How to get file extension from string in C++

Is this too simple of a solution?

#include <iostream>
#include <string>

int main()
{
  std::string fn = "filename.conf";
  if(fn.substr(fn.find_last_of(".") + 1) == "conf") {
    std::cout << "Yes..." << std::endl;
  } else {
    std::cout << "No..." << std::endl;
  }
}

The best way is to not write any code that does it but call existing methods. In windows, the PathFindExtension method is probably the simplest.

So why would you not write your own?

Well, take the strrchr example, what happens when you use that method on the following string "c:\program files\AppleGate.Net\readme"? Is ".Net\readme" the extension? It is easy to write something that works for a few example cases, but can be much harder to write something that works for all cases.

You have to make sure you take care of file names with more then one dot. example: c:\.directoryname\file.name.with.too.many.dots.ext would not be handled correctly by strchr or find.

My favorite would be the boost filesystem library that have an extension(path) function

Assuming you have access to STL:

std::string filename("filename.conf");
std::string::size_type idx;

idx = filename.rfind('.');

if(idx != std::string::npos)
{
    std::string extension = filename.substr(idx+1);
}
else
{
    // No extension found
}

Edit: This is a cross platform solution since you didn't mention the platform. If you're specifically on Windows, you'll want to leverage the Windows specific functions mentioned by others in the thread.

With C++17 and its std::filesystem::path::extension (the library is the successor to boost::filesystem) you would make your statement more expressive than using e.g. std::string.

#include <iostream>
#include <filesystem> // C++17
namespace fs = std::filesystem;

int main()
{
    fs::path filePath = "my/path/to/myFile.conf";
    if (filePath.extension() == ".conf") // Heed the dot.
    {
        std::cout << filePath.stem() << " is a valid type."; // Output: "myFile is a valid type."
    }
    else
    {
        std::cout << filePath.filename() << " is an invalid type."; // Output: e.g. "myFile.cfg is an invalid type"
    }
}

See also std::filesystem::path::stem, std::filesystem::path::filename.

actually the STL can do this without much code, I advise you learn a bit about the STL because it lets you do some fancy things, anyways this is what I use.

std::string GetFileExtension(const std::string& FileName)
{
    if(FileName.find_last_of(".") != std::string::npos)
        return FileName.substr(FileName.find_last_of(".")+1);
    return "";
}

this solution will always return the extension even on strings like "this.a.b.c.d.e.s.mp3" if it cannot find the extension it will return "".

Using std::string's find/rfind solves THIS problem, but if you work a lot with paths then you should look at boost::filesystem::path since it will make your code much cleaner than fiddling with raw string indexes/iterators.

I suggest boost since it's a high quality, well tested, (open source and commercially) free and fully portable library.

_splitpath, _wsplitpath, _splitpath_s, _wsplitpath_w

This is Windows (Platform SDK) only

For char array-type strings you can use this:

#include <ctype.h>
#include <string.h>

int main()
{
    char filename[] = "apples.bmp";
    char extension[] = ".jpeg";

    if(compare_extension(filename, extension) == true)
    {
        // .....
    } else {
        // .....
    }

    return 0;
}

bool compare_extension(char *filename, char *extension)
{
    /* Sanity checks */

    if(filename == NULL || extension == NULL)
        return false;

    if(strlen(filename) == 0 || strlen(extension) == 0)
        return false;

    if(strchr(filename, '.') == NULL || strchr(extension, '.') == NULL)
        return false;

    /* Iterate backwards through respective strings and compare each char one at a time */

    for(int i = 0; i < strlen(filename); i++)
    {
        if(tolower(filename[strlen(filename) - i - 1]) == tolower(extension[strlen(extension) - i - 1]))
        {
            if(i == strlen(extension) - 1)
                return true;
        } else
            break;
    }

    return false;
}

Can handle file paths in addition to filenames. Works with both C and C++. And cross-platform.

You can use strrchr() to find last occurence of .(dot) and get .(dot) based extensions files. Check the below code for example.

#include<stdio.h>

void GetFileExtension(const char* file_name) {

    int ext = '.';
    const char* extension = NULL;
    extension = strrchr(file_name, ext);

    if(extension == NULL){
        printf("Invalid extension encountered\n");
        return;
    }

    printf("File extension is %s\n", extension);
}

int main()
{
    const char* file_name = "c:\\.directoryname\\file.name.with.too.many.dots.ext";
    GetFileExtension(file_name);
    return 0;
}

So, using std::filesystem is the best answer, but if for whatever reason you don't have C++17 features available, this will work even if the input string includes directories:

string getextn (const string &fn) {
  int sep = fn.find_last_of(".\\/");
  return (sep >= 0 && fn[sep] == '.') ? fn.substr(sep) : "";
}

I'm adding this because the rest of the answers here are either strangely complicated or fail if the path to the file contains a dot and the file doesn't. I think the fact that find_last_of can look for multiple characters is often overlooked.

It works with both / and \ path separators. It fails if the extension itself contains a slash but that's usually too rare to matter. It doesn't do any filtering for filenames that start with a dot and contain no other dots -- if this matters to you then this is the least unreasonable answer here.

Example input / output:

/ => ''
./ => ''
./pathname/ => ''
./path.name/ => ''
pathname/ => ''
path.name/ => ''
c:\path.name\ => ''
/. => '.'
./. => '.'
./pathname/. => '.'
./path.name/. => '.'
pathname/. => '.'
path.name/. => '.'
c:\path.name\. => '.'
/.git_ignore => '.git_ignore'
./.git_ignore => '.git_ignore'
./pathname/.git_ignore => '.git_ignore'
./path.name/.git_ignore => '.git_ignore'
pathname/.git_ignore => '.git_ignore'
path.name/.git_ignore => '.git_ignore'
c:\path.name\.git_ignore => '.git_ignore'
/filename => ''
./filename => ''
./pathname/filename => ''
./path.name/filename => ''
pathname/filename => ''
path.name/filename => ''
c:\path.name\filename => ''
/filename. => '.'
./filename. => '.'
./pathname/filename. => '.'
./path.name/filename. => '.'
pathname/filename. => '.'
path.name/filename. => '.'
c:\path.name\filename. => '.'
/filename.tar => '.tar'
./filename.tar => '.tar'
./pathname/filename.tar => '.tar'
./path.name/filename.tar => '.tar'
pathname/filename.tar => '.tar'
path.name/filename.tar => '.tar'
c:\path.name\filename.tar => '.tar'
/filename.tar.gz => '.gz'
./filename.tar.gz => '.gz'
./pathname/filename.tar.gz => '.gz'
./path.name/filename.tar.gz => '.gz'
pathname/filename.tar.gz => '.gz'
path.name/filename.tar.gz => '.gz'
c:\path.name\filename.tar.gz => '.gz'