CODEWITHSUNDEEP
c++functional-programmingc++20
Sam Martin
Sam Martin

Reputation: 73

Why is the pipe operator not working when I use ranges in C++20?

I am currently working through examples from the book Expert C++.

In chapter 7, they offer the following code for mapping a function to a matrix:

#include <vector>
#include <ranges>
#include <iostream>

using IntMatrix = std::vector<std::vector<int>>;

int count_evens(const std::vector<int>& number_line) {
    return std::count_if(number_line.begin(),
                         number_line.end(), [](int num){return num % 2 == 0;});
}

std::vector<int> count_all_evens(const IntMatrix& numbers)
{
    return numbers | std::ranges::views::transform(count_evens); // ERROR APPEARS HERE AT |
}

int main()
{
    IntMatrix m{{1, 2, 3}, {4, 5, 6}};
    for (auto item : count_all_evens(m)) {
        std::cout << item << " ";
    }
    std::cout << std::endl;
    return 0;
}

I am getting an error on line 14 that says:

could not convert 'std::ranges::views::__adaptor::operator|<const std::vector<std::vector<int> >&>((* & numbers), std::ranges::views::transform.std::ranges::views::__adaptor::_RangeAdaptor<std::ranges::views::<lambda(_Range&&, _Fp&&)> >::operator()<int (&)(const std::vector<int, std::allocator<int> >&)>(count_evens))' from 'std::ranges::transform_view<std::ranges::ref_view<const std::vector<std::vector<int> > >, int (*)(const std::vector<int>&)>' to 'std::vector<int>'

Does anyone else have this issue? I am using the g++10 compiler.

Upvotes: 6

Views: 5789

Answers (3)

MarioC
MarioC

Reputation: 41

Today I read a book "Functional Programming in C++" and met the same issue.

After spending a couple of hours, I found out it was related to the library "ranges".

The complete library should be: https://github.com/ericniebler/range-v3 instead of std::ranges. Be careful on the namespaces.

Maybe "C++20 ranges are the core subset of range-v3", they are sometimes mixed up... (Source: https://www.reddit.com/r/cpp/comments/kueb7n/what_are_the_differences_between_boostrange/)

The code worked for me, C++ 14/17/20, with g++ 10.3.0.

#include <iostream>
#include <vector>
#include <iomanip>

#include <range/v3/view/transform.hpp>
#include <range/v3/range/conversion.hpp>

auto vector_plus_one(const std::vector<uint32_t>&) -> std::vector<uint32_t>;

int main(int argc, char* argv[])
{
    std::vector<uint32_t> v_in { 1, 2, 3 };
    for (auto &n : v_in) {
        std::cout << std::setw(5) << n;
    }
    std::cout << std::endl;

    auto v_out = vector_plus_one(v_in);

    for (auto &n : v_out) {
        std::cout << std::setw(5) << n;
    }
    std::cout << std::endl;
    return 0;
}

uint32_t elementwise_plus_one(const uint32_t& input)
{
    return input + 1;
}

std::vector<uint32_t> vector_plus_one(const std::vector<uint32_t>& v_input)
{
    return v_input | ranges::views::transform(elementwise_plus_one) | ranges::to_vector;
}

Upvotes: 4

Ludovic Aubert
Ludovic Aubert

Reputation: 10526

Use ranges::copy()

following code compiles with g++10 (C++20) and outputs

1 2

ranges::copy() accepts as argument a range and an iterator to the beginning of the output.

#include <vector>
#include <ranges>
#include <iostream>
#include <algorithm>
using namespace std;

using IntMatrix = vector<vector<int> >;

int count_evens(const vector<int>& number_line) {
    return std::count_if(number_line.begin(),
                         number_line.end(), [](int num){return num % 2 == 0;});
}

vector<int> count_all_evens(const IntMatrix& numbers)
{
    auto r = numbers | ranges::views::transform(count_evens);
    int n = r.size();
    vector<int> v(n);
    ranges::copy(r, begin(v));
    return v;
}

int main()
{
    IntMatrix m{{1, 2, 3}, {4, 5, 6}};
    for (auto item : count_all_evens(m)) {
        std::cout << item << " ";
    }
    std::cout << std::endl;
    return 0;
}

Upvotes: 2

Yakk - Adam Nevraumont
Yakk - Adam Nevraumont

Reputation: 275510

std::vector<int> count_all_evens(const IntMatrix& numbers)
{
  auto view = numbers | std::ranges::views::transform(count_evens);
  return {view.begin(), view.end()};
}

there is a proposal to make this suck less.

std::vector<int> count_all_evens(const IntMatrix& numbers)
{
  auto view = numbers | std::ranges::views::transform(count_evens);
  return std::ranges::to<std::vector<int>>(view);
}

You can also get fancy

template<class Range>
struct to_container {
  Range&& r;
  template<class Container>
  operator Container()&&{ return {r.begin(), r.end()}; }
};
template<class Range>
to_container(Range&&)->to_container<Range>;

std::vector<int> count_all_evens(const IntMatrix& numbers)
{
  return to_container{ numbers | std::ranges::views::transform(count_evens) };
}

Upvotes: 8

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