concept是给模板添加限制条件，如果条件不符，代码无法通过编译。类似SFINAE的语法糖。

1. 示例一

#include <algorithm>
#include <iostream>
#include <concepts>
#include <type_traits>template<class T>
concept Int = std::same_as<T, int> || std::same_as<T, unsigned>|| std::same_as<T, long> || std::same_as<T, long long>;template<Int T>T plus(T a, T b){return a + b;
}Int auto minus(Int auto a, Int auto b){return a - b;
}int main()
{int a = 9;int b = 8;std::cout << plus(a, b) << std::endl;    // 17std::cout << plus(99, b) << std::endl;   // 107//std::cout << plus(a+0.3, b) << std::endl;std::cout << minus(a, b) << std::endl;   // 1std::cout << minus(99, b) << std::endl;  // 91
}

2. 示例二

2.1 不使用concept进行限制时，square可以传入数字、也可以传入自定义类型

#include <iostream>template<class T>
T square(const T& val){return val * val;
}class Foo{
public:Foo(int val):val_(val){std::cout << "Foo ctor called.\n";}~Foo(){std::cout << "Foo dtor called.\n";}int operator*(const Foo& foo) const {return val_ * foo.val_;}friend std::ostream& operator<<(std::ostream& os, const Foo& foo){os << foo.val_;return os;}private:int val_;
};int main(){std::cout << square(4) << std::endl;std::cout << square(14u) << std::endl;std::cout << square(1.4f) << std::endl;std::cout << square(1.4) << std::endl;Foo foo(5);std::cout << square(foo) << std::endl;
}

输出：

16
196
1.96
1.96
Foo ctor called.
Foo ctor called.
25
Foo dtor called.
Foo dtor called.

2.2 使用concept限制传入的参数必须是数字

// g++ main.cpp -std=c++20 -O2
#include <iostream>
#include <concepts>
#include <type_traits>template<class T>
concept NumberType = std::is_integral_v<T> || std::is_floating_point_v<T>;NumberType auto square(const NumberType auto& val){return val * val;
}class Foo{
public:Foo(int val):val_(val){std::cout << "Foo ctor called.\n";}~Foo(){std::cout << "Foo dtor called.\n";}int operator*(const Foo& foo) const {return val_ * foo.val_;}friend std::ostream& operator<<(std::ostream& os, const Foo& foo){os << foo.val_;return os;}private:int val_;
};int main(){std::cout << square(4) << std::endl;std::cout << square(14u) << std::endl;std::cout << square(1.4f) << std::endl;std::cout << square(1.4) << std::endl;Foo foo(5);//std::cout << square(foo) << std::endl;  // 编译报错error: no matching function for call to 'square(Foo&)'
}

输出：

16
196
1.96
1.96
Foo ctor called.
Foo dtor called.