Handling fatal errors (or asserts) in constexpr functions

Question

What should I do in the following function if the argument type T is wrong?

    template <class T>
    constexpr inline size_t GetObjectSize(const T & val)
    {
        if constexpr (std::is_arithmetic<T>::value)
        {
            return sizeof(val);
        }

        if constexpr (std::is_class<T>)
        {
            return 5u;
        }

        //there should be compile time error.
    }

    int * p;
    //compile time error
    GetObjectSize(p);

Possible alternatives are 1) throwing an exception 2) assert 3) static_assert

1) Exception of what type should I throw?

2) it is implementation defined and is not guaranteed to be a costexpr.

3) static_assert(false) always fails independently of T.

In this case perhaps specialization is better? And as always, doing switch or if checks on type is usually an anti-pattern. — Some programmer dude
– Some programmer dude, Commented Feb 25, 2019 at 12:49
@Someprogrammerdude yes, specialization looks better, because I need specialization for static_assert, see the answer. — Alexey Starinsky
– Alexey Starinsky, Commented Feb 25, 2019 at 13:15
@Someprogrammerdude it was not quite correct. static_assert(false) can be used with 'if else if else'. — Alexey Starinsky
– Alexey Starinsky, Commented Feb 25, 2019 at 13:21
@Someprogrammerdude static_assert(false) works with MSVC2017, but with GCC it does not. — Alexey Starinsky
– Alexey Starinsky, Commented Feb 25, 2019 at 14:48

StoryTeller - Unslander Monica · Accepted Answer · 2019-02-25 13:09:35Z

3

static_assert(false) always fails independently of T.

Then make it dependent on T.

template<typename>
struct always_false { enum {value = 0}; };

// ...

if constexpr(...) {
}
else {
    static_assert(always_false<T>::value, "Some useful description");
}

Yes, it's a a magician's trick. But the dry letter of the law is okay with it. Sadly there isn't really a better way to print a useful diagnostic in a dependent branch of an if constexpr.

answered Feb 25, 2019 at 13:09

StoryTeller - Unslander Monica

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6 Comments

Alexey Starinsky Over a year ago

Are you sure static_assert should be in 'else' branch?

StoryTeller - Unslander Monica Over a year ago

@AlexeyStarinsky - Of course. Otherwise it will be a part of all branches.

StoryTeller - Unslander Monica Over a year ago

Moreover, your branches should really be an if else if else chain for this to work.

Alexey Starinsky Over a year ago

Yes, static_assert(false) works if there is 'if else if else'. So the answer is static_assert(false).

StoryTeller - Unslander Monica Over a year ago

@AlexeyStarinsky - Which is why I did not suggest static_assert(false), but to make the condition formally dependent too.

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