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C++20:从0到1学懂concept

目录

  • 1.concept语法- 1.1 替换typename- 1.2 requires关键字- 1.4 concept与auto
  • 2.编译器支持
  • 3.总结

C++20引入了concept(概念),是对模板参数(编译时评估)的一组约束。你可以将它们用于类模板和函数模板来控制函数重载和特化。一些优点包括:

  • 对模版参数强制类型约束
  • 提高代码可读性(替换了较长的SFINAE代码)
  • 提供更友好的报错信息
  • 通过限制可以使用的类型来防止意外的模板实例化

往期C++20的系列文章:

1.C++那些事之C++20协程开篇

2.盘点C++20模块那些事

注:本篇所有代码已更新于星球。


下面进入正文,以一个比较简单加法为例。

#include <iostream>

struct Foo {};

template <typename T>
T Add(T a, T b) {
  return a + b;
}

int main() {
  std::cout << Add(1, 2) << std::endl;
  Foo f1, f2;
  std::cout << Add(f1, f2) << std::endl;
  return 0;
}

对于Foo来说,是不支持加法的,于此同时也是不可以直接

std::cout << 

,因此在编译时报一大堆错误,包含

operator<<

operator+

,但这并不是我们期望的错误信息,我们比较期望的是编译器给我们最直观的错误信息,即:这个结构体能不能相加。

add.cc: In function 'int main()':
add.cc:13:13: error: no match for 'operator<<' (operand types are 'std::ostream' {aka 'std::basic_ostream<char>'} and 'Foo')
   13 |   std::cout << Add(f1, f2) << std::endl;
      |   ~~~~~~~~~ ^~ ~~~~~~~~~~~
      |        |          |
      |        |          Foo
      |        std::ostream {aka std::basic_ostream<char>}
In file included from /usr/local/Cellar/gcc/13.2.0/include/c++/13/iostream:41,
                 from add.cc:1:
/usr/local/Cellar/gcc/13.2.0/include/c++/13/ostream:110:7: note: candidate: 'std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(__ostream_type& (*)(__ostream_type&)) [with _CharT = char; _Traits = std::char_traits<char>; __ostream_type = std::basic_ostream<char>]'
  110 |       operator<<(__ostream_type& (*__pf)(__ostream_type&))
  
/usr/local/Cellar/gcc/13.2.0/include/c++/13/ostream:801:5: note:   template argument deduction/substitution failed:
/usr/local/Cellar/gcc/13.2.0/include/c++/13/ostream: In substitution of 'template<class _Ostream, class _Tp> _Ostream&& std::operator<<(_Ostream&&, const _Tp&) [with _Ostream = std::basic_ostream<char>&; _Tp = Foo]':
add.cc:13:26:   required from here
/usr/local/Cellar/gcc/13.2.0/include/c++/13/ostream:801:5: error: no type named 'type' in 'struct std::enable_if<false, void>'
add.cc: In instantiation of 'T Add(T, T) [with T = Foo]':
add.cc:13:19:   required from here
add.cc:7:12: error: no match for 'operator+' (operand types are 'Foo' and 'Foo')
    7 |   return a + b;
      |          ~~^~~

当我们使用concept实现之后:

template <typename T>
concept Addable = requires(T a, T b) { a + b; };

template <typename T>
  requires Addable<T>
T Add(T a, T b) {
  return a + b;
}

便可以得到我们关心的编译错误:

add_concept.cc: In function 'int main()':
add_concept.cc:17:19: error: no matching function for call to 'Add(Foo&, Foo&)'
   17 |   std::cout << Add(f1, f2) << std::endl;
      |                ~~~^~~~~~~~
add_concept.cc:10:3: note: candidate: 'template<class T>  requires  Addable<T> T Add(T, T)'
   10 | T Add(T a, T b) {
      |   ^~~
add_concept.cc:10:3: note:   template argument deduction/substitution failed:
add_concept.cc:10:3: note: constraints not satisfied
add_concept.cc: In substitution of 'template<class T>  requires  Addable<T> T Add(T, T) [with T = Foo]':
add_concept.cc:17:19:   required from here
add_concept.cc:6:9:   required for the satisfaction of 'Addable<T>' [with T = Foo]
add_concept.cc:6:19:   in requirements with 'T a', 'T b' [with T = Foo]
add_concept.cc:6:42: note: the required expression '(a + b)' is invalid
    6 | concept Addable = requires(T a, T b) { a + b; };
      |                                        ~~^~~

下面,我们来针对上面这个例子深入学习concept语法。

1.concept语法

语法:

template <template-parameter-list>
concept concept-name = constraint-expression;

我们来对比一下实际的例子,Addable是concept-name,constraint-expression是

requires(T a, T b) { a + b; }

template <typename T>
concept Addable = requires(T a, T b) { a + b; };

使用方式为:

#include <concepts>

这个concept可以放在多个地方,如下:

  • typename的位置
  • requires后面
  • auto前面

1.1 替换typename

约束模版参数,替换typename。

// template <typename T>  typename->Addable
template <Addable T>
T Add(T a, T b) {
  return a + b;
}

1.2 requires关键字

我们在函数模板中使用 requires 关键字。它可以访问我们的模板T是否是可以相加的,如果模板可以处理相加,它将返回 true。

requires可以放在模版中,也可以放在函数之后,但是不可以放在类之后。于此同时它有两个写法:

