If you freeze bacon, then thaw it out after it's expiration date, how long is it good for?
up vote 3 down vote. A .o object file file (also .obj on Windows) contains compiled object code (that is, machine code produced by your C or C++ compiler), together with the names of the functions and other objects the file contains. Object files are processed by the linker to produce the final executable.
A file extension is the set of three or four characters at the end of a filename; in this case, .obj. File extensions tell you what type of file it is, and tell Windows what programs can open it. Windows often associates a default program to each file extension, so that when you double-click the file, the program launches automatically.