How to get file extension from string in C++

Question

Given a string \"filename.conf\", how to I verify the extension part?

I need a cross platform solution.

Answer 1

Here's a function that takes a path/filename as a string and returns the extension as a string. It is all standard c++, and should work cross-platform for most platforms.

Unlike several other answers here, it handles the odd cases that windows' PathFindExtension handles, based on PathFindExtensions's documentation.

wstring get_file_extension( wstring filename )
{
    size_t last_dot_offset = filename.rfind(L'.');
    // This assumes your directory separators are either \ or /
    size_t last_dirsep_offset = max( filename.rfind(L'\\'), filename.rfind(L'/') );

    // no dot = no extension
    if( last_dot_offset == wstring::npos )
        return L"";

    // directory separator after last dot = extension of directory, not file.
    // for example, given C:\temp.old\file_that_has_no_extension we should return "" not "old"
    if( (last_dirsep_offset != wstring::npos) && (last_dirsep_offset > last_dot_offset) )
        return L"";

    return filename.substr( last_dot_offset + 1 );
}