Exceptions and Stack Unwinding in C++
In the C++ exception mechanism, control moves from the throw statement to the first catch statement that can handle the thrown type. When the catch statement is reached, all of the automatic variables that are in scope between the throw and catch statements are destroyed in a process that is known as stack unwinding. In stack unwinding, execution proceeds as follows:
Control reaches the try statement by normal sequential execution. The guarded section in the try block is executed.
If no exception is thrown during execution of the guarded section, the catch clauses that follow the try block are not executed. Execution continues at the statement after the last catch clause that follows the associated try block.
If an exception is thrown during execution of the guarded section or in any routine that the guarded section calls either directly or indirectly, an exception object is created from the object that is created by the throw operand. (This implies that a copy constructor may be involved.) At this point, the compiler looks for a catch clause in a higher execution context that can handle an exception of the type that is thrown, or for a catch handler that can handle any type of exception. The catch handlers are examined in order of their appearance after the try block. If no appropriate handler is found, the next dynamically enclosing try block is examined. This process continues until the outermost enclosing try block is examined.
If a matching handler is still not found, or if an exception occurs during the unwinding process but before the handler gets control, the predefined run-time function terminate is called. If an exception occurs after the exception is thrown but before the unwind begins, terminate is called.
If a matching catch handler is found, and it catches by value, its formal parameter is initialized by copying the exception object. If it catches by reference, the parameter is initialized to refer to the exception object. After the formal parameter is initialized, the process of unwinding the stack begins. This involves the destruction of all automatic objects that were fully constructed—but not yet destructed—between the beginning of the try block that is associated with the catch handler and the throw site of the exception. Destruction occurs in reverse order of construction. The catch handler is executed and the program resumes execution after the last handler—that is, at the first statement or construct that is not a catch handler. Control can only enter a catch handler through a thrown exception, never through a goto statement or a case label in a switch statement.
Stack Unwinding Example
The following example demonstrates how the stack is unwound when an exception is thrown. Execution on the thread jumps from the throw statement in C to the catch statement in main, and unwinds each function along the way. Notice the order in which the Dummy objects are created and then destroyed as they go out of scope. Also notice that no function completes except main, which contains the catch statement. Function A never returns from its call to B(), and B never returns from its call to C(). If you uncomment the definition of the Dummy pointer and the corresponding delete statement, and then run the program, notice that the pointer is never deleted. This shows what can happen when functions do not provide an exception guarantee. For more information, see How to: Design for Exceptions. If you comment out the catch statement, you can observe what happens when a program terminates because of an unhandled exception.
#include <string>
#include <iostream>
using namespace std;
class MyException{};
class Dummy
{
public:
Dummy(string s) : MyName(s) { PrintMsg("Created Dummy:"); }
Dummy(const Dummy& other) : MyName(other.MyName){ PrintMsg("Copy created Dummy:"); }
~Dummy(){ PrintMsg("Destroyed Dummy:"); }
void PrintMsg(string s) { cout << s << MyName << endl; }
string MyName;
int level;
};
void C(Dummy d, int i)
{
cout << "Entering FunctionC" << endl;
d.MyName = " C";
throw MyException();
cout << "Exiting FunctionC" << endl;
}
void B(Dummy d, int i)
{
cout << "Entering FunctionB" << endl;
d.MyName = "B";
C(d, i + 1);
cout << "Exiting FunctionB" << endl;
}
void A(Dummy d, int i)
{
cout << "Entering FunctionA" << endl;
d.MyName = " A" ;
// Dummy* pd = new Dummy("new Dummy"); //Not exception safe!!!
B(d, i + 1);
// delete pd;
cout << "Exiting FunctionA" << endl;
}
int main()
{
cout << "Entering main" << endl;
try
{
Dummy d(" M");
A(d,1);
}
catch (MyException& e)
{
cout << "Caught an exception of type: " << typeid(e).name() << endl;
}
cout << "Exiting main." << endl;
char c;
cin >> c;
}
/* Output:
Entering main
Created Dummy: M
Copy created Dummy: M
Entering FunctionA
Copy created Dummy: A
Entering FunctionB
Copy created Dummy: B
Entering FunctionC
Destroyed Dummy: C
Destroyed Dummy: B
Destroyed Dummy: A
Destroyed Dummy: M
Caught an exception of type: class MyException
Exiting main.
*/