C++ - 没有运算符“<<"匹配这些操作数 directory_iterator() [英] C++ - no operator &quot;&lt;&lt;&quot; matches these operands directory_iterator()

问题描述

我复制了 directory_iterator 页面上的示例代码，所以这就是我所拥有的:

#include "pch.h"//为了视觉工作室的利益#include <fstream>#include #include <文件系统>命名空间 fs = std::filesystem;int main(){fs::create_directories("sandbox/a/b");std::ofstream("沙盒/file1.txt");std::ofstream("沙箱/file2.txt");for (auto& p : fs::directory_iterator("sandbox"))std::cout <<p＜＜'\n';//这一行在第一个 '<<'是错误发生的地方fs::remove_all("沙盒");返回0；//由我包含}

错误信息是:

Severity Code 描述 Project File Line Suppression State错误(活动)E0349 无操作符<<"匹配这些操作数

由于我是 C++ 的新手，我可能弄错了，但我对错误的理解基本上是 p 在我的情况下是无法打印到控制台的东西cout.

如果我直接在页面上运行该示例，则该示例有效，因此它没有任何问题，这也是我不期望的.那么问题来了，为什么我会看到这个错误?

我使用最新版本的 Visual Studio 和 C++ 2017.

解决方案

directory_entry 不直接支持流；它通过 path 的 operator<< 加上隐式转换到 const std::filesystem::path & 来获取它.>

由于 LWG 问题 2989 和 3065，path的操作符已经变成了隐藏的朋友，不能再用来流式传输可转换为path，例如 directory_entry.

解决方法是直接请求 .path() 而不是依赖于隐式转换.我已经修复了 cppreference 示例.

<小时>

与 LWG 核实后，此更改似乎是无意的.我已提交LWG 3171.

I've copied the example code on the directory_iterator page, so this is what I have:

#include "pch.h" //for visual studios benefit
#include <fstream>
#include <iostream>
#include <filesystem>
namespace fs = std::filesystem;

int main()
{
    fs::create_directories("sandbox/a/b");
    std::ofstream("sandbox/file1.txt");
    std::ofstream("sandbox/file2.txt");
    for (auto& p : fs::directory_iterator("sandbox"))
        std::cout << p << '\n'; //this line on the first '<<' is where the error occurs
    fs::remove_all("sandbox");
    return 0; //included by me
}

And the error message is:

Severity    Code    Description Project File    Line    Suppression State
Error (active)  E0349   no operator "<<" matches these operands 

As I'm new to C++ I might have gotten things wrong, but my understanding of the error is basically that p in my case is something which can't be printed out to the console using cout.

The example works if I run it directly on the page so there isn't something wrong with it, which I wouldn't expect either. So question is, why am I seeing this error?

I use the latest version of Visual Studio, along with C++ 2017.

解决方案

directory_entry doesn't have streaming support directly; it gets it via path's operator<< plus an implicit conversion to const std::filesystem::path &.

As a result of LWG issues 2989 and 3065, path's operators have been made into hidden friends, and can no longer be used to stream things convertible to a path, such as a directory_entry.

The fix is to ask for .path() directly rather than depend on the implicit conversion. I've fixed the cppreference example.

---

After checking with LWG, this change appears to be unintended. I've filed LWG 3171.

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