concepts

Before Concepts

template<typename T>
T myMax(T a, T b) {
    return a < b ? a : b;
}
 
int main() {
    auto m = myMax(1, 2);
    auto m2 = myMax(1, 2.9);
    // error: no matching function for call to 'myMax(int, double)'
    // note:  deduced conflicting types for parameter 'T' ('int' and 'double')
}

After Concepts

• Constraints and concepts (since C++20)

We get a better error message, 讓我們知道到底出了什麼問題。

#include <concepts>
 
template<typename T>
concept supportLessThan = requires (T x) {x < x;};
 
template<typename T>
requires std::copyable<T> && supportLessThan<T>
T myMax(T a, T b) {
    return a < b ? a : b;
}
 
int main() {
    auto m = myMax(1, 2);
    auto m2 = myMax(1, 2.9);
    // error: no matching function for call to 'myMax(int, double)'
    // note: candidate: 'template<class T>  requires (copyable<T>) && (supportLessThan<T>) T myMax(T, T)'
}

#include <iostream>
 
auto is_even(std::integral auto n) {
    return n % 2 == 0;
}
 
int main() {
    std::cout << is_even(30000000000000);
}