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<<
与
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
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👇:
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
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