anyone know a C++ api for reading/writing object code?

we have this project that the professor wants us to do an assembler so here is the problem how can i read write an object file in my assembler? is there is any api to do that stuff for me?
in which i pass the instructions i want to be put in that object file and it writes the file and load it to see the instructions
any help will be much appreciated completely clueless here thanks in advance

EDIT
i forgot to mention that i need the object file in windows format

>is there is any api to do that stuff for me?
...

Woudln't that defeat the purpose of doing it (which I think was Prelude's point)?

Download the source code for NASM and start reading.

But since your homework is to write your own assembler, then you can define your own "object" file format however you want to.