Concepts (C++)

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 a required part of C++20.

Main uses
The main uses of concepts are:


 * introducing type-checking to template programming
 * simplified compiler diagnostics for failed template instantiations
 * selecting function template overloads and class template specializations based on type properties
 * constraining automatic type deduction

Constraint types and usage
There are five different places in a function template signature where a constraint can be used (labeled below as C1 to C5):


 * : A type-constraint. This kind replaces  or   for declaring a type template parameter. When using a concept instead of the former two the type is constraint.
 * : 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.
 * : A constrained placeholder type. The same syntax is available for placeholder variable aka.  variable. C++20 added abbreviated function templates which use   as a placeholder type in the parameter declaration. A constrained placeholder type allows to put constraints on the automatically deduced return type of a function or a variable.
 * : A trailing requires-clause. This form is similar to  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  and   can be used in all kinds of templates.

Example: equality_comparable
The following is a declaration of the concept "equality_comparable" from the  header of a C++20 standard library. This concept is satisfied by any type  such that for lvalues   and   of type , the expressions   and   as well as the reverse   and   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:

Compiler diagnostics
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:

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 ; _Compare = __gnu_cxx::__ops::_Iter_less_iter]': error: no match for 'operator-' (operand types are 'std::_List_iterator ' and 'std::_List_iterator ') 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 ]' note:  concept 'RandomAccessIterator' was not satisfied

Overload resolution
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.

Type deduction
Concepts may be used instead of the unconstrained type deduction placeholder in variable declarations and function return types:

Implementation status
Concepts TS, as specified in ISO/IEC TS 19217:2015, are implemented as an experimental feature in GCC 6. C++20 concepts are fully implemented in GCC 10, MSVC 19.30, and Clang 10.

History
A different form of Concepts, popularly known as "C++0x Concepts", was temporarily accepted into the working paper for C++11 but was removed in 2009. 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).

In contrast to this abandoned proposal, the C++20 version of Concepts is sometimes referred to as "Concepts Lite".

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.

Concepts v1 was merged into the C++20 draft.

"The One Range" version of Range feature that depend on concepts was also merged into C++20.