How can I know when or why the value of stage changes from 0 to 1 in the veins example?

Question

When I run the example in veins 5.0, I know Multi stage can be defined by numInitStages() function. But I don't know when or why the value of stages changes from 0 to 1 in the BaseModule.cc files.

void BaseModule::initialize(int stage)

Answer 1

The simulation environment is responsible for calling subsequent stages during initialization.
For example, let's assume that one defines for a module:

int numInitStages() const { return 3; }

When the simulation is starting, the simulation environment calls:

initialize(0); 
initialize(1); 
initialize(2); 

What is important: the simulation environment calls initialize(0) for every module, then it calls initialize(1) for every module whose numInitStages() returns 2 or more, then it calls initialize(2) for every module whose numInitStages() returns 3 or more, etc.
Thanks to this, we are sure that initialize(1) will be called after calling initialize(0) in all modules. As a consequence, we may decide - for example - that an address is assigned in stage=1, and a socket that uses this address is opened in stage=2.

Reference: OMNeT++ Simulation Manual