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本章讨论C++ concept特性。

为什么添加concept特性？

在Theory\Programming-languageDesign-of-programming-language.md#What's new in the programming language中，提出了“首先搞清楚设计者引入这些新特性的意图、这些新特性所能够解决的问题，然后再去了解它们的grammar”，所以在学习concept特性的grammar之前，我们实现要搞清楚c++的designer引入这个feature的原因。关于这个问题，我首先从generic programming的角度来进行分析，我觉得这个角度是最高的、最本质的，后面会结合具体的问题来进行分析。

在 generic programming 中，就已经提出了“concept”的概念，但是C++直到C++20版本才显式地支持它了，在之前的版本，programmer能够通过类似于SFINAE + enable_if + type_trait的方式来间接地表达它，那我们就需要考虑:

1) 为什么添加concept特性?

2) concept相较于 SFINAE + enable_if + type_trait 的优势是什么?

3) concept能否完全地实现 SFINAE + enable_if + type_trait 提供的功能?

NOTE: 前面的这几个问题其实是存在一定的重复的。

Complete support for generic programming

阅读维基百科 generic programming，我们会发现，generic programming中已经提出了concept概念，concept是完整地实现generic programming所必须的，关于这一点，在Generic Programming in ConceptC++中进行了非常详细地说明，下面截取了其中一部分：

ConceptC++ is an extension to the C++ Programming Language that introduces first-class support for Generic Programming. It is the first attempt to introduce complete support for Generic Programming into a mainstream programming language. ConceptC++ is a strong candidate for inclusion into the upcoming ANSI/ISO C++ Standard, dubbed C++0x.

C++20之前，这个语言并没有显式定义concept的语法，C++20引入了这个feature，允许用户显式地定义concept。

Summary

concept是explicit的，它是对named requirement的formal definition，它能够让我们的code更加地expressive、readable。

wikipedia Concepts (C++)

维基百科的Concepts (C++)对concept的介绍是比较简洁清晰的，可以作为prime来进行阅读。

C++ concept是对GP concept的一种实现，同时结合C++的实现进行了很多的扩展。

Concepts are named Boolean predicates on template parameters, evaluated at compile time. A concept may be associated with a template (class template, function template, or member function of a class template), in which case it serves as a constraint: it limits the set of arguments that are accepted as template parameters.

Main uses

NOTE: 原文的main use章节的总结是非常好的

The main uses of concepts are:

1) Introducing type-checking to template programming

NOTE: type requirement是GP的核心内容

2) Simplified compiler diagnostics for failed template instantiations

3) Selecting function template overloads and class template specializations based on type properties

NOTE: static polymorphism，能够替代SFINAE

4) Constraining automatic type deduction

NOTE:

1、automatic type deduction是C++17引入的

Example: EqualityComparable

template<typename T>
concept EqualityComparable = requires(T a, T b) {
    { a == b } -> std::same_as<bool>;
    { a != b } -> std::same_as<bool>;
};

void f(const EqualityComparable auto&); // constrained function template declaration

template <EqualityComparable T>
void f(const T&); // constrained function template declaration

f(42); // OK, int satisfies EqualityComparable

Compiler diagnostics

#include <iostream>
#include <algorithm>
#include <list>


int main() {
    std::list<int> l = {2, 1, 3};
    std::sort(l.begin(), l.end());
}

g++ --std=c++11 test.cpp

编译错误如下：

In file included from /usr/include/c++/4.8.2/algorithm:62:0,
                 from test.cpp:2:
/usr/include/c++/4.8.2/bits/stl_algo.h: In instantiation of ‘void std::sort(_RAIter, _RAIter) [with _RAIter = std::_List_iterator<int>]’:
test.cpp:8:33:   required from here
/usr/include/c++/4.8.2/bits/stl_algo.h:5461:22: 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

NOTE: 在cppreference Constraints and concepts 中对此有说明，因为

Violations of constraints are detected at compile time, early in the template instantiation process, which leads to easy to follow error messages.

Overload resolution

NOTE: concept可以作为 SFINAE and tag dispatching 的替代方案，它能够实现相同的功能

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 auto in variable declarations and function return types:

auto     x1 = f(y); // the type of x1 is deduced to whatever f returns
Sortable auto x2 = f(y); // the type of x2 is deduced, but only compiles if it satisfies Sortable

TO READ

https://www.modernescpp.com/index.php/c-20-concepts-the-details

https://www.drdobbs.com/cpp/the-c0x-remove-concepts-decision/218600111

https://www.modernescpp.com/index.php/c-20-define-the-concept-regular-and-semiregular