Create safer templates with concepts and constraints

Templates are great for writing code that works with different types. For example, this function will work with any numeric type:

template <typename T>
T arg42(const T & arg) {
    return arg + 42;
}

But what happens when you try to call it with a non-numeric type?

const char * n = "7";
cout << "result is " << arg42(n) << "\n";

Output:

Result is ion

This compiles and runs without error, but the result is unpredictable. In fact, the call is dangerous and it could easily crash or become a vulnerability. I would much prefer the compiler generate an error message so I can fix the code.

Now, with concepts, I can write it like this:

template <typename T>
requires Numeric<T>
T arg42(const T & arg) {
    return arg + 42;
}

The requires keyword is new for C++20. It applies constraints to a template....