Re: throwing exception in recursive function.
Peter_APIIT wrote:
Hello. I would like if it's correct to throw an exception >in a
recursive funcion. I mean, imagine my recursive function >has been
called 5 times itself, and inside that call, it throws an >exception.
Will this cause any problem?
Assume the recurisve called is from stack 1 to stack 5 where it throw
an exception.
I wonder does the exception unwind the stack from stack 5 until stack
1.
If there is a matching handler in the stack-frames above stack-1, and
there is no matching handler in the recursive function, then yes, the
stack will be unwound from stack-5 to stack-1 and as much further as
needed.
Stack unwinding works like this: Starting from the current function,
going up towards main(), the compiler checks if there is a suitable
catch handler for the current exception. Once the first suitable catch
handler is found, the stack is unwound up to that point, and execution
continues in the catch handler.
If the compiler is unable to find any suitable catch handler, then the
program is aborted, but it depends on the compiler if the stack is
first unwound or not.
The compiler does not care if there are recursive functions present on
the stack or not, because each function has its own stack-frame anyway.
Here is a small program to show the stack unwinding:
#include <iostream>
using namespace std;
struct recurse
{
recurse() : level(++counter)
{
cout << "Entering recursion level " << level << endl;
}
~recurse()
{
cout << "Leaving recursion level " << level << endl;
}
int level;
static int counter;
}
int recurse::counter = 0;
void foo(int max_levels)
{
recurse r;
if (r.level == max_levels)
{
cout << "Reached maximum recursion depth. Throwing now." << endl;
throw 42;
}
else
{
cout << "Calling foo() recursively" << endl;
foo(max_levels);
cout << "Recursive call of foo() returned normally" << endl;
}
}
int main()
{
try {
cout << "Calling recursive function foo(5)" << endl;
foo(5);
cout << "Recursive call of foo() returned normally" << endl;
}
catch (int)
{
cout << "Caught exception in main()" << endl;
}
return 0;
}
This program should print:
Calling recursive function foo(5)
Entering recursion level 1
Calling foo() recursively
Entering recursion level 2
Calling foo() recursively
Entering recursion level 3
Calling foo() recursively
Entering recursion level 4
Calling foo() recursively
Entering recursion level 5
Reached maximum recursion depth. Throwing now.
Leaving recursion level 5
Leaving recursion level 4
Leaving recursion level 3
Leaving recursion level 2
Leaving recursion level 1
Caught exception in main()
Thanks.
Bart v Ingen Schenau
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