c++ - 在移出边界之前停止 directory_iterator

Question

我正在尝试遍历目录并读取其中的图像，同时允许跳过每第 n 个文件。

我的代码目前看起来像这样:

// Count number of files in directory and reserve memory (take skipped slices into account)
std::experimental::filesystem::directory_iterator it_begin{path};
int count = count_if(it_begin, std::experimental::filesystem::directory_iterator(), [](const std::experimental::filesystem::directory_entry & d) {return !is_directory(d.path());});
auto maxCount = count - (count % (1 + skipSlices));
m_imageStack.reserve(maxCount);

// Read image stack and check for validity
int n = 0;
for (std::experimental::filesystem::v1::directory_iterator it(path); it != std::experimental::filesystem::v1::directory_iterator(); std::advance(it, 1 + skipSlices))
{
    std::string filename{std::experimental::filesystem::v1::path(*it).string()};
    cv::Mat image = cv::imread(filename);
    m_imageStack.push_back(image);

    n = n + 1 + skipSlices;
    if (n == maxCount) break;
}

如果 skipSlices = 1，我只想读取每 2 张图像，等等。为了不超出范围，我在 for 循环中添加了一个中断条件。我现在的解决方案非常糟糕，我想摆脱中断，而是在 for 循环中使用更正确的停止条件。但是，我找不到告诉迭代器在前进太多之前停止的方法。有什么解决办法吗？

Answer 1

只需编写一个有限制的 advance 版本即可。

namespace detail {
    template<class It, class Dist>
    void advance_with_limit_impl(It& it, It end, Dist n, std::random_access_iterator_tag) {
        if(n > 0) it += std::min(end - it, n);
        else it += std::max(end - it, n);
    }

    template<class It, class Dist>
    void advance_with_limit_impl(It& it, It end, Dist n, std::bidirectional_iterator_tag) {
        if(n > 0) { while(n != 0 && it != end) { --n; ++it; } }
        else { while (n != 0 && it != end) { ++n; --it; } }
    }

    template<class It, class Dist>
    void advance_with_limit_impl(It& it, It end, Dist n, std::input_iterator_tag) {
        while(n != 0 && it != end) { --n; ++it; }
    }
}

template<class It>
void advance_with_limit(It& it, It end, 
                        typename std::iterator_traits<It>::difference_type n) {
    detail::advance_with_limit_impl(it, end, n, 
                               typename std::iterator_traits<It>::iterator_category());
}

然后只需使用 advance_with_limit(it, {}, 1 + skipSlices)。