Constexpr variable and division

Question

I'm trying to evaluate this simple expression at compile time using C++11 new constexpr feature:

template <int a, int b>
class Test
{
   static constexpr double c = a / b;
};

But here's what Clang keeps telling me:

Constexpr variable 'c' must be initialized by a constant expression

The weird thing is that the following compiles well:

template <int a, int b>
class Test
{
   static constexpr double c = a / 2.f;
};

Do you guys have any idea on why a/b is not a constant expression, and how could I evaluate this at compile time?

Using Clang compiler with -std=c++1y and -stdlib=libc++

Update

The following example causes the error with the original code:

Test<10,0> test1 ;

while:

Test<10,1> test1 ;

does not.

[Works with Clang 3.5](http://coliru.stacked-crooked.com/a/e7e6dc72912ed4b5), and with my install of 3.7. — chris, Mar 23 '15 at 23:35
Unrelated but: You want to divide two integers and store the result as a double. The division result will be an integer, e.g. 1/2 is not 0.5 but 0. Is this really what you had in mind? — usr1234567, Mar 24 '15 at 05:51
@usr1234567 you are right, I forgot the cast in the example. — static_cats, Mar 24 '15 at 11:12
I am retracting my close vote since the OP has clarified the source of the problem this is now reproducible. — Shafik Yaghmour, Mar 24 '15 at 16:59
@chris the OP clarified and it is now clear this failed due to undefined behavior. — Shafik Yaghmour, Mar 24 '15 at 17:00
@ShafikYaghmour, I really should have seen that coming, thanks. — chris, Mar 24 '15 at 17:02

score 5 · Accepted Answer · edited May 23 '17 at 12:06

The reason why:

Test<10,0> test1 ;

fails is because you have undefined behavior due to division by zero. This is covered in the draft C++ standard section 5.6 [expr.mul] which says:

If the second operand of / or % is zero the behavior is undefined

and constant expressions specifically exclude undefined behavior. I am not sure what version of clang you are using but the versions I have available online do provide a divide by zero warning (see it live):

note: division by zero
static constexpr double c = a / b;
                              ^

score 2 · Answer 2 · answered Mar 24 '15 at 11:21

2

Solved. One of the template instance had b=0. Somehow, Clang didn't warn me I was dividing by zero. And +Inf is not a constant expression.

answered Mar 24 '15 at 11:21

static_cats

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Constexpr variable and division

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