Recursive calculation of variable template value

user_634821 0
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This is what I'm tying (and failing) to make work. The idea is that count<N> has to be equal to N. This is a simplified case of a more complex calculation, don't mind the apparent silliness of this code.

template <>
constexpr size_t count<0> = 0;

template <auto N>
constexpr size_t count = 1 + count<static_cast<size_t>(N) - 1>;

int main()
{
    return count<1>;
}

It doesn't compile:

<source>:5:23: error: expected initializer before '<' token

 constexpr size_t count<0> = 0;
                       ^

If I swap the general case with the specialization, it also doesn't compile. Is it even doable with variable templates?

Try it online

c++14 c++17 templates c++
1 Answers

Here's the correct version of the code:

template <auto N>
constexpr size_t count = 1 + count<static_cast<size_t>(N) - 1>;

template <>
constexpr size_t count<static_cast<size_t>(0)> = 0;

Observe the following:

  1. The primary template must be declared before any specializations. No exceptions.
  2. The specializations count<0> and count<static_cast<size_t>(0)> are different because their arguments have different types. In your original code, where you provide an explicit specialization count<0>, this specialization is not used by the recursion that occurs in count<1> as specified in your primary template definition, which is why the infinite recursion occurs.
user_4812670
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