title: Delete certain content from txt file using C++
body: <p>I am seeking for help on how to delete certain content from txt file using C++. 
  I know how to open a txt file and write to it, but I am not sure about how to delete certain content in a file. eg, the user opens a file, and want to write "apple" in the file, later they add "banana" in the file, and then hopes to delete "apple". </p>