CODEWITHSUNDEEP
c++templatessequenceexpansionvariable-templates
Jonathan Mee
Jonathan Mee

Reputation: 38919

How do I expand integer_sequence?

I have a function which looks like this:

template <typename T, std::size_t... I>
std::ostream& vector_insert(std::ostream& lhs, const char* delim, const T& rhs, std::index_sequence<I...>) {
    std::ostream_iterator<float> it(lhs, delim);

    ((*it++ = at(rhs, I)), ...);
    return lhs;
}

This is my final attempt and I'm still failing on my expansion of the integer_sequence I'm hoping someone can tell me how to write a line that will effectively expand to:

*it++ = at(rhs, 0U), *it++ = at(rhs, 1U), *it++ = at(rhs, 2U)

Other things I've tried are:

  1. *it++ = at(rhs, I...)
  2. *it++ = at(rhs, I)...
  3. (*it++ = at(rhs, I))...

All of them are giving me the error:

error C3520: I: parameter pack must be expanded in this context

How do I expand this thing?

EDIT:

@AndyG has pointed out that this seems to be a bug.

Upvotes: 3

Views: 957

Answers (1)

Fran&#231;ois Andrieux
Fran&#231;ois Andrieux

Reputation: 29022

This seems like a compiler bug with Visual C++. I'm not aware of any easy fix for it other than simplifying the expression in which the parameter pack is expanded. Converting to a recursive approach seems to reliably work around the problem. For example :

#include <array>
#include <iostream>
#include <iterator>

template <typename T>
const auto& at(const T& v, size_t i) { return v[i]; }

// End of recursion
template<class T>
void vector_insert_impl(std::ostream_iterator<int> &, const char*, const T&)
{}

// Recursion case
template<class T, std::size_t N, std::size_t... I>
void vector_insert_impl(std::ostream_iterator<int> & iter, const char* delim, const T&rhs)
{
    *iter++ = at(rhs, N);

    // Continue the recursion
    vector_insert_impl<T, I...>(iter, delim, rhs);
}

template <typename T, std::size_t... I>
std::ostream& vector_insert(std::ostream& lhs, const char* delim, const T& rhs, std::index_sequence<I...>) 
{
    std::ostream_iterator<int> it(lhs, delim);

    // Call the recursive implementation instead
    vector_insert_impl<T, I...>(it, delim, rhs);

    return lhs;
}

int main() {
    std::array<int, 5> v = { 1, 2, 3, 4, 5 };
    vector_insert(std::cout, " ", v, std::make_index_sequence<v.size()>());
}

Here, the parameter pack I is only expanded in the context of providing template parameters which VC++ has no trouble with.

Upvotes: 2

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