Initializing `constexpr` Array with Pattern

Question

I would like to initialize a constexpr array with a pattern that is generated using variadic template parameters. For simplicity, consider the problem of initializing a constexpr unsigned static array with the sizes of a list of types, say, unsigned, short, char, int, long. How can I do this so that all of the computation is done during compile time? I need the solution to play nice with the C++ type system, so I cannot use macros.

The best I could come up with is shown below, ~~but compilation using g++ -std=c++11 -Wall -Ofast -S (using g++ 4.7) and inspection of the assembly clearly reveals that the values are pushed onto the stack during runtime. Any ideas?~~ and works fine.

Using an array initializer as follows would work if I could somehow tell the expansion n +1 about expansion n.

static constexpr unsigned foo[] = { compute_element<Args>::value... };

Edit: Wait, never mind, I had a brainfart. The line above works fine...

Here is the ~~code~~ answer:

#include <iostream>

template <class... Args>
struct foo
{
    static constexpr unsigned bar[] = { sizeof(Args)... };
};

int main()
{
    std::cout << foo<unsigned, short, char, int, long>::bar[2] << std::endl;
    return 0;
}

Thank you very much for your time!

It's encouraged that if you find an answer to your questions to post it as such. :) — Xeo
– Xeo, Commented Jul 16, 2012 at 17:41

void-pointer · Accepted Answer · 2012-07-16 20:21:33Z

5

Here is the answer. Keep in mind that due to limitations in C++, I think that this can only be done in-compile time to create arrays that are of the same size of the variadic template parameter pack.

#include <iostream>

template <class... Args>
struct foo
{
    static constexpr unsigned bar[] = { sizeof(Args)... };
};

int main()
{
    std::cout << foo<unsigned, short, char, int, long>::bar[2] << std::endl;
    return 0;
}

answered Jul 16, 2012 at 20:21

void-pointer

15.1k11 gold badges45 silver badges63 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Initializing `constexpr` Array with Pattern

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related