Initialize array of pointers to member functions at compile time using constexpr

Question

I wanted to write the following code but got an error "no matches converting function 'handler' to type 'void (struct Parser::*)()". I use g++ 5.3.1 with options -std=c++14

With -std=17 I can use consrexpr T& std::array::operator[] (...) but now I wrote my class array.

What is wrong in this code?

#include <cstddef>
template <typename T, size_t _size> struct array {
T elements[_size];
constexpr size_t size() const noexcept { return _size; }
constexpr bool empty() const noexcept { return size() == 0; }
constexpr T& operator[](size_t index) noexcept { return elements[index]; }
};

struct Parser {
template <size_t index> void handler ();
};

template<> void Parser::handler<0> () {
}

template<> void Parser::handler<1> () {
}


constexpr auto createArrayHandlers () {
    array <void (Parser::*)(), 2> ans{&Parser::handler<0>, &Parser::handler<1>};
    for (size_t i = 0; i < ans.size(); ++i) {
        ans[i] = &Parser::handler<0>; //ok 
        ans[i] = &Parser::handler<i>; //error
    }
    return ans;
}
constexpr auto table = createArrayHandlers();
int main() {
    //table[parse_cmd(read_from_socket())]();
    return 0;
}

Now i have replaced this code with the following, but it executes at runtime

std::array<void (Parser::*)(), xxx> table {};
template <size_t i> size_t fillMemeberHandlerArray () {
    fillMemeberHandlerArray<i-1>();
    table[i] = &Parser::handler<i>;
    return i;
}

template<> size_t fillMemeberHandlerArray<0> () {
    table[0] = &Parser::handler<0>;
    return 0;
}
size_t const initTable = fillMemeberHandlerArray<table.size () - 1>();

Template parameter should be constant expression, i (from for) is not a constant expression. you may use std::index_sequence to initialize you array. — Jarod42
– Jarod42, Commented Apr 14, 2016 at 7:56

ildjarn · Accepted Answer · 2016-04-14 07:50:51Z

2

#include <utility>
#include <array>

namespace detail {
    template<std::size_t... Is>
    constexpr auto createArrayHandlers(std::index_sequence<Is...>) noexcept
     -> std::array<void(Parser::*)(), sizeof...(Is)>
    {
        return {{&Parser::handler<Is>...}};
    }
}

constexpr auto createArrayHandlers() noexcept
{
    return detail::createArrayHandlers(std::make_index_sequence<2>{});
}

constexpr auto table = createArrayHandlers();

Online Demo

As this does not use operator[] of the container to populate it, this works fine with std::array<> instead of rolling an approximation yourself.

answered Apr 14, 2016 at 7:50

ildjarn

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Comments

Jarod42 · Accepted Answer · 2016-04-14 07:59:54Z

1

Template parameter should be constant expression, i (from for) is not a constant expression.

You may do

constexpr array<void (Parser::*)(), 2> createArrayHandlers () {
     return {{&Parser::handler<0>, &Parser::handler<1>}};
}

or use std::index_sequence to handle several compile time values.

answered Apr 14, 2016 at 7:59

Jarod42

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