Concepts are an extension to the templates feature provided by the C++ programming language. Concepts are named Boolean predicates on template parameters, evaluated at compile time. A concept may be associated with a template (class template, function template, member function of a class template, variable template, or alias template), in which case it serves as a constraint: it limits the set of arguments that are accepted as template parameters.
Originally dating back to suggestions for C++11, the original concepts specification has been revised multiple times before formally being standardised in C++20.
The main uses of concepts are:
There are five different places in a function template signature where a constraint can be used (labeled below from 1 through 5):[1]
Concept1: A type-constraint. This kind replaces class or typename for declaring a type template parameter. When using a concept instead of the former two the type is constraint.Concept2: A requires-clause. Whenever a type-constraint does not work, for example, because the concept takes multiple parameters, a requires-clause can be used to apply more elaborated constraints.Concept3, Concept4: A constrained placeholder type. The same syntax is available for placeholder variable aka. auto variable. C++20 added abbreviated function templates which use auto as a placeholder type in the parameter declaration.[2] A constrained placeholder type allows to put constraints on the automatically deduced return type of a function or a variable.Concept5: A trailing requires-clause. This form is similar to Concept2 with one notable exception. A trailing requires-clause can be applied to a function in a class template. This allows the function to remain a regular, template-free function, which can be enabled or disabled depending on the functions trailing requires-clause.The constraint forms Concept1 and Concept2 can be used in all kinds of templates.
The following demonstrates using a concept as an upper bound for inheritance constraints on types, by creating a concept ExtendsPlayer satisfied only by classes which inherit from a base class Player, blocking any type that does not.
using std::is_base_of_v;using std::vector;
class Player ;
template
// T is required to be a type whose inheritance upper bound is Player,// blocking any type that does not inherit from Playertemplate
This is similar to constrained generics in Java, and is equivalent to the following example:
class Player
public class Example
The following is a possible definition of the concept std::equality_comparable from the <concepts> header of the C++ Standard Library. This concept is satisfied by any type T such that for lvalues a and b of type T, the expressions a == b and a != b as well as the reverse b == a and b != a compile, and their results are convertible to a type that satisfies the concept "boolean-testable":
A function template constrained on this concept may be declared as follows:
or
And may be called as usual:
If a programmer attempts to use a template argument that does not satisfy the requirements of the template, the compiler will generate an error. When concepts are not used, such errors are often difficult to understand because the error is not reported in the context of the call, but rather in an internal, often deeply nested, implementation context where the type was used.
For example, requires that its first two arguments be random-access iterators. If an argument is not an iterator, or is an iterator of a different category, an error will occur when attempts to use its parameters as bidirectional iterators. Consider std::list<T>, which implements a doubly-linked list, and is not random-access:
// std::list is typically a doubly-linked list, whose iterators are not random-accesslist
Typical compiler diagnostic without concepts is over 50 lines of output, beginning with a failure to compile an expression that attempts to subtract two iterators:
In instantiation of 'void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = std::_List_iterator<int>; _Compare = __gnu_cxx::__ops::_Iter_less_iter]': error: no match for 'operator-' (operand types are 'std::_List_iterator<int>' and 'std::_List_iterator<int>') std::__lg(__last - __first) * 2, [..]
If concepts are used, the error can be detected and reported in the context of the call:
error: cannot call function 'void std::sort(_RAIter, _RAIter) [with _RAIter = std::_List_iterator<int>]' note: concept 'RandomAccessIterator' was not satisfied
Concepts can be used to choose function template overloads and class template specializations based on properties of their template arguments, as an alternative to SFINAE and tag dispatching. If an argument satisfies more than one concept, the overload associated with the more constrained concept is chosen.
Concepts may be used instead of the unconstrained type deduction placeholder in variable declarations and function return types:
Concepts TS, as specified in ISO/IEC TS 19217:2015, are implemented as an experimental feature in GCC 6.[3] C++20 concepts are fully implemented in GCC 10,[4] MSVC 19.30,[5] and Clang 10.[6]
A different form of Concepts, popularly known as "C++0x Concepts"[7], was temporarily accepted into the working paper for C++11 but was removed in 2009.[8] In addition to concepts themselves, "C++0x Concepts" included concept maps (a feature that could make it possible, for example, for the concept Stack to accept, automatically mapping Stack operations such as to differently named operations on, such as) and axioms (a facility to specify semantic properties such as associativity or commutativity, allowing the compiler to take advantage of these properties without proof). The proposed syntax looked something like the following:
concept TotalOrder
The original proposal was ultimately abandoned due to resolution ambiguity in compilation as well as increased complexity in compile-time behavior, while axioms could not be verified during compilation. In contrast to this abandoned proposal, the C++20 version of Concepts is sometimes referred to as "Concepts Lite".[9]
During the C++ standards committee meeting in March 2016, the evolution working group moved to merge Concepts into the mainline C++17 standard, but the motion was defeated in full committee.[10]
Concepts v1 was merged into the C++20 draft.[11]
"The One Range" version of Range feature that depend on concepts was also merged into C++20.
In C#, a generic type constraint is expressed with a where clause, which can be as expressive as concepts but are not named.[12]
public class MyGenericClass
Java has wildcard generics, which are not as expressive as concepts but can represent bounds on types.[13]
public static
Kotlin, does not support Java-style type wildcards. However, it represents ? instead represented as * (for example, List<*>). It otherwise has C#-style where clauses:[14]
Rust also uses where clauses to bound traits.
struct MyStruct
impl