly make use of that specialization. [ Example:
template <class T> struct A;
template <class T> using B = typename A<T>::U;
template <class T> struct A {
typedef B<T> U;
};
B<short> b; // error: instantiation of B<short> uses own type via A<short>::U
— end example ]
If you want this, stick to your Felix, Ocaml, or Haskell. You will easily realize that very few (none?) sucessful languages have type systems as rich as those three. And in my opinion, if all languages were the same, learning new ones wouldn't be worth it.
template<typename T>
using list<T> = variant< unit, tuple<T, list<T>> >;
In C++ doesn't work because an alias template doesn't define a new type. It's purely an alias, a synonym, and it is equivalent to its substitution. This is a feature, btw.
That alias template is equivalent to the following piece of Haskell:
type List a = Either () (a, List a)
GHCi rejects this because "[cycles] in type synonym declarations" are not allowed. I'm not sure if this is outright banned in C++, or if it is allowed but causes infinite recursion when substituted. Either way, it doesn't work.
The way to define new types in C++ is with the struct, class, union, and enum keywords. If you want something like the following Haskell (I insist on Haskell examples, because I don't know the other two languages), then you need to use those keywords.
newtype List a = List (Either () (a, List a))
I think you may need to review your type theory, as several of your assertions are incorrect.
Let's address your main question (and backhanded point) - as others have pointed out type recursion of the type you requested is not allowed. This does not mean that c++ does not support type recursion. It supports it perfectly well. The type recursion you requested is type name recursion, which is a syntactic flair that actually has no consequence on the actual type system.
C++ allows tuple membership recursion by proxy. For instance, c++ allows
class A
{
A * oneOfMe_;
};
That is type recursion that has real consequences. (And obviously no language can do this without internal proxy representation because size is infinitely recursive otherwise).
Also C++ allows translationtime polymorphism, which allow for the creation of objects that act like any type you may create using name recursion. The name recursion is only used to unload types to members or provide translationtime behavior assignments in the type system. Type tags, type traits, etc. are well known c++ idioms for this.
To prove that type name recursion does not add functionality to a type system, it only needs to be pointed out that c++'s type system allows a fully Turing Complete type calculation, using metaprogramming on compiletime constants (and typelists of them), through simple mapping of names to constants. This means there is a function MakeItC++:YourIdeaOfPrettyName->TypeParametrisedByTypelistOfInts that makes any Turing computible typesystem you want.
As you know, being a student of type theory, variants are dual to tuple products. In the type category, any property of variants has a dual property of tuple products with arrows reversed. If you work consistently with the duality, you do not get properties with "new capabilities" (in terms of type calculations). So on the level of type calculations, you obviously don't need variants. (This should also be obvious from the Turing Completeness.)
However, in terms of runtime behavior in an imperative language, you do get different behavior. And it is bad behavior. Whereas products restrict semantics, variants relax semantics. You should never want this, as it provably destroys code correctness. The history of statically typed programming languages has been moving towards greater and greater expression of the semantics in the type system, with the goal that the compiler should be able to understand when the program does not mean what you want it to. The goal has been to turn the compiler into the program verification system.
For instance, with type units, you can express that a particular value isn't just an int but is actually an acceleration measured in meters per square seconds. Assigning a value that is a velocity expressed in feet per hour divided by a timespan of minutes shouldn't just divide the two values - it should note that a conversion is necessary (and either perform it or fail compilation or... do the right thing). Assinging a force should fail compilation. Doing these kinds of checks on program meaning could have given us potentially more martian exploration, for instance.
Variants are the opposite direction. Sure, "if you code correctly, they work correctly", but that's not the point with code verification. They provably add code loci where a different engineer, unfamiliar with current type usage, can introduce the incorrect semantic assumption without translation failure. And, there is always a code transformation that changes an imperative code section from one that uses Variants unsafely to one that use semantically validated non-variant types, so their use is also "always suboptimal".
The majority of runtime uses for variants are typically those that are better encapsulated in runtime polymorphism. Runtime polymorphism has a statically verified semantics that may have associated runtime invariant checking and unlike variants (where the sum type is universally declared in one code locus) actually supports the Open-Closed principle. By needing to declare a variant in one location, you must change that location everytime you add a new functional type to the sum. This means that code never closes to change, and therefore may have bugs introduced. Runtime polymorphism, though, allows new behaviors to be added in separate code loci from the other behaviors.
(And besides, most real language type systems are not distributive anyway. (a, b | c) =/= (a, b) | (a, c) so what is the point here?)
I would be careful making blanket statements about what makes a type system good without getting some experience in the field, particularly if your point is to be provocative and political and enact change. I do not see anything in your post that actually points to healthy changes for any computer language. I do not see features, safety, or any of the other actual real-world concerns being addressed. I totally get the love of type theory. I think every computer scientist should know Cateogry Theory and the denotational semantics of programming languages (domain theory, cartesian categories, all the good stuff). I think if more people understood the Curry-Howard isomorphism as an ontological manifesto, constructivist logics would get more respect.
But none of that provides reasons to attack the c++ type system. There are legitimate attacks for nearly every language - type name recursion and variant availability are not them.
EDIT: Since my point about Turing completeness does not seem to be understood, nor my comment about the c++ way of using type tags and traits to offload type calculations, maybe an example is in order.
Now the OP claims to want this in a usage case for lists, which my earlier point on the layout easily handles. Better, just use std::list. But from other comments and elsewhere, I think they really want this to work on the Felix->C++ translation.
So, what I think the OP thinks they want is something like
template <typename Type>
class SomeClass
{
// ...
};
and then be able to build a type
SomeClass< /*insert the SomeClass<...> type created here*/ >
I've mentioned this is just a naming convention wanted. Nobody wants typenames - they are transients of the translation process. What is actually wanted is what you will do with Type later on in the structural composition of the type. It will be used in typename calculations to produce member data and method signatures.
So, what can be done in c++ is
struct SelfTag {};
Then, when you want to refer to self, just put this type tag there.
When it's meaningful to do the type calculation, you have a template specialisation on SelfTag that will substitute SomeClass<SelfTag> instead of substituting SelfTag in the appropriate place of the type calculation.
My point here is that the c++ type system is Turing Complete - and that means a lot more than what I think the OP is reading everytime I've written that. Any type calculation may be done (given constraints of compiler recursion) and that really does mean that if you have a problem in one type system in a completely different language, you can find a translation here. I hope this makes things even clearer about my point. Coming back and saying "well you still can't do XYZ in the type system" would be clearly missing the point.
C++ does have the "curiously recurring template pattern", or CRTP. It's not specific to C++11, however. It means you can do the following (shamelessly copied from Wikipedia):
template <typename T>
struct base
{
// ...
};
struct derived : base<derived>
{
// ...
};
#jpalcek answered my question. However, my actual problem (as hinted at in the examples) can be solved without recursive aliases like this:
// core combinators
struct unit;
struct point;
template<class T,class U> struct fix;
template<class T, class U> struct tup2;
template<class T, class U> struct var2;
template <> struct
fix<
point,
var2<unit, tup2<int,point> >
>
{
// definition goes here
};
using the fix and point types to represent recursion. I happen not to require any of the templates to be defined, I only need to define the specialisations. What I needed was a name that would be the same in two distinct translation units for external linkage: the name had to be a function of the type structure.
#Ex0du5 prompted thinking about this. The actual solution is also related to a correspondence from Gabriel des Rois many years ago. I want to thank everyone that contributed.