How to implement a function that safely cast any larger type to a smaller type in C++ using templates?

Question

I'm trying to write a function that checks if the variable being cast can fit in the destination type, and assert() if not. Right now this is what I came up with. I didn't test it yet. I would like to make the template figure out the type of the varible being passed automatically, with something like typeid, although I don't really know what typeid really is. Is that possible? Also, I don't know much about templates.

template<typename from_T, typename to_T>
static inline to_T safe_cast(from_T variable)
{
    assert(variable >= std::numeric_limits<to_T>::min());
    assert(variable <= std::numeric_limits<to_T>::max());

    return static_cast<to_T>(variable);
}

Well, if that is actually some function that already does this that I don't know of I will be glad to hear.

bolov · Answer 1 · 2021-02-27T02:45:47.013

C++ Core Guidelines already has a gsl::narrow

// narrow() : a checked version of narrow_cast() that throws if the cast changed the value

You can see the Microsoft implementation here

// narrow() : a checked version of narrow_cast() that throws if the cast changed the value
template <class T, class U>
    constexpr T narrow(U u) noexcept(false)
{
    constexpr const bool is_different_signedness =
        (std::is_signed<T>::value != std::is_signed<U>::value);

    const T t = narrow_cast<T>(u);

    if (static_cast<U>(t) != u || (is_different_signedness && ((t < T{}) != (u < U{}))))
    {
        throw narrowing_error{};
    }

    return t;
}

You can see the explanation of the implementation on this SO post (it's for an older version of the implementation, but nothing substantially changed, so the answer still applies).

How to implement a function that safely cast any larger type to a smaller type in C++ using templates?

1 Answers1