How to pass constexpr as a template parameter?

Question

I have a templated class MyClass and I want to run it for various parameters in order to measure some values. I know the exact parameters before the compilation hence I assume there must be a way of achieving the goal.

My code so far:

template <int T>
class MyClass { /*...*/ };


constexpr int PARAMS[] = {1,2,3 /*, ...*/};
for (constexpr auto& t: PARAMS) {
    MyClass<t> myClass;
    // ... do sth
}

However the compiler (gcc v4.9.2, C++11) doesn't accept that. I also tried using const instead of constexpr which doesn't work as well.

Is it possible to something like this? I really don't want to use macros at all.

The loop is performed at runtime, not at compile-time. Templates are all resolved at compile-time. — Some programmer dude, Dec 06 '15 at 17:19
You need a compile-time loop (which can be implemented via compile-time recursion or variadic template parameter pack expansion). — Constructor, Dec 06 '15 at 17:24

Piotr Skotnicki · Accepted Answer · 2015-12-06T18:12:40.347

6

#include <utility>
#include <cstddef>

constexpr int PARAMS[] = { 1, 2, 3, /*...*/ };

template <int N>
void bar()
{
    MyClass<N> myClass;
    // do sth
}

template <std::size_t... Is>
void foo(std::index_sequence<Is...>)
{
    using dummy = int[];    
    static_cast<void>(dummy{ 0, (bar<PARAMS[Is]>(), 0)... });

    // (bar<PARAMS[Is]>(), ...); since C++1z
}

int main()
{
    foo(std::make_index_sequence<sizeof(PARAMS)/sizeof(*PARAMS)>{});
    //                          <std::size(PARAMS)> since C++1z
    //                          <PARAMS.size()> for std::array<int,N> PARAMS{};
}

DEMO

edited Dec 06 '15 at 18:12

answered Dec 06 '15 at 17:34

Piotr Skotnicki

46,953
7
118
160

Can you explain why the `0`'s are needed? Oh I think I get it now... that's comma operator in there I guess... – Chris Beck Dec 06 '15 at 17:41
@ChrisBeck the 0s will be used to fill that temporary array after pack expansion (the first one is to prevent a 0-length array error), http://stackoverflow.com/a/25683817/3953764 – Piotr Skotnicki Dec 06 '15 at 17:44
`static_cast` is just to suppress unused variable warning? I guess it must be the same as `(void) unused_var` like I have always seen... – Chris Beck Dec 06 '15 at 17:56
@ChrisBeck yes, it's to suppress warnings – Piotr Skotnicki Dec 06 '15 at 17:57

How to pass constexpr as a template parameter?

1 Answers1