  • requires 条件

例如:

template <typename T>
  requires Addable<T>
T Add(T a, T b) { ... }
  • requires 表达式

例如:

template <typename T>
concept Addable = requires(T a, T b) { a + b; };
1.2.1 放在模版中

函数:

template <typename T>
  requires Addable<T>
T Add(T a, T b) {
  return a + b;
}

类:

template <typename T>
 requires Addable<T> 
class Bar {
 public:
  T Add(T a, T b) { return a + b; }
};
1.2.2 函数尾随 Requires 子句

函数:

template <typename T>
T Add(T a, T b)
  requires Addable<T>
{
  return a + b;
}

对于类则不支持这种写法,会报错:error: expected unqualified-id before 'requires' 28 | requires Addable

template <typename T>
class Bar requires Addable<T> 
{
 public:
  T Add(T a, T b) { return a + b; }
};
1.2.3 requires基本示例

以数据库当中的类型为例,数据库中有不同类型,我们将其划分为:null、binary、number等,我们想要对传递的类型执行打印操作,于是有了下面的示例:

#include <concepts>

class NumberType {};
class BaseBinaryType {};
class NullType {};
class FloatingPointType : public NumberType {};
class IntegerType : public NumberType {};
class BinaryType: public BaseBinaryType {};

template <typename T>
  requires std::is_base_of_v<NumberType, T> || std::is_base_of_v<BaseBinaryType, T>
void PrintValue(T v) {}

int main() {
  PrintValue(FloatingPointType{});
  PrintValue(NullType{});
  return 0;
}

对于requires我们可以使用

||

,上面示例中出现了NullType,它不满足

requires

,因此会编译出现:

concept_requires.cc:16:13: error: no matching function for call to 'PrintValue(NullType)'
   16 |   PrintValue(NullType{});
      |   ~~~~~~~~~~^~~~~~~~~~~~
concept_requires.cc:12:6: note: candidate: 'template<class T>  requires (is_base_of_v<NumberType, T>) || (is_base_of_v<BaseBinaryType, T>) void PrintValue(T)'
   12 | void PrintValue(T v) {}
      |      ^~~~~~~~~~
concept_requires.cc:12:6: note:   template argument deduction/substitution failed:
concept_requires.cc:12:6: note: constraints not satisfied
concept_requires.cc: In substitution of 'template<class T>  requires (is_base_of_v<NumberType, T>) || (is_base_of_v<BaseBinaryType, T>) void PrintValue(T) [with T = NullType]':
concept_requires.cc:16:13:   required from here
concept_requires.cc:12:6:   required by the constraints of 'template<class T>  requires (is_base_of_v<NumberType, T>) || (is_base_of_v<BaseBinaryType, T>) void PrintValue(T)'
concept_requires.cc:11:45: note: no operand of the disjunction is satisfied
   11 |   requires std::is_base_of_v<NumberType, T> || std::is_base_of_v<BaseBinaryType, T>
      |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

1.4 concept与auto

当auto出现时,我们可以将其与concept一起使用,例如:

auto add(auto x, auto y) {
  return x + y;
}

我们可以变为:

template <typename T>
concept Addable = requires(T a, T b) { a + b; };

auto add2(Addable auto x, Addable auto y) {
  return x + y;
}

编译时会出现:

concept_auto.cc:17:20: error: no matching function for call to 'add2(Foo&, Foo&)'
   17 |   std::cout << add2(f1, f2) << std::endl;
      |                ~~~~^~~~~~~~
concept_auto.cc:10:6: note: candidate: 'template<class auto:18, class auto:19>  requires (Addable<auto:18>) && (Addable<auto:19>) auto add2(auto:18, auto:19)'
   10 | auto add2(Addable auto x, Addable auto y) {
      |      ^~~~
concept_auto.cc:10:6: note:   template argument deduction/substitution failed:
concept_auto.cc:10:6: note: constraints not satisfied
concept_auto.cc: In substitution of 'template<class auto:18, class auto:19>  requires (Addable<auto:18>) && (Addable<auto:19>) auto add2(auto:18, auto:19) [with auto:18 = Foo; auto:19 = Foo]':
concept_auto.cc:17:20:   required from here
concept_auto.cc:8:9:   required for the satisfaction of 'Addable<auto:18>' [with auto:18 = Foo]
concept_auto.cc:8:19:   in requirements with 'T a', 'T b' [with T = Foo]
concept_auto.cc:8:42: note: the required expression '(a + b)' is invalid
    8 | concept Addable = requires(T a, T b) { a + b; };

2.编译器支持

需要GCC(10.0+),Clang(10.0+),编译选项:

-std=c++20

/

-std=c++2a

https://en.cppreference.com/w/cpp/compiler_support

3.总结

自C++20提供的concept之后,我们不再需要enable_if/SFINAE的机制、函数重载来做一些模版约束检查了,使用concept可以帮你搞定这个操作,它提供了一种更清晰和强大的模板参数约束机制,使得模板代码更易于编写、理解和维护。通过在编译时进行类型检查,它有助于提高代码的稳健性和可读性。

源码获取👇:

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往期回顾:

热度更新,手把手实现工业级线程池

C++ 多值返回:从版本1到版本6秒杀

标签: c++20

本文转载自: https://blog.csdn.net/guangcheng0312q/article/details/135470276
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