An MM file is a source code file that can contain both Objective-C and Objective-C++ programming code. It is used on MacOS and differentiates from .M files that contains Objective-C code only. When the compiler sees the .mm extension, it is able to compile the C++ classes present in the file. Since a single source code file can contain a mix of both C and C++ languages, renaming the .m files to .mm extension allows the C++ features to set in. Applications that can open .MM file include Apple TextEdit, Microsoft Notepad, Notepad++, and Microsoft Wordpad.