Bad Re-throw compiles but crash at runtime

Question

Following code will compile but crash at run time:

int main() {

    try {
        throw;
    }
    catch(...){
        cout<<"In catch";
    }
     return 0;

}

Result: “Unhandled exception at 0x7c812a5b in hello.exe: Microsoft C++ exception: [rethrow] @ 0x00000000”

Why compiler allows the code to compile? it looks not so difficult job for compiler to check if this code is part of catch block or not.

Answer 1

Not so easy. You could have called this function from inside a catch block in some other function.

A concept of so-called exception handler is described in this answer.

Answer 2

How should it know the caller of the function will not catch the exception?

Answer 3

There are probably millions of errors that compilers could catch, if the compiler writers put enough work into them. But those compiler writers have to make judgements about whether that work is worthwhile. In this case, they decided not (and I agree with them).

Answer 4

You can put the throw in a function which is called from a catch block. Sometimes useful if you have a common handling for a class of exceptions:

void handleXExceptions()
{
   try {
      throw;
   } catch (XA&) {
      ...
   } catch (XB&) {
      ...
   } catch (X&) {
      assert("Update handleXExceptions" == NULL); 
   }
}

void f() {
   try {
      ...
   } catch (X&) {
      handleXExceptions();
   }
}

void g() {
   try {
      ...
   } catch (X&) {
      handleXExceptions();
   }
}

Answer 5

Simply because the code is legal. You could say the same about:

int* p=0;
*p = 0;

and thousands of other examples. It's legal but very wrong.

Answer 6

Thanks, I think i got the point, it is something like throwing null exception (there is nothing to re throw) and compiler is not supposed to do null checks for us.

Answer 7

From C++ Standard (15.1.8)

If no exception is presently being handled, executing a throw-expression with no operand calls std::terminate()

As standard allows it and gives clear semantics, a compiler can only conform to it.