迭代元组向量 c+17 样式不起作用？

Question

我目前正在寻找可以迭代这个元组向量的简洁方法..

这就是我目前正在做的事情？

#include <experimental/filesystems>
#include <tuple>
#include <iostream>
#include <vector>


std::tuple<std::experimental::filesystem::path, std::experimental::filesystem::file_status, std::size_t>
file_info(const std::experimental::filesystem::directory_entry &entry)
{

    const std::experimental::filesystem::file_status fs(std::experimental::filesystem::status(entry));
    return {entry.path(),
                fs,
                std::experimental::filesystem::is_regular_file(fs) ? std::experimental::filesystem::file_size(entry.path()) : 0u};

}



int main ()
{
    std::experimental::filesystem::path _path(string_dir_to_test_files);  // string_dir_to_test_files is just a string 
    std::experimental::filesystem::directory_entry dir_path(_path);
    if (std::experimental::filesystem::exists(_path))
    {
        std::cout << "exists() = " << std::experimental::filesystem::exists(_path) << std::endl;
        std::cout << "Number of files in directory: " << number_of_files(_path) << std::endl;
        std::vector<std::tuple<std::experimental::filesystem::path,std::experimental::filesystem::file_status, std::size_t>> items;
        std::transform(std::experimental::filesystem::directory_iterator(_path),{},back_inserter(items),file_info);
        for( auto const& index : items)
        {
            std::cout << std::get<0>(index) << std::endl; // This line could be written better?
        }
    }
    else
    {
        std::cout << "No data to build database with!" << std::endl;
        exit(1);
    }

    return 0;
}

尽管它完成了工作，但它似乎并不 "reader" 友好，有没有其他方法可以让我以更 reader 友好的方式迭代向量的元素方式？

我试图使 for 循环更具可读性的一种方法是像这样：

for (auto const& [path, status, size] : items) {
    std::cout << path << std::endl;
}

但这不知何故给我这个错误：

/home/noob/soundcloud/src/include/database/database.cpp: In constructor ‘database::database()’:
/home/noob/soundcloud/src/include/database/database.cpp:31:25: error: expected unqualified-id before ‘[’ token
         for(auto const& [path, status, size] : items)
                         ^
/home/noob/soundcloud/src/include/database/database.cpp:31:25: error: expected ‘;’ before ‘[’ token
/home/noob/soundcloud/src/include/database/database.cpp:31:26: error: ‘path’ was not declared in this scope
         for(auto const& [path, status, size] : items)
                          ^~~~
/home/noob/soundcloud/src/include/database/database.cpp:31:26: note: suggested alternatives:
In file included from /usr/include/c++/6/experimental/filesystem:39:0,
                 from /home/noob/soundcloud/src/include/database/database.h:9,
                 from /home/noob/soundcloud/src/include/database/database.cpp:1:
/usr/include/c++/6/experimental/bits/fs_path.h:79:9: note:   ‘std::experimental::filesystem::v1::path’
   class path
         ^~~~
/usr/include/c++/6/experimental/bits/fs_path.h:79:9: note:   ‘std::experimental::filesystem::v1::path’
/home/noob/soundcloud/src/include/database/database.cpp:31:32: error: ‘status’ was not declared in this scope
         for(auto const& [path, status, size] : items)
                                ^~~~~~
/home/noob/soundcloud/src/include/database/database.cpp:31:32: note: suggested alternatives:
In file included from /usr/include/c++/6/experimental/filesystem:41:0,
                 from /home/noob/soundcloud/src/include/database/database.h:9,
                 from /home/noob/soundcloud/src/include/database/database.cpp:1:
/usr/include/c++/6/experimental/bits/fs_ops.h:274:15: note:   ‘std::experimental::filesystem::v1::status’
   file_status status(const path& __p, error_code& __ec) noexcept;
               ^~~~~~
/usr/include/c++/6/experimental/bits/fs_ops.h:274:15: note:   ‘std::experimental::filesystem::v1::status’
/home/noob/soundcloud/src/include/database/database.cpp:31:40: error: capture by copy of incomplete type ‘<unresolved overloaded function type>’
         for(auto const& [path, status, size] : items)
                                        ^~~~
/home/noob/soundcloud/src/include/database/database.cpp: In lambda function:
/home/noob/soundcloud/src/include/database/database.cpp:31:46: error: expected ‘{’ before ‘:’ token
         for(auto const& [path, status, size] : items)
                                              ^
/home/noob/soundcloud/src/include/database/database.cpp: In constructor ‘database::database()’:
/home/noob/soundcloud/src/include/database/database.cpp:31:46: error: expected ‘;’ before ‘:’ token
/home/noob/soundcloud/src/include/database/database.cpp:31:46: error: expected primary-expression before ‘:’ token
/home/noob/soundcloud/src/include/database/database.cpp:31:46: error: expected ‘)’ before ‘:’ token
/home/noob/soundcloud/src/include/database/database.cpp:31:46: error: expected primary-expression before ‘:’ token
make[2]: *** [src/include/database/CMakeFiles/database.dir/database.cpp.o] Error 1
make[1]: *** [src/include/database/CMakeFiles/database.dir/all] Error 2
make: *** [all] Error 2

Answer 1

在 C++17 中，你可以这样做：

for (auto const& [path, status, size] : items) {
    std::cout << path << std::endl;
}

Answer 2

在 C++11 中你可以这样做

for (auto const& tup : items)
    std::experimental::filesystem::path path;
    std::tie(path, std::ignore, std::ignore) = tup;
    std::cout << path << std::endl;
}

迭代元组向量 c+17 样式不起作用？

Iterating through a vector of tuples c+17 style don't work?

c++

for-loop

code-readability

c++17

structured-bindings