error: no match for call to '(const std::ranges::__sort_fn)

Question

I was practicing vectors and ranges in c++ 20 was stuck at following state.

#include <iostream>
#include <vector>
#include <random>
#include <ranges>
#include <algorithm>

namespace ranges = std::ranges;

struct Model
{
    double next_event_time;
};

std::vector<Model> generate_examples(int number)
{
    // A uniform random number generator object
    // Used as the source of randomness
    std::default_random_engine generator;

    // Calls () operator on generator to get uniformly-distributed integers
    // then transforms the obtained values to output the disired distribution

    // Will uniformly generate values between 0 ~ 1
    std::uniform_real_distribution<double> distribution(0.0, 1.0);

    std::vector<Model> models;

    for (auto i = 0; i < number; i++)
    {
        models.push_back(Model{.next_event_time = distribution(generator)});
    }

    return models;
}

Model get_next_model(const std::vector<Model> &models)
{
    ranges::sort(models | std::views::transform([](const Model &x) { return x.next_event_time; }));

    return models[0];
}

int main()
{
    std::vector<Model> models = generate_examples(10);

    for (const auto &model : models)
        std::cout << model.next_event_time << std::endl;
}

I compiled the code with g++ 10.2 and got error

 error: no match for call to '(const std::ranges::__sort_fn) ~~~
ranges::sort(models | std::views::transform([](const Model &x) { return x.next_event_time; }));

Instead of std::views::transform, I also tried

1. lambda expression
2. ranges::sort(models, {}, &Model::next_event_time)

But they all produced similar no match for call to error. Why is this happening?

Answer 1

Your function should be as this:

Model get_next_model( std::vector<Model> models)
{

    ranges::sort(models, ranges::less{}, [](const Model &x) { return x.next_event_time; });

    return models[0];
}

There were two problems:

1. You cannot sort const object (so remove const&)
2. The signature of sort requires way of sorting (ranges::less) before projections. And transform has no sense here