How to eliminate "divide by 0" error in template code

Question

I'm using a pair of integer template parameters to specify a ratio, since I can't use a double as a template parameter. The conversion into a double is protected against divide-by-zero with a ternary. This worked in an earlier version of the compiler, but Visual Studio 2013 gives an error:

error C2124: divide or mod by zero

Here's a simplified version of the code:

template<int B1, int B2>
class MyClass
{
    const double B = (B2 == 0) ? 0.0 : (double) B1 / (double) B2;
    // ...
};

MyClass<0, 0> myobj;

I really want B to be optimized out of expressions that use it when it's zero, so I need the single-line definition. I know I can just use template parameters <0, 1> to get around it, but I wonder if there's a way to just convince the compiler that my expression is safe?

Answer 1

What I'm told worked:

 const double B = (B2 == 0 ? 0.0 : (double) B1) /
                  (B2 == 0 ? 1.0 : (double) B2);

This avoids a reliance on short circuit evaluation preventing the divide by 0; having the conditional selections happen before the division.

---

Original idea / Perhaps something like this...? (I think B should be static const or constexpr, but I'm sure you can sort that...)

template<int B1, int B2>
struct MyClass
{
    const double B = (double) B1 / (double) B2;
};

template <int B1>
struct MyClass<B1, 0>
{
    const double B = 0.0;
};

If there's lots of other stuff you want in MyClass and don't want to duplicate or put in a base etc., you could move the B calculation into a supporting template using the specialisation approach above.

Answer 2

Visual Studio is not able to typecast B1, B2 in ternary operation at compile time, but explicitly casting will work.

template<int B1, int B2>
class MyClass
{
    double d1 = (double)B1;
    double d2 = (double)B2;
    const double B = (B2 == 0) ? 0.0 : d1/d2;
    // ...
};

MyClass<0, 0> myobj;