(C++) How do I get a filename's extension?

include <string>

include <cassert>


std::string getExtension(const std::string& fileName)
{
  const int length = fileName.length();
  for (int i=0; i!=length; ++i)
  {
    if (fileName[i]=='.') 
    {
      return fileName.substr(i+1,length);
    }
  }
  return fileName;
}
int main()
{
  assert(getExtension("Test.txt")=="txt");
}

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• assert
• string