a.h is included in b.h and I include b.h in my source file, do I need to include a.h in my source file as well? - header-files

Question

I am going through an example in a c++ book. And did not understand a certain case.
The main.cpp includes stdexcept.h to use runtime exception. And also includes ErrorHandlingModule.h.
ErrorHandlingModule.h already includes stdexcept.h because of the runtime error parameter it has in the function prototype in it.
The book says I have to include stdexcept.h also in main.cpp. In the source code of the book it is also written that way.
When I remove sdtexcept.h from main.cpp compile passes just fine and program works ok still.
But why the book says that?
Thanks for any answers.
MAIN.CPP:
#include <iostream>
#include <stdexcept>
#include "ErrorHandlingModule.h"
#include "Prompt.h"
// ....
int main(int argc, char* argv[])
{
SAMSErrorHandlingModule::initialize();
do
{
try
{
char Operator = getOperator();
float Operand = getOperand();
cout << Accumulate( Operator, Operand ) << endl;
}
catch( runtime_error RuntimeError )
{
SAMSErrorHandlingModule::handleRuntimeError(RuntimeError);
}
catch(...)
{
SAMSErrorHandlingModule::handleNotaNumberError();
};
}while (SAMSPrompt::UserWantsToContinueYorN("More?"));
return 0;
}
ERRORHANDLINGMODULE.H
#include <stdexcept>
#ifndef _ERROR_HANDLING_MODULE_H
#define _ERROR_HANDLING_MODULE_H
namespace SAMSErrorHandlingModule
{
using namespace std;
void initialize( void );
int handleNotaNumberError( void );
int handleRuntimeError( runtime_error theRuntimeError );
}
#endif // _ERROR_HANDLING_MODULE_H

Answer 1

I think the reason is maintainance. At least, I would do that for this reason. If you eventually decide to switch to "ErrorHandlingModule.h" to something else (e.g. you implement a better error handling strategy and want to use the new one) you don't want the other code into main to break because of this.
This means that if you are using something from stdexcept in your main, apart from using it for input to/output from ErrorHandlingModule, you should include it. Otherwise, I would not include it, since it is not needed when ErrorHandlingModule is not used.
As you noted, this has nothing to do with compilation, as your program compiles just fine if you don't include it into the main. With this respect, however, notice that even if it compiles, it is not always "semantically" correct. In fact, your code might compile even if you remove the inclusion from "ErrorHandlingModule.h" (since stdexcept is included before including ErrorHandlingModule.h). This is the reason why I always prefer to have "..." includes before <...> ones.
Also check this question for a similar discussion.

Answer 2

No, you do not need the #include for stdexcept in both files. The pre-processor will copy the header file into the c file before running through the C compiler. Multiple includes of the same header file can be very problematic which is why the example code uses #ifndef for the header file you are creating to make sure that the header code is only included once. Like Stefano said though, if you no longer include stdexcept and then stop using the ERRORHANDLINGMODULE header file your code will no longer compile.