Std::Filesystem' Has Not Been Declared After Including <Experimental/Filesystem>

std::filesystem' has not been declared after including experimental/filesystem

You use std::filesystem::directory_iterator. std::filesystem::directory_iterator and the entire namespace std::filesystem are declared in the header <filesystem>.

You haven't included the header <filesystem>. Instead, you've included <experimental/filesystem>. This header does not declare std::filesystem::directory_iterator. Instead, it declares std::experimental::filesystem::directory_iterator.

You can consistently use either the standard filesystem library, or the experimental filesystem library from the technical specification, but you must not mix them together. If you're targeting C++17, then you should use <filesystem>.

I will get the error that filesystem: No such file or directory

The ideal solution is to upgrade your compiler to a version that has non-experimental support for C++17. Otherwise use the experimental TS or Boost.

C++ an VS error: experimental/filesystem header providing std::experimental::filesystem is deprecated by Microsoft and will be REMOVED

I had the same problem and then, when I removed the experimental filesystem and added:

#include <filesystem>
namespace fs = std::filesystem;

I would get the error:

namespace “std” has no member “filesystem”

The solution was to go to project properties and do as instructed in the link.
VS2017: E0135 namespace "std" has no member "filesystem"

Using std::experimental::filesystem with g++ 9.3.0

I would use the version of C++ standard in combination with a pre-processor directive to distinguish between them as it the filesystem library has been officially introduced in C++17

#if __cplusplus >= 201703L
  #include <filesystem>
  namespace fs = std::filesystem;
#else
  #include <experimental/filesystem>
  namespace fs = std::experimental::filesystem;
#endif

In case this occurs to you with another library in C++17 you can use __has_include

#if __has_include(<some_lib>)
  #include <some_lib>
  namespace sl = std::some_lib;
#else
  #include <experimental/some_lib>
  namespace sl = std::experimental::some_lib;
#endif

C++ experimental filesystem library incomplete?

You are confused, because std::experimental::filesystem (<experimental/filesystem>) contains the Filesystem TS, not the filesystem additions to C++17 which are found in std::filesystem (<filesystem>) if your standard library implements them (although the latter is based on the former).

In the TS there is a std::experimental::filesystem::is_regular_file free function (which you link to in the question). This free std::filesystem::is_regular_file also exists in C++17.

However in C++17 the std::filesystem::directory_entry class also has a member function is_regular_file.

This member function is absent in the TS version std::experimental::filesystem::directory_entry.

The TS is not exactly the same as what was then later added to the standard library for C++17.

VS2017: E0135 namespace std has no member filesystem

A couple of options to investigate.

• Check the language standard. You need C++17 or above:
  
  
• If your version of visual studio doesn't support std::filesystem yet, you can try std::experimental::filesystem.

I have the following version and std::filesystem works (with the C++17 language selection shown above):

Microsoft Visual Studio Community 2017 Preview
Version 15.8.0 Preview 1.1
VisualStudio.15.Preview/15.8.0-pre.1.1+27705.2000

Link errors using filesystem members in C++17

Add the flag -lstdc++fs:

$ g++-7 test.cpp -std=c++17 -lstdc++fs

gcc 7.2 supports C++17 experimental filesystem namespace only. I do not know, maybe gcc 7.3 supports std filesystem namespace already.

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