What’s the best way to check if a file exists in C++? (cross platform)

Question

I have read the answers for What's the best way to check if a file exists in C? (cross platform), but I'm wondering if there is a better way to do this using standard c++ libs? Preferably without trying to open the file at all.

Both stat and access are pretty much ungoogleable. What should I #include to use these?

Solution 1

Use boost::filesystem:

#include <boost/filesystem.hpp>

if ( !boost::filesystem::exists( "myfile.txt" ) )
{
  std::cout << "Can't find my file!" << std::endl;
}

Solution 2

Be careful of race conditions: if the file disappears between the "exists" check and the time you open it, your program will fail unexpectedly.

It's better to go and open the file, check for failure and if all is good then do something with the file. It's even more important with security-critical code.

Details about security and race conditions: http://www.ibm.com/developerworks/library/l-sprace.html

Solution 3

I am a happy boost user and would certainly use Andreas' solution. But if you didn't have access to the boost libs you can use the stream library:

ifstream file(argv[1]);
if (!file)
{
    // Can't open file
}

It's not quite as nice as boost::filesystem::exists since the file will actually be opened...but then that's usually the next thing you want to do anyway.

Solution 4

If your compiler supports C++17 you don't need boost, you can simply use std::filesystem::exists

#include <iostream> // only for std::cout
#include <filesystem>

if (!std::filesystem::exists("myfile.txt"))
{
    std::cout << "File not found!" << std::endl;
}

Solution 5

Use stat(), if it is cross-platform enough for your needs. It is not C++ standard though, but POSIX.

On MS Windows there is _stat, _stat64, _stati64, _wstat, _wstat64, _wstati64.

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Comments

[Rob]

<io.h> for access (which might actually be _access).

[c0m4]

Yes, as therefromhere pointed out.

[c0m4]

Seems to be a bit of a hazzle to install a huge third party library to do something that should be simple

[Andreas Magnusson]

Boost is a library where much of what will eventually be a part of C++ standard library is developed. Many of the people involved with boost are people involved with the C++ standard. So boost isn't just any third party library. If you're programming in C++ you should have boost installed!

[c0m4]

Is io.h normaly available on windows and linux even if its not standard?

[rlerallut]

I seem to recall that b::fs::exists returns "true" on non-existent files on network shares: "\\machine\share\this_file_doesnt_exist" => true. Last time I checked was on boost 1.33, use caution...

[Alex B]

access() is POSIX function that is available via <unistd.h> on Linux.

[Nemanja Trifunovic]

If your compiler comes with a tr1 implementation, you don't even need to install Boost. It will be in the std::tr1::filesystem

[Andreas Magnusson]

Actually ASFAIK it didn't make TR1 but will be added at a later stage. I also didn't find any references to it in the official TR1 draft: open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1836.pdf

[activout.se]

<sys/types.h> and <sys/stat.h> See msdn.microsoft.com/en-us/library/14h5k7ff(VS.71).aspx

[scigor]

But with this code you would also jump into the if clause if you don't have permissions for the file, although it exists. In most cases it won't matter, but still worth mentioning.

[Ela782]

I think they said at BUILD2013 that a standard-committee working group is working on that and they will include it (most likely boost::fs v2) as a package separate from C++14 but at similar time.

[Pritesh Acharya]

boost::filesystem::exists checks the existance of absolute path. how to check the existence of relative path.?

[Andreas Magnusson]

@PriteshAcharya: boost::filesystem::exists(boost::filesystem::absolute(relati‌​vePath))

[FelixJongleur42]

Noticed that good() also yields true if the given argument denotes a directory, see stackoverflow.com/questions/9591036/…

[Marcel Tricolici]

unfortunately boost filesystem cannot be used in windows phone :((

[HelloWorld]

I agree, even a lot of parts are moved to - or influenced C++ - IMHO boost is not the solution for everything. I appreciate non-templates when possible. Keep your source safe and simple.

[Andreas Magnusson]

@HelloWorld: I fail to see any templates in the code above. Is the snippet above not the simplest possible?

[Rastikan]

Good tip, but ended up not using it because boost::filesystem::exists ( path.cpp) requires yet another module or library std::locale::locale(char const*)

[Michael]

What if you're writing a ls-like program ? I'm guessing the original poster here doesn't want to open the file, at all. Posix's stat function is supposed to give you informations about the file's permissions though, so it would fix that problem.

[gsamaras]

Nice answer +1 for NOT USING BOOST, since it's an overkill, however it wasn't trivial to write that from what is provided here, so I just posted an answer. Check it please.

[pavon]

Not the downvoter, but the question asked for a cross-platform solution, and stat doesn't exist on all platforms.

[void.pointer]

Surprised no one has mentioned this, but this will return 'true' for directories too. myfile.txt could be a directory on linux or windows. If you want to require it be a file, you can use boost::filesystem::is_regular_file().

[John]

The link is not available any more.