c-resources/CPlusPlus20ForProgrammers-m.../examples/ch15/fig15_09.cpp

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// fig15_09.cpp
// Testing custom concepts with static_assert.
#include <iostream>
#include <string>
template<typename T>
concept Numeric = std::integral<T> || std::floating_point<T>;
template<typename T>
auto multiply(T a, T b) {
static_assert(Numeric<T>);
return a * b;
}
int main() {
using namespace std::string_literals;
multiply(2, 5); // OK: int is Numeric
multiply(2.5, 5.5); // OK: double is Numeric
multiply("2"s, "5"s); // error: string is not Numeric
}
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