c++ - Why can't I zip an lvalue generator? - Stack Overflow

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The following simple program compiles and works:

std::generator<int> get_ints(int n)
{
    for (int i{0}; i < n; ++i)
        co_yield i;
}

int main(int argc, char* argv[])
{
    std::vector<float> floats { 0.0f, 1.0f, 2.0f, 3.0f };

    for (auto [i, f] : std::views::zip(get_ints(4), floats))
        std::println("{}: {}", i, f);
}

However, if I instead store the generator in a variable and attempt the same, it does not compile:

std::generator<int> get_ints(int n)
{
    for (int i{0}; i < n; ++i)
        co_yield i;
}

int main(int argc, char* argv[])
{
    std::vector<float> floats { 0.0f, 1.0f, 2.0f, 3.0f };
    auto ints = get_ints(4);

    for (auto [i, f] : std::views::zip(ints, floats))
        std::println("{}: {}", i, f);
}

From my understanding this fails, because std::ranges::viewable_range<std::generator<int>&> evaluates to false, while std::ranges::viewable_range<std::generator<int>&&> evaluates to true. But why is the first case forbidden? I know that generator is not a range, but it behaves like one and every other range type that I tested (although I only tested a few) satisfies this constraint when passed as an lvalue reference. So why makes generator special in this case?

The following simple program compiles and works:

std::generator<int> get_ints(int n)
{
    for (int i{0}; i < n; ++i)
        co_yield i;
}

int main(int argc, char* argv[])
{
    std::vector<float> floats { 0.0f, 1.0f, 2.0f, 3.0f };

    for (auto [i, f] : std::views::zip(get_ints(4), floats))
        std::println("{}: {}", i, f);
}

However, if I instead store the generator in a variable and attempt the same, it does not compile:

std::generator<int> get_ints(int n)
{
    for (int i{0}; i < n; ++i)
        co_yield i;
}

int main(int argc, char* argv[])
{
    std::vector<float> floats { 0.0f, 1.0f, 2.0f, 3.0f };
    auto ints = get_ints(4);

    for (auto [i, f] : std::views::zip(ints, floats))
        std::println("{}: {}", i, f);
}

From my understanding this fails, because std::ranges::viewable_range<std::generator<int>&> evaluates to false, while std::ranges::viewable_range<std::generator<int>&&> evaluates to true. But why is the first case forbidden? I know that generator is not a range, but it behaves like one and every other range type that I tested (although I only tested a few) satisfies this constraint when passed as an lvalue reference. So why makes generator special in this case?

• c++
• generator
• std
• std-ranges
• c++23

Share Improve this question Follow edited Jan 14 at 17:12 TylerH 21.1k7777 gold badges7979 silver badges112112 bronze badges asked Dec 16, 2024 at 18:12 CarstenCarsten 11.6k77 gold badges4343 silver badges6565 bronze badges

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1 Answer 1

Sorted by: Reset to default 8

std::generator is a move-only range with a deleted copy constructor, you need to move it into the zip_view:

for (auto [i, f] : std::views::zip(std::move(ints), floats))
    std::println("{}: {}", i, f);

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