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

Question

我在练习vectors，ranges在c++ 20卡在了下面的状态。

#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;
}

我用 g++ 10.2 编译了代码并得到了错误

 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; }));

而不是std::views::transform，我也试过

lambda 表达式
ranges::sort(models, {}, &Model::next_event_time)

但它们都产生了类似的 no match for call to 错误。为什么会这样？

Answer 1

你的函数应该是这样的：

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

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

    return models[0];
}

有两个问题：

您无法对 const 对象进行排序（因此删除 const&）
signature of sort 需要在投影前进行排序 (ranges::less)。转换在这里没有意义

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

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

c++

c++20

std-ranges

g++10