下面进入正文，以一个比较简单加法为例。
代码语言：javascript复制#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：
代码语言：javascript复制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：
代码语言：javascript复制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：
代码语言：javascript复制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：
代码语言：javascript复制#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。
代码语言：javascript复制// 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：
代码语言：javascript复制template <typename T>
requires Addable<T>
T Add(T a, T b) { ... }
- requires 表达式
例如：
代码语言：javascript复制template <typename T>
concept Addable = requires(T a, T b) { a b; };
1.2.1 放在模版中
函数：
代码语言：javascript复制template <typename T>
requires Addable<T>
T Add(T a, T b) {
return a b;
}
类：
代码语言：javascript复制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：
代码语言：javascript复制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
代码语言：javascript复制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：
代码语言：javascript复制#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：
代码语言：javascript复制auto add(auto x, auto y) {
return x y;
}
我们可以变为：
代码语言：javascript复制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：
代码语言：javascript复制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。
