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
– Some programmer dude, Commented Dec 6, 2015 at 17:19
You need a compile-time loop (which can be implemented via compile-time recursion or variadic template parameter pack expansion). — Constructor
– Constructor, Commented Dec 6, 2015 at 17:24

Piotr Skotnicki · Accepted Answer · 2015-12-06 18:12:40Z

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 6, 2015 at 18:12

answered Dec 6, 2015 at 17:34

Piotr Skotnicki

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4 Comments

Chris Beck Over a year ago

Can you explain why the 0's are needed? Oh I think I get it now... that's comma operator in there I guess...

Piotr Skotnicki Over a year ago

@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), stackoverflow.com/a/25683817/3953764

Chris Beck Over a year ago

static_cast<void> is just to suppress unused variable warning? I guess it must be the same as (void) unused_var like I have always seen...

Piotr Skotnicki Over a year ago

@ChrisBeck yes, it's to suppress warnings

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