Explicit instantiation of function template with `using` or otherwise [duplicate]

Question

using for class templates works like a charm

 template<class T,int N> struct VecNT{ T arr[N]; }; using Vec5d = VecNT<double,5>; // doing great job!

but it seems it does not work for functions at all

 template<class T,int N> T sumNT(T* xs){ T sum=0; for(int i=0;i<N;i++){sum+=xs[i];}; return sum; }; using sum5d = sumNT<double,5>; // ERROR: sumNT<double,5> does not name a type using sum5d(double* xs) = sumNT<double,5>(T* xs); // ERROR: expected nest-name-specifier before 'sum5d'

So how to make sum5d as an specialized/instantiated alias for sumNT<double,5> ?

Have you tried e.g. auto sum5d = &sumNT<double,5>;? This should make a function pointer which can be used then like a function ID. (Or double (*sum5d)(double*) = &sumNT<double,5>;?) — Scheff's Cat
– Scheff's Cat, Commented Aug 29, 2019 at 7:08

Alan Birtles · Accepted Answer · 2019-08-29 07:20:16Z

2

You can just declare a function pointer for your alias:

template<class T,int N> T sumNT(T* xs){ T sum=0; for(int i=0;i<N;i++){sum+=xs[i];}; return sum; }; constexpr auto sum5d = &sumNT<double,5>; int main() { double d[5]; sum5d(d); }

GCC and Clang manage to optimise away the function pointer and call the original function directly, MSVC doesn't: https://godbolt.org/z/1_fs83

answered Aug 29, 2019 at 7:20

Alan Birtles

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2 Comments

Prokop Hapala Over a year ago

thanks. Just for curiosity what constexpr exactly does here? Does it help the optimization (like to bake the constant template parameters, and optimize out the function pointer?

Alan Birtles Over a year ago

constexpr is for a constant that the compiler can generate at compile time. Ofent (and in this case) compilers will generate the same code with just const, the main difference is that if you mark something as constexpr which the compiler can't generate at compile time it will fail to compile. see stackoverflow.com/questions/14116003/… and stackoverflow.com/questions/42107744/what-is-constexpr-in-c

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