alexeykuzmin0 alexeykuzmin0 - 4 months ago 21
C++ Question

How to write a variadic template recursive function?

I'm trying to write a variadic template

constexpr
function which calculates sum of the template parameters given. Here's my code:

template<int First, int... Rest>
constexpr int f()
{
return First + f<Rest...>();
}

template<int First>
constexpr int f()
{
return First;
}

int main()
{
f<1, 2, 3>();
return 0;
}


Unfortunately, it does not compile reporting an error message
error C2668: 'f': ambiguous call to overloaded function
while trying to resolve
f<3,>()
call.

I also tried to change my recursion base case to accept 0 template arguments instead of 1:

template<>
constexpr int f()
{
return 0;
}


But this code also does not compile (message
error C2912: explicit specialization 'int f(void)' is not a specialization of a function template
).

I could extract first and second template arguments to make this compile and work, like this:

template<int First, int Second, int... Rest>
constexpr int f()
{
return First + f<Second, Rest...>();
}


But this does not seem to be the best option. So, the question is: how to write this calculation in an elegant way?

UP: I also tried to write this as a single function:

template<int First, int... Rest>
constexpr int f()
{
return sizeof...(Rest) == 0 ? First : (First + f<Rest...>());
}


And this also does not work:
error C2672: 'f': no matching overloaded function found
.

Answer

Your base case was wrong. You need a case for the empty list, but as the compiler suggests, your second try was not a valid template specialization. One way to define a valid instantiation for zero arguments is to create an overload that accepts an empty list

template<class none = void>
constexpr int f()
{
    return 0;
}
template<int First, int... Rest>
constexpr int f()
{
    return First + f<Rest...>();
}
int main()
{
    f<1, 2, 3>();
    return 0;
}

EDIT: for completeness sake also my first answer, that @alexeykuzmin0 fixed by adding the conditional:

template<int First=0, int... Rest>
constexpr int f()
{
    return sizeof...(Rest)==0 ? First : First + f<Rest...>();
}
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