Two C++ Questions

Question

Question 1: In a challenge, it says ## can also be used as a preprocessor other than just #. But I do ##include <iostream> and I get an error. Can someone explain it?

Question 2: Can someone explain this C++ code:

#include <iostream>
using namespace std;

void foo(int x) try{
int y=2;
throw 1;
}

catch(int e)
{
cout << e << endl << x << endl; //how can it access x but not y?
//cout << y << endl; //doesn't work?
}

int main() {
foo(3);
return 0;
}

Answer 1

1. It is really also used by preporcessor, but has different role: https://docs.microsoft.com/en-us/cpp/preprocessor/token-pasting-operator-hash-hash?view=msvc-160 , https://www.cprogramming.com/reference/preprocessor/token-pasting-operator.html
2. It is because the code after `try` block and the code after the `catch (int)` are in different blocks and thus are using their own namespace (block of a code inside which the names are accessible and outside which are not (unless some spacial command like calling a function and passing variable values or addresses as arguments)) which is initially the same as the namespace of the parent block. You can try how it is actually working: declare and initialize a variable, then do something with it; now place the variable declaring and initializing into braces; now remove that braces and place them around the code which uses this variable.

3. Please, ask one question (or several hardly related to each other questions) per question.

Answer 2

Thanks Martin Taylor. I didn’t know about token pasting operator, thanks. Thanks get