How do a test Engineer carry out unit testing ?

How do a test Engineer carry out unit testing of the c++ code.   As we have learned in OOP a class e.g. is dependent upon so many outer variables and other objects as well, so how will the test engineer test it independently?   Is he suppose to write another piece of code to test each class separately? ....is there any off the shelf tool available for this purpose?

 Unit testing is a two way work as developer is doing unit testing by himself when he is writing code. When quality assurance team or test engineer performs test, they mention error containing module/block to developer for testing because test engineer can't do unit testing of code. It is done by the develper. When developer is asked then he again checks the code for issues.  Code analysis, code checkers and code optimizers are also used for this purpose.