Testing User Defined Header File and other questions - c++

Question

I have a question about testing user-defined header file.
So, here is the example.
#include <iostream>
using namesapce std;
#include "header.h"
#include "header.h" // I wonder the reason why I need to write this down two time for testing ifndef test.
int main()
{
cout << "Hello World!" << endl;
cin.get();
}
I learned that I need to write down user-defined header file's name in driver.cpp two times. But, I cannot understand the reason why I need to do that.
And, here is the second question.
#include <iostream>
#include <fstream>
using namesapce std;
int main()
{
fstream fin;
fin.open("info.txt");
if(!fin.good()) throw "I/O Error\n"; // I want to know what throw exactly do.
else cout << "Hello World!\n";
cin.get();
}
So, my question is the function of throw.
I realized that if I use throw instead of cout, my compiler would be terminated.
I want to know what does throw exactly do.
Since I am new to be here, I might have made some mistake about formatting and some rules, so feel free to point them out if I made it.
Thank you !

Answer 1

To your first question imagine following example:
MyHelper.cpp
int sum( int first , int second )
{
return first + second;
}
test.cpp
#include<iostream>
#include"MyHelper.cpp"
#include"MyHelper.cpp"
int main()
{
std::cout << sum(2,4) << std::endl;
}
Now as you know the precompiler replaces the line #include"MyHelper.cpp" with the content of the corresponding file.
// here comes the code from iostream header
// ...
int sum( int first , int second )
{
return first + second;
}
int sum( int first , int second )
{
return first + second;
}
int main()
{
std::cout << sum(2,4) << std::endl;
}
This is the code which the compiler receives. As You see the symbol sum is now defined more than once. So the compiler will exit with an error like:
t.c: In function ‘int sum(int, int)’:
t.c:7:9: error: redefinition of ‘int sum(int, int)’
int sum( int first , int second )
^
t.c:3:9: note: ‘int sum(int, int)’ previously defined here
int sum( int first , int second )
^
In other words if something with your includeguard is wrong then the compiler will report it to you with this error.
Second question:
throws is used to throw exceptions. I think there are thousands of tutorials to exceptions. simply ask your search engine of choice for "C++ exceptions tutorial" and follow one or two of them. Like following one for example:
http://www.learncpp.com/cpp-tutorial/152-basic-exception-handling/
(scroll a bit down to "A more realistic example" if you're only interested in use cases)