Differentiate between the message and method.-c++ interview question with solution

Differentiate between the message and method.
Message:
* Objects communicate by sending messages to each other.
* A message is sent to invoke a method.

Method
* Provides response to a message.
* It is an implementation of an operation.

What do you mean by Stack unwinding? 
It is a process during exception handling when the destructor is called for all local objects between the place where the exception was thrown and where it is caught.

What are the conditions that have to be met for a condition to be an invariant of the class? 
* The condition should hold at the end of every constructor.
* The condition should hold at the end of every mutator (non-const) operation.

Name some pure object oriented languages. 
Smalltalk, Java, Eiffel, Sather. .